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1973

Functional Studies of Descending Sympathetic Pathways in the Spinal Cord

Robert Dale Foreman Loyola University Chicago

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This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License. Copyright © 1973 Robert Dale Foreman FUNCTIONAL STUDIES OF DESCENDING SYMPATHEI'IC PATHWAYS IN THE SPDIAL CORD

by Robert Dale Foreman

A Disaertation Submitted to the Faculty of the Graduate School of Loyola University in Partial Fulfillment of the Requirements for the Degree of

Doctor of Philosophy

June 1973

·L l , Univers:t·J ,_~_cdi,a1 Center [ 10rary - oyo1a · Dedicated to my parents-­ Dad and Mom Foreman

ii :BIOGRAPHY

Robert D. Foreman was born on April 27, 1946, at Orange City, Iowa. He was raised on a farm near Montevideo, Minnesota and attended a small country school during his elementary school years. After moving back to Orange City, Iowa, he attended Western Christian High School at Hull, Iowa. :Before his college career began, he worked one year as a con­ struction worker in Artesia, California and later as a truck driver for a paint company in Orange City, Iowa. He attended Northwestern

College in Orange City for two years, then transferred to Central College in Pella, Iowa. He was awarded his :Bachelor of Arts degree from Central College in the summer of 1969. After graduation he married Charlotte Poppen, who also attended Central College. She taught fifth grade at Lind.op Elementary School in Broadview, throughout the four years of graduate school. Robert began his graduate studies in the summer of 1969 in the Department of Physiology at Stritch School of Medicine. He worked under the direction of Robert D. Wurster and was supported by a Research Training Grant of the National Institutes of General Medical Sciences, of the National Institutes of Health.

iii ACKNOWLEDGEMENTS

I would like to take this opportunity to express my apprecia­

tion to Dr. Robert D. Wurster for his guidance and direction throughout the pa.st four years. His ability for teaching and for challenging me

to think creatively, ma.de this dissertation possible. Above all, his

inspiration and example will guide my attitudes and future goals as a physiologist.

I would also like to express appreciation to the faculty of the

Department of Physiology for sharing their knowledge and offering their criticism in molding my graduate education, as well as research. The preparation of this dissertation would not have been possible without the help of my dear wife, Charlotte. She spent many long hours typing the dissertation and preparing many of the figures.

She was also a great source of moral support and encouragement through­ out. the past four years. A word of thanks is expressed to my colleagues who shared these past four years of graduate school with me. A special word of appreciation is expressed to Dr. J. Andrew Armour, who inspired thought and shared his opinions about this dissertation and about life in general.

iv PU:BLICATIONS

1. FOREMAN, R.D. and R.D. Wurster. Localization of descending sympathetic spinal pathways. The Physiologist 14: 143, 1971. 2. ·Norris, J.E., R.D. Wurster and R.D. FOREMAN. Responses of the right ventricle during stimulation of ventral roots. The Physiologist 14: 203, 1971. 3. FOREMAN, R.D. and R.D. Wurster. Electrophysiological character­ istics of descending sympathetic spinal pathways. The Physiologist 15: 137, 1972.

4. FOREMAN, R.D. and R.D. Wurster. The influence of afferent stim­ ulation on descending sympathetic pathways of the spinal cord. Fed. Proc. 32: 1016, 1973. 5. FOREMAN, R.D. and R.D. Wurster. Localization and functional characteristics of descending sympathetic spinal pathways. Amer. J. Physiol., In Press, 1973. 6. Norris, J.E., R.D. FOREMAN, and R.D. Wurster. Inotropic and chronotropic responses of the canine heart during stimulation of the first five ventral thoracic roots. Circ. Res., Submitted, 1973.

v TABLE OF CONTENTS

Chapter Page

I. INTRODUCTION •••••••••••••••••••••••••••••••••••••••••• 1

II. LITERATURE REVIEW••••••••••••••••••••••••••••••••••••• 4 A. Brain Stem Controlling the Cardiovascular System••••••••••••••••••••••••••••••••••••••••• 4 1. Medulla••••••••••••••••••••••••••••••••• 4 2. Hypothalamus •••••••••.•••••••••••••••••• 11 :B. Pathways Transmitting Cardiovascular ActivitY••••••••••••••••••••••••••••••••••••••• 15 c. Reflex Activation of the Cardiovascular ·System...... 19 1. Somato-sympathetic Reflexes...... 19 2. Baroceptor Reflexes From the Carotid Sinus••••••••••••••••••••••••••••••••••• 26

III. METHODS AND MATERIALS••••••••••••••••••••••••••••••••• 30 A. Animal Preparation••••••••••••••••••••••••••••• 30 B. Lateral Spinal Cord Stimulations ••••••••••••••• 31 c. Reflex and Tonic Experiments ••••••••••••••••••• 33 n. Electrophysiology of the Descending Pathway •••• 35

IV. RESULTS••••••••••••••••••••••••••••••••••••••••••••••• 44 A. Lateral Spinal Cord Stimulation•••••••••••••••• 44 B. Bilateral Carotid Occlusions ••••••••••••••••••• 53 c. Electrophysiological Characteristics ••••••••••• 65 v. DISCUSSION •••••••••••••••••••••••••••••••••••••••••••• 82

VI. CONCLUSIONS ••••••••••.•••••••••••••••••••••••••••••••• • 106 Bll3LIOGRAPHY...... 109

vi LIST OF TABLES

Tables P88'! I. LATERAL SPINAL CORD STil1ULATIONS...... 49

vii LIST OF FIGURES

Figure Page

1. DIAGRAM OF INSTRUMENTS••••••••••••••••••••••••••••••••• 38 2. CALIBRATION GRAPH FOR DETER?1INING CONDUCTION TIME •••••• 39 3. CALIBRATION GRAPHS FOR DETERMINING AMPLITUDES OF EVOKED RESPONSES••••••••••••••••••••••••••••••••••••••• 42 4. CO:NTROL TRACINGS OF RANDOM NOISE AND SPONTANEOUS ACTIVITY••••••••••••••••••••••••••••·•••••••••••••••••• 43 STIMULATION OF THE DORSOLATERAL FUUICULUS •••••••••••••• 45 6. BLOOD PRESSURE RESPONSES FROM STIMULATION OF T1 DORSOLATERAL FUNICULUS••••••••••••••••••••••••••••••••• 47 7. ALTERATION IN BLOOD PRESSURE FROM STilIDLATION OF T2 AND T) DORSOLATERAL FUNICULI...... 48 8. . STI}'ITJLATION OF THE DORSOLATERAL FUNICULUS AFTER UNILATERAL AND BILATERAL LESIONS••••••••••••••••••••••• 52 9. BLOOD PRESSURE RESPONSES DURING BILATERAL CAROTID OCCLUSIONS...... 54 10. BLOOD PRESSURE TRACINGS DURING BILATERAL CAROTID OCCLUSIONS••••••••••••••••••••••••••••••••••••••••••••• 58 11. ALTERATIONS IN BLOOD PRESSURE DURING UNILATERAL CAROTID OCCLUSIONS...... 60 12. BILATERAL CAROTID OCCLUSIONS WITH SHAM PREPARATION ••••• 61

TRANSVERSE SECTION OF THE SPINAL CORD•••••••••••••••••• 63

viii LIST OF FIGURES

Figure Page ALTERATIONS IN HEART RATE BEFORE AND AFTER SPINAL CORD LESIONS...... 64

c2 DORSOLATERAL FUNICULUS STI}IULATION •••••••••••••••••• 66 16. T2 DORSOLATERAL FUNICULUS STD1ULATION •••••••••••••••••• 68 STIMULATION CAUDAL AND ROSTRAL TO CS DORSOLATERAL FUNICULUS LESION••••••••••••••••••••••••••••••••••••••• 70

18. IPSILATERAL T8 STIMULATION OF DORSOLATERAL FUNICULUS •••••••••••••••••••••••••••••••••••••••••••••• 71

IPSILATERAL AND CONTRALATERAL c2 STil1ULATION 2 MM VENTROLATERAL TO THE DLS•••••••••••••••••••••••••• 73

20. IPSILATERAL T2 STHIULATION OF THE DLS BEFORE AND AFTER LESION••••••••••••••••••••••••••••••••••••••••••• 75 21. CONTRALATERALT STIMULATION OF THE DLS BEFORE AND AFTER LEsron ••••••••••••••••••••••••••••••••••••••• 11

22. IPSILATERAL c8 STIMULATION BEFORE AND AFTER LESION ••••• 7S CONTRALATERAL c8 STIJ>IDLATION BEFORE AND AFTER LESION ••• 80 24. IPSILATERAL TS DLS STUIULATION BEFORE AND AFTER LESION •••••••••••••••••••••• e •••••••••••••••••••••••••• 82

ix CHAPTER I INTRODUCTION

Anatomical descriptions of the autonomic nervous system in the spinal cord have been based on the classical studies of Gaskell

(50, 51) and Langley (91). They noted that the sympathetic component of the autonomic nervous system is located between the first thoracic and first or second lumbar segments, forming thoracolumbar outflow. Gaskell divided the sympathetic nervous system into three major com­ ponents. The first component is the afferent nerve which enters the dorsolateral sulcus and terminates on the preganglionic neuron which

is the second component of the sympathetic reflexes. The preganglionic neuron was described as being analagous to the interneuron of the somatic reflex. The third component of the reflex was the postga.ng­

lionic neuron which originated in the paravertebral sympathetic trunk or prevertebral ganglia and terminated on the effector organs. The brain stem (66) and cortex(6o) have been analyzed to de­ termine how the supraspinal central nervous system controls the peripheral sympathetic nervous system. From a study of these two divisions of the sympathetic nervous system, it became apparent that the connections between the supraspinal central nervous system and

1 2 the preganglionic neurons have not been extensively examined.. This observation provided the stimulus for functionally locating and analyzing descending pathways in the spinal cord which transmit sympathetic activity. Systemic blood pressure was utilized to assess the direct effects of sympathetic spinal pathway stimulations on the cardio­ vascular system. The cardiovascular system was used in this study, be­ cause it was of interest to determine how the sympathetic spinal path­ ways were involved in transmitting sympathetic activity from supra­ spinal levels to the cardiovascular system. Along with blood pressure responses, evoked responses were used to evaluate the electrophysiological characteristics of the descending sympathetic pathways. Evoked responses were used to de­ termine conduction latencies and the interaction of somato-sympathetic reflexes.

Direct application of these results from animals to humans may not be possible, but interruption of sympathetic pathways of the spinal cord have been reported clinically in humans (41, 46, 70). Ventral cordotomies for relief of intractable pain have interfered with blood pressure activity. In certain clinical cases individuals suffering from tabes dorsalis have had bouts of postural hypotension.

It was suggested that sympathetic pathways in the spinal cord were involved, but the evidence was not substantial.

In summary, experiments reported in these studies were designed to examine the location, as well as functional and electrophysiological 3 characteristics of the descending sympathetic spinal tracts. Ex­ perimental conditions which permit the analysis of these phenomena

include electrical stimulation, reflex stimulation, lesions and electrophysiological recording techniques. CHAPTER II LITERATURE REVIEW

A. Brain Stem Controlling the Cardiovascular System

1. Medulla Complexities of the neuronal organization and patterns in the

central nervous system have made the analysis of central cardio­ vascular control difficult. The reticular regions of the medulla have been the most thoroughly studied for determining mechanisms which control the cardiovascular system from supraspinal regions. Location of medullary cardiovascular control has been identified as diffuse cells in the dorsolateral reticular formation in the rostral two-thirds of the medulla. One realizes the complexities of the reticular for­ mation after examining the anatomical structures in the brain which were described by Scheibel and Scheibel (134). Using Golgi methods, they analyzed the complex reticular formation. Suffice it to say that

the patterns of synaptic interaction were complex and involved as­

cending, as well as descending pathways in the medullary region. Anatomical and physiological research concerning cardio­ vascular control of the medullary region has been examined and analyzed with several hypotheses being derived from the data presented.

4 5 The basis for a medullary vasomotor "center" originated with

the serial transections of the brain stem by Owsjannikow (102) and Dittmar (37). They showed that blood pressure remained relatively stable until a section was made through the rostral medulla which

caused a marked fall in blood pressure. Sections in the lower medulla further decreased systemic blood pressure until a level was reached which was characteristic of spinal animals. These early investigators demonstrated that a central mechanism for maintainance of a normal level of blood pressure was located three to four millimeters rostral to the calamus scriptorius. Using stimulation techniques, Ranson and Billingsley (121) located pressor and depressor points on the floor of the fourth ventricle of the medulla. They localized a pressor area in the in­ ferior fovea at the apex of the ala cinera and a depressor region in the area prostrema., just lateral to the obex. This was the first con~ vincing evidence of two regions in the medulla which control pressor and depressor activity of the cardiovascular system. Earlier work (115, 116) suggested a vasotonic, as well as pressor and depressor

"centers" but the evidence was not convincing. After injection of d-tubocurarine into the jugular vein of the animal, it was noted that reflex activity was depressed but no change occurred in the tonic level~ of blood pressure. Injecting d-tubocurarine into the systemic circulation could have affected peripheral autonomic or effector mechanisms without having any significant medullary effects. Concepts of pressor and depressor areas on the floor of the 6 fourth ventricle were seriously questioned by Scott (137) in 1925. He observed that destruction of these superficial areas by using a cautery did not interfere with normal pressor and depressor functions, but did affect afferent vasomotor pathways. Points of discrepancy between Ranson and Billingsley (121) and Scott (137) were clarified when deeper structures of the medulla were stimulated with fine needle electrodes attached to a Horsley­

Clarke stereotaxic apparatus. Wang and Ranson (149) explored the entire brain stem from the pons to the decussation of the pyramidal tract. Monnier (100) also explored the brain stem and noted pressor responses were very marked in a region limited to the dorsal level of the lateral reticular formation and the adjacent floor of the fourth ventricle, especially at the level of the inferior fovea. The depress­ or area was located more ventral and medial to the pressor area in the medulla. However, in examining their data, one notes there is a pre­ ponderance of pressor or depressor activity, but intermingled with these areas are responses which show the opposite responses. The concept of pressor and depressor "centers" in the medulla were confirmed by Chen, Lim, Wang and Yi (25, 26), but these investi­ gators argued that the pressor area is actually a region which is concerned with excitation of the entire sympathetic nervous system. They observed changes not only in blood pressure, but also in the intestine and renal vessels, the plain msucle of the spleen, small intestine, colon, bladder, pupillary sphincter, nictitating membrane, tail hair, bronchioles, liver, adrenal glands, and sweat glands. 7 Certainly all these organs and tissues could be activated when stimulating the medulla, but the question is whether one is stimulat­ ing the origin of these fibers, afferent fibers or efferent fibers from different levels of the central nervous system. Alexander (3) re-evaluated the functional significance of the presser and depressor regions as reticular systems rather than utiliz­ ing punctate stimulation. After brain stem transections he observed the ·effects on blood pressure and also on the recorded electrical activity from the inferior cardiac nerve and cervical sympathetic nerves, which he assumed to be the representative outflows to the vasomotor and cardioaccelerator regions of the cardiovascular system. He reported that transection of the rostral medulla removed a signifi­ cant portion of the rostral pressor region but left the depressor region intact, resulting in a considerable fall in arterial pressure and a reduction in the tonic discharge of fibers in the inferior cardiac nerve. A transection caudal to the medulla near the first cervical segment restored some activity in the inferior cardiac nerve.

He concluded that there is a bulbar presser area which contributes to the tonic activity in sympathetic vasoconstrictor and cardioaccelerator preganglionic fibers and this tonic area is the same area he described for presser reflexes. He suggested that the depressor area can inhibit sympathetic activity which was observed in the nerve recordings. Bach (5) noted that along with blood pressure responses he could activate respiratory changes e.nd knee jerk responses by stimu­ lating the lateral reticular formation of the medulla. He suggested 8

that the medullary reticular formation, in which the vasomotor area is located, is a rather non-specific system. Thus, the question of a pure vasopressor or depressor region within the medulla is debatable. Reassessment of the cardiovascular responses to electrical stimulation of the brain stem was done by Peiss (104). Until this time

only the mean blood pressure responses to medullary stimulation had been analyzed, while nothing had been described concerning cardio­

augmentor influences. Certain generalizations were made concerning

electrical stimulation of the medulla. Peiss (104) noted the most common response observed was vasoconstriction which he associated with cardiac and adrenal effects. With analysis of pulse pressure which was correlated with the cardioaugmentor and cardioaccelerator effects of sympathetic trunk stimulation (4, 119), he suggested that the vasomotor region contained cells or fibers which increased myocardial contractility. He also noted that after bilateral vagotomy, no cardio­ accelerator responses were observed when the dorsal half of the medulla was stimulated.

Pressor and depressor "centers" have been described but with no emphasis placed on the influence of the medullary region on cardio­ acceleration. The first significant analysis of this aspect of cardiovascular control was described by Peiss (105). In cats anesthetized with alpha-chloralose, stimulation of the dorsolateral and ventrolateral medullary reticular formation produced maximal short-latency vasoconstriction, myocardial augmentation and cardio­ acceleration. However, in cats anesthetized with sodium pentobarbital 9 and bilaterally vagotomized, stimulation of the dorsal medulla pro­ duced very little cardioacceleration, but stimulation of the ventro­ lateral medulla produced cardioacceleration which was comparable to responses of animals anesthetized with alpha-chloralose. In other reports, Peiss and Manning (108) reported that sodium pentobarbital depressed the hypothalamus, without having any noticeable effect on the medulla. Using these observations, Peiss suggested that cardio­ acceleration from stimulation of the dorsal medulla ascends to the hypothalamus before the impulses are transmitted to the heart. The evidence from these observations suggests that the cardiovascular system is not controlled solely by the medulla, but includes supra­ medullary levels of integration. Opposition to the hypothesis Peiss proposed concerning cardio­ acceleration from afferent fibers in the dorsolateral medulla, was presented by Wang and Chai (147). Under similar experimental con­ ditions these investigators observed cardioacceleration in the dorso­ lateral reticular formation with nembutal, alpha.-chloralose or midcollicular transection.

Another important point observed in the experiments of Chai and Wang (23) was vasopressor and cardioaccelerator responses to stimulation on either the right or left side of the dorsal reticular formation. Stimulation of the left dorsolateral formation signifi­ cantly increased pulse pressure and blood pressure with a small concomitant increase in heart rate. However, stimulation of the right dorsolateral formation caused a large increase in heart rate with 10 a much smaller increase in pulse pressure and blood pressure. This observation coincides with that of Randall et al. (119) who reported similar differences from cardiac sympathetic stimulation of the right and left stellate ganglia. This suggests that the intramedullary pathways for cardioacceleration and augmentation may descend in separate spinal tracts to the preganglionic neurons.

An actual attempt has been made to record from the reticular neurons of the medulla by using glass microelectrodes (117, 126). Przybyla (117) noted two types of cardiovascular neurons; one type was steadily firing neurons which decreased in firing rate with epinephrine administration; the second type was frequency modulated neurons which changed firing rate with respiration but also showed a reduction in firing rate with epinephrine administration. These cells were localized in the dorsolateral reticular formation. One must note only.fourteen neurons were analyzed in the experiments of Przybyla and Wang (117). Although the evidence for medullary presser and depressor areas, as well as possible cardioaccelerator areas are important for control of the cardiovascular system, one cannot ignore supramedullary regions of cardiovascular control. Peiss (107) noted that with mid­ collicular transaction, blood pressure fell 20 to 40 mm Hg. These observations were also made by Keller (74) in hypothalectomized dogs. Therefore, higher levels of the central nervous system play a signi­ ficant role in controlling tonic blood pressure. 11 2. Hypothalamus

Koizumi and Brooks (81) have summarized the importance of the hypothalamus by stating that "the' hypothalamus should not be considered exclusively as a part of the autonomic system any more than it should be thought of as part of the endocrine system. It is the brain region responsible for integration of basic behavioral patterns which involve correlated somatic, autonomic, and endocrine function". The descrip­ tion of the hypothalamus which follows will describe only the influences of the hypothalamus on the cardiovascular system, but this will be done with full realization that the hypothalamus has much broader capacities in controlling homeostasis. A very detailed plotting of stimulation sites which activated vasopressor responses and other sympathetic activity from the hypo­ thalamus to the pons and medulla was done by Ranson and his associates

(71, 123, 149)• By systematic stimulation with small electrodes, they observed that a functional pathway left the lateral hypothalamus and descended to the reticular formation of the medulla (149). The functional pathway passed through the central and tegmental midbrain, and tegmental pons as it descended to the medulla. Previous investigators described the cardiovascular responses as presser or depressor responses by analyzing the changes in mean pressure. Manning and Peiss (97) showed that stimulation of the lateral and posterior hypothalamus and subthalamus caused vaso­ constriction, augmentation of myocardial contraction and cardio­ acceleration. These responses could sometimes be produced separately 12 but usually vasoconstriction was predominate. Supramedullary control of the cardiovascular system and especially cardioacceleration was described by Peiss (105). He re­ ported that stimulation of the dorsal medulla would result in cardio­ acceleration, which could be decreased or abolished by placing electrolytic lesions just caudal to the hypothalamus or could be abolished by midcollicular transection. He suggested that impulses for cardioacceleration must ascend to the hypothalamus and then de­ scend to the peripheral sympathetic nervous system. Manning (95, 96) examined the effects of gross lesions in the medullary region leaving only the hypothalamo-spinal pathways intact. After massive lesions were placed in the medulla, blood pressure levels were not signifi­ cantly altered and significant vasopressor responses could be elicited from hypothalamic stimulation. After decerebration, blood pressure responses were abolished and control blood pressure levels fell to that of acute spinal animals. He concluded that the cardio­ vascular system could be controlled from the hypothalamus without having the medullary dorsolateral reticular formation intact. This concept was disputed when it was reported t.hat cardioacceleration could be elicited from the dorsal medulla in midcoliicular cats (23, 146). Even though the evidence for cardiovascular control from the hypothalamus, without the medullary reticular formation, is not clearly established, there is evidence which suggests that a hypo­ thalamo-spinal pathway descends through the ventrolateral medulla 13 (94t 105, 146, 148, 150). There is also anatomical evidence suggesting parallel hypothalamo-spinal fibers regulate the cardiovascular system.

Beattie, Brow and Long (9) placed lesions in the posterior hypothalamus and allowed the fibers to degenerate. These fibers, which remained essentially uncrossed, descended from the hypothalamus and passed through the anterior columns of the white matter of the spinal cord and terminated in the lateral horn of the gray matter. Similar observations on hypothalamo-spinal fibers were made by Smith (141). After localizing blood pressure responses to hypo­ thalamic stimulation, a large lesion was placed in the region which caused the responses, and the fibers were allowed to degenerate. He noted that many fibers terminated in their descent through the brain stem, but some fibers were observed in the intermediomedial and inter­ mediolateral regions of the spinal cord. These observations further suggested that. the hypothalamus may have the capacity to control cardiovascular functions without having to be relayed to the medullary vasomotor regions. One major defect in this type of degeneration study is that one is not certain whether these fibers relay impulses which control cardiovascular or sympathetic activity. Enoch and Kerr (42, 43) using smaller lesions in the hypothalamus did not observe degeneration in the spinal cord.

In discussing hypothalamic influence upon tonic cardiovascular discharge, one must note the studies done by Pitts, Bronk and Larrabee

(16, 111, 112). They stimulated the hypothalamus and recorded blood pressure simultaneously with sympathetic activity from the nerves 14 to the heart and blood vessels. Stimulation of the hypothalamus initially increased sympathetic activity and was followed one or two sec later by an increase in blood pressure. An increase in the in­ tensity or frequency of hypothalamic stimulation, increased the number of sympathetic fibers activated and frequency of responses within the fibers. They observed that the frequency of discharge was proportional to the frequency of stimulation. In their study, they also reported that one or two preganglionic discharges were recorded for every 20 to 25 impulses• They suggested that the hypothalamus has the cap­ ability of controlling the cardiovascular system, however, Bronk et al. (16) reported that the sympathetic activity was not changed significantly if the hypothalamus was separated from the medulla.

This interpretation was carefully examined by Peiss (107). He pointed out that the tonic activity had decreased and the amplitude of the reflex responses had increased after the brain stem transection and suggested that the hypothalamus does influence tonic and reflex sympathetic activity.

Evidence in the previous discussion suggests that there is a complex interaction between the hypothalamus and the medulla. The medulla may be capable of controlling the cardiovascular system but the hypothalamus does have important influences which may directly affect the medulla or may possibly descend by hypothalamo-spinal fibers to the preganglionic nerves. 15 B. Pathways Transmitting Cardiovascular Responses The location and characteristics of descending sympathetic pathways have not been extensively examined. One of the earliest de­ scriptions of location of the pathways subserving vasoconstrictor activity was reported by Foerster (46). He localized a descending vasoconstrictor pathway in a relatively restricted zone of the spinal cord situated at the junction of the anterolateral and posterolateral columns. Sections of these tracts in the lower cervical segments of the spinal cord abolished vasoconstrictor reflexes in the body. Chen, Lim, Wang and Yi (26) stimulated medullary sympathetic regions on both sides of the midline to observe sympathetic responses in various organs including the cardiovascular system. After sympathe­ tic responses were obtained, the ventrolateral column of the spinal cord was destroyed. The sympathetic region on the ipsilateral or contralateral side was again stimulated but no sympathetic responses were observed on the side of the lesion. They concluded that the sympathetic tract arises either from a ventromedial part of the vestib­ ular group of nuclei or from dorsolateral cells in the reticular formation and descend uncrossed in the ventrolateral column. Further studies on the effect of placing lesions in the cervical lateral funiculus while stimulating the hypothalamus was de­ scribed by Wang and Ranson (150). Placement of lesions in either the anterolateral funiculus or just dorsal to the dentate ligament at the

to segments significantly reduced blood pressure elevations to c1 c2 hypothalamic stimulation. 16

In clinical observations, Johnson, Roth and Craig (70)

examined vasomotor function following unilateral and bilateral cordot­

omies in humans. They noted a wide distribution of autonomic fibers in the anterolateral columns. One major problem with this work was that they could not determine the extent of their lesions. Stimulation of the anterior sigmoid gyri (73) in the cat in­ creased blood pressure. After lesions were made in the dorsolateral funiculi of the cervical spinal cord, subsequent blood pressure re­ sponses during stimulation were decreased or abolished. The first series of experiments which examined the sympathetic descending pathway by directly stimulating the surface of the spinal

cord were performed by Kerr and Alexander (76). They stimulated the surface of the thoracic spinal cord from the dorsolateral sulcus to a point midway between the dentate ligament and ventrolateral sulcus.

They noted the greatest concentration of vasopressor fibers were

located two to three millimeters ventrolateral to the dorsolateral sulcus. The depth of the vasopressor pathway was examined by pene­ trating the spinal cord in 0.5 mm increments and stimulating at each point. Blood pressure responses were attenuated or abolished after the electrode bad penetrated the spinal cord one millimeter. Blood pressure responses were obtained again when the electrode penetrated the region of the intermediolateral cell column. Indirect evidence for localization of descending sympathetic pathways was presented by Dahlstrom and Fuxe (35) utilizing histo­ chemical fluorescence techniques. They suggested that the dorso- 17 lateral funiculus contains fibers which show fluorescence suggestive of 5-hydroxytryptamine. They also showed that the ventrolateral funiculus, as well as diffuse regions of the dorsolateral funiculus,

had fibers .which contained norepinephrine. These investigators ob­ served connections between these monaminergic fibers with cells which

lie in the reticular f orma.tion of the medulla. Experiments on descending pathways in the cervical cord have

been reported by Illert and Gabriel {65). Their most recent work in­ dicated that excitatory pathways are localized in the dorsolateral funiculus and inhibitory pathways are localized in the ventrolateral

funiculus. Sympathetic activity in the renal nerve and blood pressure increased when the dorsolateral funiculus was stimulated, and decreased when the ventrolateral pathway was stimulated.

Other pathways have been suggested which could possibly carry sympathetic nerve impulses from supraspinal levels to the peripheral

sympathetic nervous system. Papez (103) described three pathways in the spinal cord which descend from the medullary reticular formation. These pathways were described as the medial, ventral and lateral reticulospinal pathways. Physiological evidence to suggest that these pathways were important in sympathetic or cardiovascular control of the peripheral nervous system was inconclusive, since this paper was mainly concerned with Marchi stain for degenerated fibers.

Landau {88) examined the corticospinal tract and noted that -small increments in blood pressure could be activated by stimulation of the corticospinal tract at the medullary level. These responses 18 were small and showed a significant variation in blood pressure responses from different areas which were stimulated in the cortico­ spin.a.l tract.

The literature concerning descending pathways in the spinal cord which transmit impulses from the supraspinal regions of the central nervous system to the prega.nglionic neurons has only suggested possible pathways. No investigations were made to determine whether these de­ scending pathways transmitted tonic activity or reflex activity. There are no reports concerning the conduction velocity or the synaptic mechanisms between the descending pathways and the preganglionic neurons. 19 c. Reflex Activation of the Cardiovascular System 1. Somato-Sympathetic Reflexes Concepts of spinal and supraspinal reflexes elicited by sensory nerve stimulation began with observations on the effects of stimulation on the cardiovascular system. Hunt (63) noted that weak stimulation of the sciatic nerve caused reflex depressor effects on blood pressure and more intense stimulation activated a reflex pressor response. He attributed these differential responses to different types of afferent fibers with the pressor fibers having a higher threshold and depressor fibers having a lower threshold. Ranson (120, 122) disagreed with the Hunt hypothesis of dif­ ferent afferent fibers causing either a depressor or pressor effect on blood pressure, depending on the threshold of the afferent fibers. He proposed the concept that the pressor reflex was related to a central summation of afferent impulses which had the capacity to overcome the influences of the same afferent fibers which reflexly activated the depressor response by using stimulations of less intensity.

These two divergent descriptions concerning pressor and de­ pressor responses were re-evaluated by Gordon (54). His results supported the differential fiber concept described by Hunt (63). He reported his conclusions after examining the effects of cocaine, cold and asphyxia on the sensory nerves. The effects of cocaine (52) and asphyxia (27) and their influence on sensory nerves had been described. These descriptions permitted Gordon to suggest that small fibers caused pressor effects and large fibers activated the depressor responses. 20 Johansson (69) did a comprehensive study on the circulatory responses to stimulation of somatic afferent fibers. Circulatory response patterns were examined by studying changes of regional blood flow resistance in skeletal muscle, kidney, intestine and skin. A£­ ferent muscle nerve stimulation activated a generalized reflex inhibition of tonic sympathetic vasoconstrictor fibers which caused vasodilatation in all the vascular regions analyzed. The depressor responses were dependent on the prevailing level of tonic vasocon­ strictor activity. Differential pressor and depressor responses to sensory nerve stimulation were also examined by Khayutin (77). Stimulation of the tibial nerve, with low and high voltages, in unanesthetized cats, which were immobilized with succinylcholine, activated only pressor responses. A£ter the anitials were decerebrated, the magnitude of blood pressure responses to stimulation of A fibers or A and C fibers were markedly decreased. In some animals the presser response changed to a depressor response when the tibial nerve was activated with low intensity stimulation. These observations indicate that de­ cerebration and anesthetic drugs impair the normal function of the nerve processes in the central nervous system. In further analysis of the influence of decerebration and anesthetic agents, Fedina (44) attempted to clarify the different effects. Fedina suggested that anesthesia or decerebration decreases or reverses the presser reflex responses by activating inhibitory mechanisms in the medullary reticular formation. The anesthetics or 21 decerebration may also impede or interrupt impulses of the slow con­ ducting A and C afferent fibers which are transmitted in ascending spinal tracts.

Blood pressure responses and sympathetic discharges during stimulation of the sciatic nerve were described by Koizumi et al.

(84). The sciatic nerve was stimulated at various frequencies while sympathetic discharges were recorded from the lumbar white rami; systemic blood pressure.was also recorded. Stimulation of the sciatic nerve at 1 to 10 Hz depressed white rami sympathetic discharges and de­ creased blood pressure, while stimulation at 50 to 100 Hz increased discharges and blood pressure. Following spinal transection only increased sympathetic discharges and pressor responses were obtained with stimulation frequencies between 2 and 100 Hz. These observations give further evidence for the complex mechanisms involved in the con­ trol of supraspinal afferent activity on blood pressure and sympathetic discharges.

Further analysis of the-somato-sympathetic reflexes was done by recording action potentials or mass discharges from the pre- or postga.nglionic sympathetic nerves. These studies were important for describing reflex pathways, latencies of reflexes, interactions of reflexes and the involvement of the central nervous system.

Earlier literature had reported data which indicated that the sympathetic nervous system is capable of crudely adjusting to somatic or vesical afferent excitation after the supraspina.l central nervous system has been separated from the spinal cord. (2, 17, 18, 34, 39, 22 56, 87, 90, 125, 140). These earlier studies did not evaluate the contribution and organization of cord mechanisms which could possibly adjust blood pressure responses. This led to the study of spinal mechanisms by Beacham and Perl (7, 8). They recorded sympathetic reflex discharges from thoracic white rami which were evoked by single shock stimulation to dorsal roots, spinal nerves and limb nerves. Reflex latency of 3 to 15 msec suggested that there may be polysynaptic connections between the sensory fibers and the prega.nglionic nerves.

Similar reflex responses were also evoked by single shock stimulation to visceral nerves in spinal cats (48).

Anatomical evidence concerning polysynaptic pathways between the dorsal roots and the sympathetic preganglionic fibers has been re­ ported by Petras and Cummings (110). They studied sympathetic cell chroma.tolysis and dorsal root fiber degeneration after unilateral sympathectomies and dorsal rhizotomies in rhesus monkeys. They re­ ported that no evidence was found for dorsal root terminal connections with the chromatolytic cells of the intermediolateral cell column.

They noted that dorsal root fibers terminate on the neurons of the intermediomedial cell column as well as cells in the substantia gelatinosa, nucleus proprius cornus dorsalis and Clarke's Column.

From these observations they concluded that somato-sympathetic reflex pathways are polysynaptic.

The characteristics of somato-sympathetic reflex responses through spinal mechanisms and also through the supraspinal central nervous system were described by Sato et al. {133). Sato recorded 23 evoked responses from the lumbar sympathetic trunk at segments t to 5 t which were activated by single electrical shocks to the sciatic 6 nerve. An early evoked response was recoi-ded with a latency of 25 to 50 msec and a late evoked response was recorded with a latency of 80 to 120 msec. The late evoked response disappeared after spinal cord transection.

Coote and Downman (28) recorded early and late reflex dis- charges simultaneously from the inferior cardiac nerve and renal nerve while stimulating thoracic or lumbar dorsal roots. The afferent input from either the thoracic or lumbar sensory nerves activated late evoked responses in the inferior cardiac nerve which had a shorter central latency than the renal nerves. Cooling the floor of the fourth ventricle abolished the late evoked response, but did not signi­ ficantly affect the early evoked responses.

Afferent fibers have been analyzed to determine which fiber size grouping correlates with the activation of spinal and supraspinal evoked responses. Group I muscle afferent fibers do not activate evoked responses under normal conditions. Hypothermia of the spinal cord, however, is capable of causing sympathetic reflex activation when Group I afferent fibers are stimulated (19, 82, 142). It was suggested that connections between Group I afferent fibers and sympathetic neurons may be present. As the intensities of stimulation increased, afferent fibers from Group II and III activate the supraspinal sympathetic reflex (67, 130, 132). Group II afferent fibers, when stimulated alone, 24 activate sympathetic evoked responses which are small and sometimes difficult to recognize, but Koizumi and Sato (83) showed that Group II afferent fibers can excite postganglionic sympathetic nerves when

observed from single unit recordings.

A characteristic of supraspinal reflexes is the phenomenon of

the "silent period" which follows an evoked response during stimula­ tion of Group II and Group III afferent fibers. The "silent period" is characterized by inhibition of spontaneous activity for 400 to

1000 msec following an evoked discharge. Sell, Erdelyi and Schaefer

(138) first described this "silent period" by stimulating somatic afferent nerves and recording from the renal and inferior ca._-rdiac nerves.

Sato (131) systematically studied the "silent period" by using "conditioning - testing" stimuli and recording from the lumbar sympathetic trunks. Following a conditioning stimulus no test stimulus could be observed for 500 to 800 msec. These excitability - recovery curves have been observed in other pre- and postganglionic nerves.

(30, 67, 84, 132, 135). It was noted that the greatest depression following evoked response occurred with stimulation of the Group II and Group III afferent fibers. Group IV fibers did not cause signi­ ficant inhibition of spontaneous activity.

Mechanisms causing this long depression or inhibition of spontaneous activity has not been resolved. Polosa (114) attributed the "silent period" to post-excitatory depression at the preganglionic neuron. However, Iwamura et al. (67) showed that placement of a 25 discrete lesion in the ventral portion of the medullary reticular formation abolished the "silent period" without affecting the evoked discharge. Coote and Downman (29) reported that stimulations of the ventromedial region of the medulla inhibited the intercostal renal nerve reflex. Koizumi et al. (85) observed that the "silent period" became 509b longer during the inspiratory phase than the expiratory phase. She also noted that pathways for abolishing the "silent period" are not confined to certain limited structures in the medulla.

The evidence, though limited, suggests that the medullary reticular formation is essential for the appearance of the "silent period".

Group IV afferent fibers which are C fibers have also been analyzed for their importance in activation of sympathetic reflexes.

In experimental conditions, Schmidt and Weller (136) noted C fibers required 3 to 5 pulses at 15 to 30 Hz to activate a large sympathe­ tic reflex. When a single shock stimulus was given, the sympathetic reflex response produced by this stimulation was difficult to observe in a mass sympathetic discharge. Koizumi and Sato (83) noted that the C fiber reflex response is observed after a latency of 500 -

600 msec and lasts for 200 msec. Evoked response caused by Group IV activation are of greater magnitude than the earlier responses evoked by Group II and III afferent fibers.

Activation of the spinal sympathetic reflex required activa­ tion of fibers with a relatively high threshold (7, 130, 133). Reports by Sato et al. suggested that the threshold for activation of supra­ spinal sympathetic reflexes ~as much lower than the spinal sympathetic 26 reflex. However, in their analysis, they stimulated somatic nerves which were several segments from the white ramus being recorded. It was suggested that when spinal sympathetic reflexes are activated several segments from the white rami used for recording, the response is greatly reduced in size, but when activated from the same segment differences in spinal and supraspina.l reflex thresholds are not ob­ vious (7, 127, 132). A very late sympathetic reflex discharge has been observed which has a latency of 300 msec (72, 128). This response is activated by stimulation of myelinated afferent fibers and may be mediated through a supra-medullary pathway, because midcollicular transection will abolish the response. This sympathetic reflex is also extremely sensitive to higher concentrations of anesthetic drugs. In this partic­ ular experiment chloralose abolished the reflex when given to the cat several hours after the initial dose was given. Evidence from these studies suggests a spinal and supraspina.1 pathway for the somato-sympathetic reflex. The descending pathway which transmits the supraspina.l reflex to the preganglionic neuron will be analyzed in this study. 2. :Ba.roceptor reflexes from the carotid sinus Quantitative and qualitative studies of baroceptor reflexes

(58, 86) have been examined. Investigators have also analyzed the afferent projections from the carotid sinus nerve to the medullary regions of the central nervous system. These investigations were done by degeneration (31, 75) and microelectrode studies (11, 12, 32, 62, 27 92, 98, 139, 144). It was demonstrated that the primary afferent

carotid sinus nerve ends in the region of the nucleus and tractus

solitarius. One study also indicated that the primary carotid afferent

fibers may end in the region of the paramedian reticular nucleus (32,

98), but this was later disproved by showing that the hypoglossal

nerve was also being stimulated {46). There is also evidence that the

anterior hypothalamus may be activated by the baroceptor reflex {59,

143}. This survey of literature will be orientated towards central

mechanisms involved with the bilateral carotid occlusion and the pos­

sible efferent pathways transmitting baroceptor activity to the peri­

pheral sympathetic nervous system.

Discovery and the concept of baroceptive regulation of cardio­ vascular function was initiated by the identification of the depressor nerve (58). However, central control of the baroceptor reflex mechanism during bilateral carotid occlusions was not pursued to any great extent until the 1950's and the 1960's.

Two divergent views concerning central control of the baro­ ceptor mechanism were described by Manning and Wang. Manning (95, 96) reported that the vasopressor and myocardial augmentor responses to bilateral carotid occlusions were not significantly changed by placing gross bilateral electrolytic lesions in the dorsolateral regions of the meduila but disappeared after decerebration at the level of the in­ ferior colliculus. Hanning suggested that integration of cardio­ vascular responses during bilateral carotid occlusion can occur in 28 supramedullary regions without an intact medullary vasomotor region.

In the experiments performed by Wang and his associates (22,

24, 148), a different view was taken from that of Manning. Using a fine glass capillary tube, lesions of different sizes were produced in various parts of the medulla. When the lesion was placed in the presser areas of the medulla, blood pressure responses during bi­ lateral carotid occlusions were markedly attenuated or abolished. Wang attributed the differences in these two series of experiments to the techniques for placement of lesions and to animals which were not bilaterally vagotomized.

Differences between Wang and Manning were considered by

Hilton and Spyor (59) when they evaluated the participation of the anterior hypothalamus in the baroceptor reflex. They stated that

"the whole brain stem is organized in longitudinally arranged areas regulating cardiovascular function". Therefore, both regions of the brain stem, that is, the hypothalamus and medulla are important for the integrated activity of the baroceptor reflex, but the hypothalamus may not be essential for the reflex response.

Efferent pathways subserving baroc~ptor vasomotor reflexes have been suggested by indirect analysis. It has been shown that com­ plete excision of both paravertebral sympathetic chains attenuated or abolished the baroceptor reflex response in cats (6). It has also been noted that parasympathetic vasodilator fibers (10, 91) and sympathetic vasodilator nerves (49, 91) do not participate in the carotid occlusion responses. It was suggested that the sympathetic 29 vasoconstrictor nerves represent the efferent pathway of the baroceptor vasomotor reflex (58). This deduction was more clearly defined by noting that efferent impulses in the splanch.~ic nerve were significant­ ly increased by hypotension, and were abolished by hypertension (38, 53). CHAPTER III MATERIALS AND METHODS

A. Animal Prenaration

Experiments were performed on cats of either sex ranging in weight from 2.0 to 4.1 kg. The animals were anesthetized with either sodium pentobarbital (25-30 mg/kg) or a preanesthetic injection of phencyclidine hydrochloride (2 mg/kg), followed by alpha-chloralose

(20-30 mg/kg). In all preparations, a tracheotomy was performed, but the animals were allowed to breathe spontaneously throughout the surgical procedure. During lateral spinal cord stimulations and electrophysio­ logical experiments, the animals were artificially ventilated. Loose ligatures were placed around the common carotid arteries and vagus nerves. Just before stimulation procedures, all animals were bilat­ erally vagotomized. The femoral artery and vein were carefully isolated to avoid damage to the femoral nerve. The arterial cannula consisted of polyethylene tubing, (0.03011 I.D.) fitted over a 21 gauge needle. The arterial cannula was inserted into the artery to the level of the lower aorta. Blood pressure was recorded with a Statham P23 Db transducer attached to a Grass model 7 polygraph. 30 31 Laminectomies were performed at the c to c , T to T and T to T 2 4 1 4 7 9 segments of the spinal cord, depending on the procedure. Muscle masses on the right side of the spinal column were always removed at the T 1 to T4 segments or c2 to c4 segments for easier access with the stimulating electrodes. The dura mater was resected to expose the

spinal cord and dorsal roots. The dorsal roots on the right and left sides of the preparation were transected to prevent reflex activation of blood pressure. Most stimulations and recordings were done on the right side of the spinal cord. Following this basic surgical prepara­ tion the animals were divided into three separate series of experiments. The experiments included lateral spinal cord stimulations, bilateral carotid occlusions and electrophysiological recordings. B. Lateral Spinal Cord Stimulations

The laminectomy for these experiments was made between the T 1 and T segments of the spinal cord. Animals were placed in a spinal 3 cord stereotaxic apparatus by attaching clamps to the c and T spinal 7 4 processes. After the surgical procedure, animals were placed on a

Byrd respirator and were ventilated at a frequency of 16-22 inhalations per minute. The pressure during inspiration increased the chest cav­ ity to a size similar to that before positive pressure respiration was administered. Gallamine triethiodide (Flaxedil) 1.5 to 2 mg/kg was initially given to the animals and was given as needed throughout the duration of the experiment. Gallamine, at low doses, competitively blocks acetycholine at the motor end plates of striated muscle with­ out affecting the sympathetic ganglia. 32 Blood gases and the pH of the arterial blood were measured in

three animals anesthetized with alpha-chloralose. Arterial Pao2 was elevated to levels of 350 to 400 mm Hg because the animals were on a Byrd respirator which mixes o with air. Measurements made on Paco 2 2 showed that it ranged from 33 to 43 mm Hg. The pH values measured in these experiments range from 7.19 to 7.23. In some animals sodium bicarbonate was given to increase the pH values to more normal levels.

Monopolar electrodes were used for stimulation of the spinal cord in this series. The electrodes were made from stainless steel insect pins size 00, and reduced to a diameter of 15-30u using Green's

technique (55) for etching. The electrodes were held in place on the surface of the spinal cord by utilizing a David Kopf microelectrode drive. Stimulation parameters were 50-60 Hz, 1 msec duration and

O.l mA current. The monopolar electrode was connected to a Grass S48 stimulator and an indifferent electrode was placed in the vertebral muscles.

The impulse from the stimulator passed through a lOK ohm re­ sistor, which was connected to the input terminals of a differential amplifier, and was transmitted to the electrode on the surface of the spinal cord. By using the resistor between the input terminals of a differential amplifier, the difference in potential between the two terminals could be measured on the screen of the oscilloscope. The value of the potential was divided by the value of the resistor to determine the current. Current was measured by this method for two 33 reasons. One, if there was a spread of current due to fluid accumula­ tion, a decrease in the differential potential was observed from the oscilloscope. Two, since it was not possible to use the same electrode for all experiments, the differences in impedance of various electrodes would not change the current transmitted to the spinal cord, because the differential potential would be set to the same value in all ex­ periments. Mineral oil was placed on the spinal cord to prevent it from drying out and to avoid conduction through the fluid medium. Blood pressure changes were induced by punctate stimulation on the surface of the right side of the spinal cord. Stimulations were made randomly in 0.5 to 1.0 mm increments between the dorsolateral sulcus and the dentate ligament. The electrode was placed against the surface of the spinal cord with just enough pressure to cause a slight puckering. If the electrode did penetrate through the pial layer, the electrode was pulled back slightly to make certain stimulation was occurring on the surface of the spinal cord. Distances were measured by placing a small mm,ruler near the surface of the spinal cord and were observed with a 16X dissecting microscope. Distance between the dorsolateral sulcus and the dentate ligament in the cat was consistent­ ly 3 mm. All measurements were made in reference to the distance from the dorsolateral sulcus. c. Reflex and Tonic Experiments On sixteen cats, anesthetized with sodium pentobarbital, the carotid arteries were isolated and umbilical tape was placed around them. A laminectomy was made at the to T segments. Unilateral and c8 1 34 bilateral occlusions of the common carotid arteries were performed by carefully placing small artery clamps on the carotid arteries. The bilateral carotid occlusions were maintained for approximately 20 to

30 seconds. If an animal did not have a significant pressor response to bilateral carotid occlusions, the animal was discarded. Blood pressure alterations from bilateral carotid occlusions were examined before and after spinal cord lesions were placed at the c8 segments. After responses to carotid occlusions were completed, a scalpel was used to make a unilateral lesion on the surface of either the right or left side of the spinal cord. The lesion extended from the dorsolat­ eral sulcus to the dentate ligament and had a depth of approximately l mm. After carotid occlusions had been recorded following the uni­ lateral lesion, a similar lesion was placed on the contralateral side of the spinal cord and the occlusions were repeated. Finally, the spinal cord was completely transected just rostral to the bilateral lesions, and the occlusions were again applied. After the experiments, a segment of the spinal cord containing the lesions was removed and fixed in 10"~ formalin. The cord was histologically prepared by slicing it in 10 to 15 u sections and staining them with Kluver­

Ba.rrera stain (80). This technique stains myelin greenish-blue and cells pink to violet. The sections were examined to determine the extent of the lesions.

Control experiments were done for this series by using the same procedure as the experimental setup without making lesions in the spinal cord. To determine the deterioration of control blood pressure 35 levels and responses to bilateral carotid occlusions during a typical three hour experimental procedure, multiple responses to bilateral carotid occlusions were tested for this duration of time. D. Electronhysiology of the Descending Pathway

The spinai cord was exposed by laminectomy at the T to T 1 3 segments and either the c to c segments or the T to T segments. 2 3 7 9 After surgery, animals were maintained with positive pressure ventila­ tion using a Byrd respirator; they were immobilized with gallamine triethiodide. The vertebral column was stabilized by means of a spinal stereotaxic apparatus and the right side of the animal was tilted up to make the sympathetic rami more accessible. Muscle surrounding the vertebral processes and ribs was removed. Heads of the first, second and third ribs were removed exposing the rami communicantes and sympathetic trunk, retropleurally. In some experiments, it was nec­ essary to ligate small blood vessels lying dorsal and lateral to the sympathetic trunk. The identification and dissection of the rami communicantes were done with the·use of a 16 to 25X dissecting micro­ scope. Segments of the rami communicantes were identified by locating the stellate ganglion and counting caudal to that structure, or the segments were located by identifying the ventral root segments leaving the vertebral canal and following it to the sympathetic trunk.

White rami communicantes were identified anatomically and electro­ physiologically by techniques similar to ~eachem and Perl (7). If the gray rami communicantes could not be seen distinctly from the white communicantes, the white communicantes could be distinguished by persistant background activity and reflexly evoked discharge. The white

ramus, usually from the T2 spinal nerve, was transected at its junction with the sympathetic trunk and placed on bipolar stainless steel re-

cording electrodes.

Single volley potentials were recorded at the T white ramus 2 following electrical stimulation of the spinal cord at T to T , 1 2 c2 to c or T to T segments with coaxial electrodes. The coaxial 3 7 9 electrodes were made by.attaching~ insulated wire 0.006" in diameter

to a coaxial connector. The insulated wire (cathode) was then placed

inside a stainless steel tubing (anode) (.00411 I.D.), leaving an

exposed tip of approximately 0.5 mm length. The tip had been stripped

of its insulation by scraping, but no direct contact occurred between

the insulated wire and outside tubing. The coaxial electrode was utilized to reduce the stimulation artifact during activation of e­ voked potentials. The square wave stimulus was transfered to the preparation from a Grass S48 stimulator through a Grass Stimulation

Isolation Unit. The isolation unit also helped to reduce the stimulus artifact. Periodically the duration and voltage of the square wave from the stimulator was calibrated on a Tektronix 565 oscilloscope to determine its parameters. The stimulation parameters were 0.5 to

1 Hz, 1 msec and 5 - 12 volts. Evoked responses were produced by stimulating the surface of the spinal cord and were recorded from white ramus. The evoked re­ sponses were amplified by means of a Grass P9 preamplifier, displayed on a Tektronics 565 oscilloscope a..~d averaged on a Nuclear Chicago 37 Data Analyzer. The low pass filter of the preamplifier was set at

10.0 Hz and the high pass filter was set at lOK or 40K Hz.

The data from these electrophysiological experiments were averaged on a Nuclear Chicago Data Analyzer. The transient averaging mode was used to discriminate the signal of interest from the random characteristics of noise inherent in the recording instruments, as well as noise from outside sources and neuronal noise. An evoked response with the stimulus artifact was transferred from the animal through the preamplifier to the input of the computer. The evoked response was sampled over 400 data points, which were sequentially addressed over some fixed time interval. If the evoked response and stimulus arti­ fact occurs repeatedly at certain times in each analysis interval, the digital information corresponding to the input signal will be stored in the same memory data points during each interval and there will be a build up of memory counts representative of the signal. At the same time the noise signals randomly appearing as positive or negative potentials will tend to cancel out if averaged over several intervals. At all stimulation sites, evoked responses of 100 stimuli were averaged on the computer. After the evoked responses were aver­ aged, memory information from the computer was transferred to graph paper by using a Houston X-Y plotter. The tracings obtained from averaging were retraced on graph paper with non-reproducible lines in preparation for photography.

The diagram in figure 1 illustrates how the instruments were connected to amplify, average and plot the evoked responses. All the 38 FIGURE l

DIAGRAM OF INSTRUMENTS

sync. out STIMULATOR

COMPUTER SCOPE

PLOTTER stimulating electrode

AUDIO

recording '----- electrode

A schematic diagram showing the instruments used for producing, averaging and plotting evoked responses. SIU are the initials for stimulation isolation unit. 39 FIGURE 2

CALIBRATION GRAPH FOR DE'TERMINING CONDUCTION TIME

u Q) en 60 E ., .5 t- 40 c 0 ;: u ::s "C c 0 20 (J

Distance mm

Calibration graph used for determining conduction time of evoked responses (ordinate) from the distance (abscissa) on an X-Y plotter tracing. Each line on the graph represents an analysis interval of 31, 62 and 125 msec. 40 instruments were synchronized with an output signal pulse from the stimulator.

The computer and X-Y plotter were calibrated to determine the conduction time and amplitude of evoked potentials, which were plotted as mm on graph paper. Figure 2 showed the calibration curves for converting the mm on the graph paper to msec, for actual con­ duction time. This calibration was obtained by setting the analysis interval dial to 31, 62, or 125 msec intervals. A square wave from a stinrulator was transmitted to the oscilloscope and the computer.

The duration of the square wave was measured on the oscilloscope and was plotted as the ordinate. The distance for the duration of the same square wave obtained from the X-Y plotter was used as the abscissa. Thus these curves made it possible to convert the mm print out on graph pa~er to the conduction time.

The amplitude of the evoked responses was also calibrated to obtain an estimate of overall amplitude of evoked responses (fig.

3). The analysis interval dial was again set at 62 (fig. 3A) or 125

(fig. 3B) msec. The vertical scale count switch was also changed to

500 counts, 1000 counts or 2000 counts. The vertical scale counts switch selects the number of positive or negative memory counts that will vertically deflect the trace to plus or minus full scale referenced to the centerline of the computer display. The input signal was obtained from a square wave calibrator which injected a known square wave of 5, 10, 20, 50 or 100 uV through preamplifier to the computer. The signal was triggered 100 times to obtain the 41 amplitude of this signal on graph paper in mm. The deflection in mm was plotted on the abscissa and the averag"e uV of the input signal was

plotted on the ordinate. Thus the evoked response, after averaging would have the calibrated amplitude of an averag"e input from one analysis interval. In some experiments, random noise from the recording devices were averag"ed and compared with tracings which had noise. from the re~ cording devices and spontaneous activity from the T2 white rami. These two tracings are illustrated in figure 4. 42 FIGURE 3

CALIBRATION GRAPHS FOR DErERNINING AMPLITUDES OF EVOKED RESPONSES

150

> 100 ::J > ::J ti 'ti ti ::J 'ti :!:: ::J ii E -ll. ct 50 E ct

Amplitude mm Amplitude mm

Calibration graphs for determining amplitudes of evoked re­ sponses in uV {ordinate) from amplitudes in mm on a X-Y plotter tracing. Graph A represents an analysis interval of 62 msec with vertical scale settings of 500, 1000 and 2000. Graph B represents an analysis interval of 125 msec with vertical scale settings of 500,1000 and 2000. 43 FIGURE 4

CONTROL TRACINGS OF RANDOM NOISE AND SPONTANEOUS ACTIVITY

A I

8 I

Random noise (A) and spontaneous activity with random noise (B) which were averaged on the computer to determine background activity. Calibration markers: 10 uV and 5 msec. CHAPTER IV RESULTS

A. Lateral Sninal Cord Stimulation

Localization of the descending sympathetic pathway was ob­ tained by stimulating the dorsolateral funiculus and recording blood pressure responses. Figure 5 illustrates the changes in blood pressure which were obtained by stimulating the dorsolateral sulcus

(DLS), 1.5 mm, 2 mm, and 3 mm {dentate level) ventrolateral to the DLS, at the T segment. These responses are typical of nine animals 2 subjected to lateral spinal cord stimulation. The spinal cord was stimulated for 20 to 30 seconds or until the greatest pressor response was recorded for a particular stimulation. Stimulation from the DLS to 2 mm ventrolateral to the DLS. showed an increase in pulse pressure which could result from either or both cardiac augmentation and in­ creased distention of the blood vessels. Increased blood pressure re­ sulted from stimulating 3.0 mm ventrolateral to the DLS in this particular animal. Heart rate did not change significantly throughout the stimulations of the dorsolateral funiculus; this was typical for all animals studied.

Blood pressure responses resulting from stimulating all levels 44 45 FIGURE 5

STINULATION OF THE DORSOLATERAL FUNICULUS

200

100

0

I I ' I I 11 ' \II iLIL 1 'LL1_ 1 l!ll"ll "ll j,,Jbt\'t\ '\l 1111111•tt•I i ; ; I I ! I I "

Blood pressure responses obtained by stimulating the surface of the dorsolateral funiculus at the T1 segment of the spinal cord. The funiculus was stimulated at the DLS (A), 1.5 mm (B), 2 mm (c) and 3 mm (D) ventrolateral to the DLS. The lower tracing is the time marker and the dark line describes the period of stimulation. Scales to the left of the tracings represents blood pressure in mm Hg. 46 and segments of the dorsolateral funiculus in nine experiments were averaged and the results are illustrated in figures 6 and 7. Three separate graphs represent segments Ti (fig. 6), T2 (fig. 7A) and T3

(fig. 7B), which were analyzed for localization of the sympathetic pathway. The ordinate describes blood pressure and the abscissa in­ dicates the levels which were stimulated. Blood pressure preceding stimulation at each segment and level was used as blood pressure con­ trol levels for comparing responses during stimulation. Mean values of control blood pressure levels and pressor responses resulting from stimulation are tabulated in table 1. The standard error of the mean is given to the right of each mean value. Control blood pressure levels from all segments and levels which were analyzed did not show any significant differences when they were compared with the other segments.

Blood pressure increased significantly above control levels

(p<0.05) following each site of stimulation on the surface of the spinal cord in figure 6, except at the level of the dentate ligament

(3.0 mm). Blood pressure responses to stimulation of the dorsolat­ eral funiculus at the T2 (fig. 7~) and T3 (fig. 7B) segments had levels of significance similar to those in figure 6. When the sur­ face of the dorsolateral funiculus was stimulated 3.0 mm ventrolateral to the DLS (dentate ligament), blood pressure responses resulting from stimulation were not significantly different from control le- vels. This observation suggested that the level of the dentate lig­ ament is the lower limit of the sympathetic pathway. Blood pressure 47 FIGURE 6

BLOOD PRESSURE RESPONSES FROM STIMULATION

T1 DORSOLATERAL FUNICULUS

200

DLS 180 ~5 DCONTROL 2.5 3.0 STIMULATION :z:OI m E E 160

w a: ~ 140 I/) w a: 0. c 0 120 0 ...I ID

100

80 O'----DL-5-

Pressor responses (ordinate) elicited by electrical stimula­ tion on different levels (abscissa) of the surface of the T dorso­ lateral funiculus using the dorsolateral sulcus as the initial1 point of ~easurement. Upper and lower lines of each bar indicate systolic and diastolic blood pressures, respectively, and brackets represent standard errors of the mean. Note the maximum presser response at 2 mm. The open figures represent control levels and the striped figures represent blood pressure responses resulting from stimulation of the dorsoJ.ateral funiculus at designated levels. The insert indicates the points stimulated on the surface of the dorsolateral funiculus. 48 FIGURE 7

ALTF.RATIONS IN BLOOD PRESSURE FROM STilIDLATION

OF T2 AND T~ DORSOLAT~ ~CULI 21)0 A

180 0 CONTROL

~ STIMULATION :c.. ~ 160

w g 140 VJ VJ w a: a. g 120 0_, GI

100

so 0 ~-=DL=s-

200 B

180 D CONTROL mSTIMULATION r ~ 160

w a: :l 140 VJ VJ w ...a: c g120_, GI

100

80 o~-Dl-S- 1.0 1.5 ~ 2.5 ~ Blood pressure responses (ordinate) elicited by electrical stimulation at different levels (abscissa) of the surface of the T2 (A) and T3 (B) dorsolateral funiculus, using the DLS as the initial point of measurement. Upper and lower lines of each bar indicate systolic and diastolic blood pressures, respectively, and brackets represent standard errors of the mean. The open bars represent control levels and the striped bars represent blood pressure responses result­ ing from stimulation of the dorsolateral funiculus at designated levels. TABLE l LATERAL SPINAL CORD STIMULATIONS Blood Pressure (mm Hg)

Tl T2 T3 LEVEL DIASTOLE SYSTOLE DIASTOLE SYSTOLE DIASTOLE SYSTOLE

DLS c 101.4,±3. 7* 127 .8±_5.3 104.4+5.2 131.3+6.6 103.0±_5.0 131.3+6.l s 126.9+4.3 156.0±_5.3 129.0±_5.4 159.6±7.8 118.3±_5.9 147.3+6.o 1.0 mm c 90.2+6.5 125.5+7.4 106.6±_5.5 131.3,±6.8 99.8,±3.0 125.4+4.2 s 135.s+7.o 166.6+8.0 126.8+7.2 155.1+8.9 132.6,±6.4 161.2+8.l 1.5 mm c 92.4+6.5 118.6+6.9 102.7+6.4 127.1±7·4 104.4,±5.0 131.1+7.3 s 138.7+8.l 173.7+11.2 142.4+8.l 175.3+8.2 140.3±_5.4 174.0+7.5 2.0 mm c 101.0,±3.8 129.0±6.6 101.3,±5.0 127.2±6.3 101.8+6.2 130.1,±8.2 s 151.0±8.7 189.0±11.8 149.3±6.3 187.7+8.8 149.0±_5.8 184.4.±'f .1 2.5 mm c 93.2±6.8 118.4+7.2 104.9+5.8 129.0+6.5 105.8±4.3 131.8+6.8 s 120.8+4.6 150.4,±5.0 131.0+6.2 161.2,±7.6 128.7+6.3 158.3+8.3 3.0 mm c 101. 5,±5.8 129.0+7.5 106.6+6.7 132.5,±8.4 108.6,±5.0 133.6,±5.4 s 112. 7,±5.8 141.1+7.7 112.2.±4·3 138.4,±5.2 114.3+6.4 140.6+8.6

C = Control * = Standard error of the mean S = Stimulation 50 responses to stimulation at this level showed increases, decreases or no changes on the original records; however, the mean blood pressure response indicated no significant change.

Blood pressure responses resulting from stimulation 2.0 mm ventrolateral to the DLS were significantly greater (p<0.05) than the blood pressure responses obtained by stimulation at the DLS, l.O, 2.5, and 3.0 mm ventrolateral to the sulcus for all segments studied {fig. 6 and 7). However, responses to stimulations 2.0 mm ventrolateral to the DLS were not significantly greater than those elicited by stimu- lations 1.5 mm ventrolateral to the DLS. From these obse~ations one can conclude that the greatest concentration of vasopressor and possibly cardiomotor fibers are located 1.5 to 2.0 mm ventrolateral to the DLS with fibers being distributed over the entire surface of the dorso­ lateral funiculus from the DLS to the dentate ligament {3 mm).

No significant.differences in blood pressure responses to surface stimulation were observed when the T to T segments were com­ 1 3 pared at corresponding levels. For example, blood pressure responses obtained by stimulating 2.0 mm ventrolateral to the DLS were not significantly different at any of the three segments analyzed, although average blood pressure responses did decrease from T1 to T2 and T2 to T • This may indicate that there are less sympathetic fibers at 3 successively lower cord levels, because some fibers are exiting the cord to enter the sympathetic trunk.

In five experiments, blood pressure responses resulting from stimulation of the dorsolateral funiculus rostral and caudal to discrete 51 lesions were examined. Discrete lesions were made in the dorsolateral funiculi either as a unilateral lesion or bilateral lesions placed at the c8 to T1 segment of the spinal cord. Results of the blood pressure responses are illustrated in figure 8 and were obtained from stimulating the DLS and 2 mm ventrolateral to it. Blood pressure responses obtained by stimulating 3 mm ventrolateral to the DLS (dentate ligament) were excluded because the responses did not change significantly from con­ trol levels. The panel designated right illustrates blood pressure re­ sponses obtained by stimulating the dorsolateral funiculus rostral and caudal to a discrete lesion which was made from the DLS to the dentate ligament in the right or ipsilateral dorsolateral funiculus. All stimulations for figure 8 were done on the right dorsolateral funiculus.

Pressor responses were elicited by stimulating the DLS rostral to the lesion at the c8 segment, (fig. SA), however, no pressor re­ sponses were noted when the DLS was stimulated caudal to the lesion, (fig. 8C). When the stimulating electrode was moved 2 mm ventrolateral to the DLS, pressor responses were not observed by stimulating rostral to the lesion (fig. 8B), but were elicited by stimulating caudal to the lesion (fig. 8D). The data suggest that the sympathetic pathway is descending (fig. 8B and D), because no pressor responses could be activated by stimulating rostral to the lesion, however, presser responses similar to those observed in the intact preparation were observed caudal to the lesion. Pressor responses resulting from stimulation of the DLS rostral to the lesion (fig. 8A) suggested that afferent fibers may 52 FIGURE 8

ST!r-filLATION OF THE DORSOLATERAL FUNICULUS AFTER UNILATERAL AND BILATERAL LESIONS

E

B F

RIGHT LEFT

~1/fl,lfl'~. 00 [ . . . U.'li-'- ~

Blood pressure responses obtained by stimulating the right side of the spinal cord after unilateral (right) or bilateral (left) lesions were made in the dorsolateral funiculi at the Ce to Ti seg­ ment. The right dorsolateral funiculus was stimulated at the DLS, rostral (A) (E) and caudal (B) (F) to the lesion; and 2 mm ventro­ lateral to the DLS, rostral (c) (G) and caudal (D) (H) to the lesion. The scales to the left of the blood pressure tracings represent blood pressure in mm Hg. The lo~ r tracing is the time marker with the dark line designating the time of stimulation. Note the decrease in control blood pressure levels in the panel designated left. 53 be activating the sympathetic pathway on the contralateral side of the spinal cord. This possibility was examined by placing a discrete lesion on the surface of the contralateral funiculus, designated left in the panel of figure 8. Therefore, the panel designated left represents a preparation with bilateral lesions placed in the dorso­ lateral funiculi at the to T segment. No significant presser c8 1 responses were observed by stimulating the DLS rostral (fig. BE) or caudal (fig. BG) to the lesion on the right side of the spinal cord.

Blood pressure responses observed during stimulation 2 mm ventro­ lateral to the DLS (fig. 8F and 8H) were similar to the responses recorded in the unilateral preparation (fig. 8B and 8D). Bilateral lesions in the dorsolateral funiculus greatly attenuated or abolished the presser responses when stimulating the DLS rostral to c to T 8 1 lesion. These results suggest that afferent fibers in the DLS trans­ mit impulses to the contralateral descending pathway.

B. Bilateral Carotid Occlusions

After determining the location of a descending vasopressor pathway, reflex blood pressure responses were utilized to determine whether this pathway was important in relaying this activation to the preganglionic nerves. Effects of bilateral carotid occlusions on vasopressor function before and after transection of fibers in the superficial layer of the dorsolateral funiculus were studied to de­ termine reflex activity. Data of blood pressure responses from animals are illustrated in figure 9A and 9B. Panel A shows blood pressure levels after the right dorsolateral funiculus lesion was 54 FIGURE 9 BLOOD PRESSURE RESPONSES DURING BILATERAL CAROTID OCCLUSIONS

A 200

C) J: D CONTROL E E ~OCCLUSION 150

w a: ::> I/) I/) 100 w a: Q. 0 0 oi 0 $' ...I 50 m

0 INTACT RIGHT BILATERAL TOTAL LESION LESION TRANSECTION

B

200

C) CONTROL J: I D E E ~ OCCLUSION 150 - i

w a: 0 ::> I/) 0 100 I/) UJ . a: Q. o9 0 0 o* 0 ...I 50 m

0 INTACT LEFT BILATERAL TOTAL LESION LESION TRANSECTION Pressor responses from bilateral carotid occlusions before and after discrete lesions at the c8 to Tl segment in the dorsolateral funiculus (abscissa), starting with the right funiculus (A) or left funiculus (B). Note that control pressures as well as occlusion pressures decreased following each superficial lesion; complete tran­ section caused no further decrease. 55 made first and Panel B after the left dorsolateral lesion was made first. Data of right and left funiculi lesions obtained from ex­ periments represented in figure 9A and 9B were plotted separately, to make certain that any differences from making either of these lesions first would be observed. Statistical analysis for unpaired data of the control blood pressure levels and pressor responses re­ sulting from bilateral carotid occlusions showed no significant differences between the right and left lesions of the spinal cord. However, the pressor responses and control levels from the left dorsolateral lesion usually showed less change from the intact pre­ paration than was observed from the presser responses and control levels after the right dorsolateral lesion was made first. Control blood pressure levels decreased significantly when discrete lesions were made on the surface of the right (fig. 9A) or left (fig. 9B) dorsolateral funiculus or when lesions were placed bilaterally in the funiculi. Right unilateral lesions decreased systolic blood pressure from 138.8 ± 9.8 mm Hg to 118.3 ± 7.3 mm Hg (p

92.0 ± 6.8 mm Hg (p<0.025) and decreased diastolic pressure from

91.6 ± 6.4 mm Hg (unilateral lesion) to 67.0 ± 9.1 mm Hg (p

from 109.7 ± 3.8 mm Hg (unilateral lesion) to 76.5 ± 4.9 mm Hg (p<0.001). Complete transection of the spinal cord at the c to 7 segments did not significantly alter (p>0.5) control blood pressure c8 levels from that observed after bilateral lesions in figure 9A and

9B. Bilateral carotid occlusions, following unilateral or bi-

lateral lesions of the dorsolateral tracts and complete transection of the spinal cord, significantly increased blood pressure from control

levels. Systolic pressure increased from 138.8 ± 9.a mm Hg to 228.6 ±

8.6 mm Hg (p

5.2 mm Hg·to 172.1 ± 8.9 mm Hg (p<0.001) during bilateral carotid occlusions of the intact preparation represented in figure 9A. Follow-

ing right unilateral lesions, bilateral carotid occlusions caused

systolic pressure to increase from 118.3 ± 7.3 mm Hg to 160.6 ± 15.2 mm Hg (p

129.1 ± 12.1 mm Hg (p

Hg (p<0.001) and diastolic pressure increased from 124.0 + 6.4 mm Hg to 182.2 ± 4.6 mm Hg (p<0.001). After the left lesion was ma.de in the dorsolateral funiculus, systolic pressure increased from 136.2 ±

4.6 mm Hg to 190.3 ± 7.9 mm Hg (p

(p<0.05) from control blood pressure responses in figure 9B.

Presser responses resulting from bilateral carotid occlusions decreased significantly from the intact preparation after right

(p<0.001) or left (p<0.025) unilateral lesions were made. No signi­ ficant differences were obtained by comparing presser responses during bilateral carotid occlusions after bilateral lesions or complete spinal cord transections (p>0.5), however, responses to both of these spinal cord interventions were significantly decreased from responses obtained during bilateral carotid occlusions after right (p<0.001) or left (p<0.025) unilateral lesions.

Figure 10 represents typical blood pressure tracings during bilateral carotid occlusions. The panel designated left illustrates blooq pressure responses when the lesion was made in the left dorso­ lateral funiculus first and the panel designated right illustrates blood pressure responses after a lesion was made in the right dorso­ lateral funiculus first. Tracings A of the left and right panel 58 FIGURE 10

BLOOD PRESSURE TRACINGS DURING BILATERAL CAROTID OCCLUSIONS

LEFT RIGHT

A A

u

B B

c c

.....------' [ ...... 2 - ---.. ----~-i]j r --·------·--·----- D D

Blood pressure tracings during bilateral carotid occlusions when the left or right lesion was made in the dorsolateral funiculus first. Bilateral carotid occlusions were performed with intact prep­ arations {A), unilateral lesions {B), bilateral lesions (c) and com­ plete transections (D). The brackets represent blood pressure in mm Hg and the dark lines beneath the tracings show the period of occlusion. During the periods shown between the arrows, the amplifier gain was reduced by 500;6. 59 show blood pressure responses resulting from bilateral carotid occlusions in the intact preparation. Blood pressure tracings in figure lOB, lOC, and lOD illustrate the effects of unilateral lesion, bilateral lesions and complete transection respectively.

Blood pressure responses resulting from either right or left carotid artery occlusion were compared to determine if the presser responses would be eliminated independently by transecting either the right or left dorsolateral tract. Results in figure 11 indicate that blood pressure responses during right carotid occlusion or left carotid occlusion, after either the right or left unilateral lesion, do not show any significant differences when these presser responses are compared. When the preparations had intact spinal cords, there was no significant difference in blood pressure responses during right or left unilateral carotid occlusions.

Blood pressure responses during bilateral carotid occlusions were recorded for 3 hours as shown in figure 12. These occlusions were analyzed under conditions similar to the experiments with lesions, except that no lesions were made in the spinal cord. These results which are typical of three experiments, indicated that the fall in control blood pressure levels or decreased presser responses to bi­ lateral carotid occlusions were not caused by deterioration of the animal but were in fact due to interruption of sympathetic pathways.

Transverse cross sections of the spinal cord in figure 13 are representative of lesions placed in these experiments and in experi­ ments with afferent stimulation. Extreme care was used in placing 60 FIGURE 11

ALTERATIONS IN BLOOD PRESSURE DURING UNILATERAL CAROTID OCCLUSIONS

160 O control § occlusion

120

80 OT._ ___ RCO LCO RCO LCO

LEFT RIGHT

Pressor responses {ordinate) during right unilateral (RCO) or left unilateral {LCO) carotid occlusions following a left or right dorsolateral funiculus lesion. Open bars represent control blood pressure levels and striped bars represent pressor responses during unilateral occlusions. Upper lines of the graph illustrate systolic pressure and lower lines diastolic pressures. Brackets are standard errors of the mean. 61 FIGURE 12 BILATERAL CAROTID OCCLUSIONS WITH SHAM PREPARATION

200

100

0

Blood pressure responses during bilateral carotid ocqlusions in a.sham preparation. Blood pressure tracings A through E were ob­ tained at various time intervals throughout a three hour period. The scales indicate blood pressure in mm Hg. The lower tracing is the time marker and the dark line designates the period of occlusion. During the periods shown between the arrows, the amplifier gain was reduced . by 5o;6. 62 these lesions in the dorsolateral funiculus. A scalpel with approxi­

mately 1 mm long tip was utilized to carefully dissect the pathway.

After each experiment the spinal cord was examined to determine the

extent of the lesion. In most experiments the lesions were approxi­

mately 1 mm in depth and extended from the DLS to the dentate ligament.

Heart rate responses during bilateral carotid occlusions and

during control levels are illustrated in figure 14. Animals were

divided into two groups; one group of eight animals had the lesion made

in the left dorsolateral funiculus first and the second group of eight animals had the lesion nade in the right funiculus first. No signif­

icant difference was observed from the intact preparations under con­

trol conditions. After the left lesion was made first, control heart rate did not decrease significantly from the intact preparation. How­ ever, when the right lesion was made, heart rate decreased by an average of 56 beats per minute. No further decrease was observed after the spinal cord was completely transected.

Increases in heart rate during bilateral carotid occlusions were negligible for intact animals of the left unilateral group. A small increase was observed in the right unilateral group but the increase was only 11 beats per minute greater than control rate.

Changes in heart rate resulting from bilateral carotid occlusions after unil~teral lesions, bilateral lesions or complete transections were not observed in this series of experiments. One must note that heart rate was very ra.pl& and therefore, may have been near the limits for cardio­ acceleration. 63

:l!'IGURE 13

TRANSVERSE SEC'IION OF TR: SPINAL CORD

~:.;p..'.~...·.. • ~:·,~. .,-.~~l",i•!l~-,a""~"';'.Jr"M1 ' '· . ' ,. ' . . ~,

.'>;. ·;· • ., ' • •

, ' ~ 1't !'~!~~'!<. L .. f: ~; ~F~~~\ ' -.;:,,'......

.. ./ . ."~u·-~:....~ ... ;.·;;., ~v. :':...... ~.,·~~:..:.·~~ ;.·~~L.t:· .;,:

Transverse sections or the spinal cord at the Ca to T1 seg­ ment illustrating a control section (A) and a section with bilateral lesions (B) placed in the dorsolateral funiculus. The dark line marke:::r is equivalent to 1 mm. The arrows indicate the location of electrical stimulations (A) and location of bilateral lesions (B). 64 FIGURE 14

ALTERATIONS IN HEART RATE BEFORE AND AFTER SPINAL CORD LESIONS

260 !

@ rt. (control) fa ! l'i"l It. (control) I • rt. (occlusion) f 0 It. (occlusion)

220 f I t I 180 f f f f oI... __ intact unilateral bilateral complete lesion lesions transection

Heart rate responses (ordinate) during different spinal cord interventions (abscissa). Rt. refers to lesion of the right dorso­ lateral funiculus first and Lt. refers to lesion of the left dorso­ lateral funiculus first. Brackets represent standard errors of the mean. 65 c. Electrophysiological Characteristics Recordings of evoked responses from T preganglionic nerves 2 were obtained by stimulating the surface of the segments at dif­ c2 ferent levels between the DLS and dentate ligament. In figure 15 the upper tracing was obtained by stimulating the surface at the DLS; the middle and lower tracings were obtained by stimulating the surface 1 and 2 mm ventrolateral to the DLS. No significant responses were recorded from proganglionic nerves when the surface of the spinal cord was stimulated at the DLS and 1 mm ventrolateral to it, but evoked responses were recorded when the 2 to 3 mm level was stimulated. Conduction time was measured between the stimulation site at the segment and the recording site on the T preganglionic nerve c2 2 in seven animals. The average distance measured between these two sites was 87 mm. Conduction time which was determined from the initial rise of the evoked responses was 22.4 msec with a range of 17.8 msec to 27.a msec. Peaks of evoked responses were also measured for conduction time and the average value obtained was 30.6 msec with a range of

25.a msec to 43.5 msec. Evoked responses recorded from the·T white rami in figure 16 2 were elicited by stimulating the surface of the dorsolateral funiculus at the T segment. Each tracing represents a different level of the 2 dorsolateral funiculus. Stimulating the DLS (fig. 16A) while record­ ing from the T white rami activated 3 separate evoked responses 16, 2 27, and 42 msec following the stimulus artifact. These evoked responses will be discussed further when somatic afferent stimulation is de- 66 FIGURE 15

c2 DORSOLATERAL FUNICULUS STIMULATION I A

I B

c

Average evoked potentials, obtained by stimulating the dorso­ lateral sulcus (A), 1 mm ventrolateral to the sulcus {B) and 2 mm ventrolateral to the sulcus (c) recorded from the T2 white rami. The first major deflection represents the stimulus artifact followed by evoked responses. Calibration markers: lOuV and 5 msec. 67 scribed. After the stimulating electrode was moved 1 mm ventrolateral to the DLS (fig. 16B) usually no significant responses were recorded.

Stimulating 1.5 to 2 mm ventrolateral to the sulcus (fig. 16C) pro­ duced evoked responses from the preganglionic nerve 14 msec following the stimulus artifact. Evoked responses recorded from this level correlated with the levels of greatest pressor responses observed in figure 6 and 7. Usually no evoked responses were obtained 3 mm ventrolateral to the DLS (fig. 16D).

In 15 experiments, the conduction time was determined be­ tween the stimulating electrode placed 2 mm ventrolateral to the DLS and the recording electrodes on the preganglionic nerve. The average distance between the electrodes was approximately 20 mm. Conduction time from the beginning of the stimulus artifact to the initial rise on the evoked responses was 11.4 msec with a range of 9.5 msec to 14.7 msec. Conduction time was also determined from the stimulus artifact to the peak of the evoked responses; average conduction time was 15.2 msec with a range of 13··5 msec to 20.2 msec. Conduction velocity of the descending sympathetic pathway was calculated by subtacting the conduction time of the T2 segment, 2 mm ventrolateral to the DLS, from the conduction time of the c2 seg­ ment, 2 mm ventrolateral to the DLS. The value obtained from this subtraction was divided into the distance measured between the and c2 T2 segments which was approximately 67 mm. The average conduction velocity, calculat~~ from six experiments in which the c2 and T2 measurements were made in the same animal, was 5.9 m/sec with a range 68 FIGURE 16

T2 DORSOLATERAL FUNICULUS STIMULATION - .... - ---~-- ____ ....

A

B I

c ':~

D tr-w-> I

Average evoked potentials, recorded from the T2 white rami while stimulating the T2 segment of the spinal cord at the levels of the dorsolateral sulcus (A), 1 mm ventrolateral to the sulcus (B), 2 om ventrolateral to the sulcus (c) and 3 mm ventrolateral to the sulcus (dentate ligament) (D). The first major deflection represents the stimulus artifact followed by the evoked potentials. Calibration markers: 10 uV and 5 msec. 69 of 3.8 m/sec to 7.3 m/sec. These values were determined from the initial rise of the evoked responses. When the peak of the evoked responses was used to calculate conduction velocity, the average value obtained was 4.4 m/sec with a range of 2.2 m/sec to 8.6 m/sec. Evoked responses were also utilized to determine whether this sympathetic pathway is descending and not ascending. Results in figure

17 were obtained after a discrete lesion was placed in the dorsolateral funiculus at the to T segment. After the lesion was made the c8 1 sympathetic pathway was stimulated caudal (fig. 17A) and rostral (fig.

17B) to the lesion 2 :rrm ventrolateral to the DLS and evoked responses were recorded from the T preganglionic nerve. Stimulating rostral to 2 the lesion did not evoke any responses, while stimulating caudal to the lesiori evoked discharges which were similar to the intact prepar­ ations. Since the evoked responses were not produced by stimulating rostral to the lesion, these observations indicate these sympathetic pathways are descending.

Stimulation of the descending sympathetic pathway at lower segments of the spinal cord is illustrated in figure 18. The upper tracing was obtained by stimulating the DLS· (fig. 18A) at the T 8 segment while recording from the T preganglionic ramus. Two evoked 2 responses were observed following the stimulus artifact. The stimu­ lating electrode/~as moved approximately 2 mm ventrolateral to the

DLS (fig. 18D) in the lower tracing. No evoked responses were ob­ tained by stimulating at this level, therefore, this is further evidence to indicate the descending characteristic of the sympathetic 70 FIGURE 17

STIMULATION CAUDAL AND ROSTRAL TO c8 DORSOLATERAL FUNICULUS LESION

A I

B I

Averaged evoked responses recorded from the T2 white rami while stimulati1lg caudal (A) and rostral (B) to a lesion made in the dorsolater~l funiculus of the c8 segment. The dorsolateral funiculus was stitmlated 2 mm ventrolateral to the DLS. Calibration markers: 10 uV and 5 nsec. 71 FIGURE 18

IPSILA.TERAL T STIMULATION OF DORSOLATERAL FUNICULUS 8

A

1----1

B I

Averaged evoked responses recorded from the T2 white rami while stimulating the DLS (A) and 2 mm ventrolateral to the DLS (B) at the T8 segment. Note the lack of evoked responses when stimu­ lating 2 nun ventrolateral to the DLS. Calibration markers: 10 uV and 10 msec. 72 pathway.

Sympathetic pathways were also analyzed to determine if they

cross the midline of the spinal cord between the c and T segments. 2 2 Two pairs of stimulating electrodes were placed on the right and left

surface of the segment at the level of the greatest blood pressure c2 response (approximately 2 mm ventrolateral to the DLS). Recordings of

evoked responses were obtained from the T preganglionic nerves only 2 on the right side of the animals. Similar increases in blood pressure, which are demonstrated in figure 19A and 19B, were obtained by c2 stimulation before and after evoked responses were recorded. The trac-

ing in figure 19A depicts the evoked responses recorded from the T 2 preganglionic nerve while stimulating the ipsilateral side of the

spinal cord. The averaged evoked responses were recorded 25 msec

following the stimulus artifact. No evoked responses were observed in

figure 19B when the contralateral side of the spinal cord was stimu-

lated at the segment; however, the blood pressure response was c2 comparable to the response observed in figure 19A. Since pressor re-

sponses could be stimulated from both sides of the spinal cord and

evoked responses could be recorded only from the ipsilateral side,

these observations indicated that these pathways remained mainly un-

crossed between the and T segments of the spinal cord. c2 2 ! Stimulation of the DLS at the ipsilateral T segment activated 2 three separate evoked discharges in figure 20A. These responses were noted only at the DLS and cotild not be activated by stimulating from

l mm ventrolateral to the DLS to the dentate ligament. The average 73 FIGURE 19

IPSILATERAL .AND CONTRALATERAL c2 STIMITLATION 2 MM VENTROLATERAL TO THE DLS

250 I A

250 DJ ::c: ... W~··. E J E 0

B

Average evoked responses recorded from the right T2 prega.ng­ lionic nerve while stimulating the right (A) or left side (B) of the dorsolateral funiculus 2 mm ventrolateral to the dorsolateral sulcus at the C2 segment. Inserts show similar blood pressure elevations when either side of the spinal cord was stimulated independently. The first major deflection represents the stimulus artifact followed by evoked potentials. Calibration markersi 10 uV and 5 msec. 74 conduction times in six experiments for the initial rise of the first response was 12.5 msec with a range of 9.5 to 16.5 msec. Conduction times for the peak of the evoked response was an average of 14.3 msec and ranged from 11.5 msec to 21.0 msec. The second evoked response had an average conduction time of 26.4 msec (range of 20.5 to 30.5 msec) for the initial rise and a conduction time of 31.1 msec (range of 24.0 to 37.0 msec) for the peak of the evoked responses. A third evoked response appeared 40.2 msec (range of 35.0 to 52.2 msec) following the stimulus artifact and the peak of the response occur­ red 48.2 msec (range of 43.0 to 58.7 msec) after the artifact.

The possibility of afferent stimulation at the DLS, raised the question as to whether stim'..llation of the contralateral DLS at the T segment would evoke responses in the T white ramus. The 2 2 responses in figure 21A indicate that contralateral stimulation of the first evoked response was 24.9 msec (range of 23.0 to 30 msec) for the initial rise and 32.3 msec (range of 28.5 to 37.5 msec) for the peak. The second evoked response had an average conduction time of

42.8 msec (range of 36.0 to 52.2 msec) for the initial rise and 50.4 msec (range of 43.0 to 59.7 msec) for the peak. These two responses correlated with the second and third evoked responses obtained by stim­ ulating the ipsilateral DLS at the T segment (fig. 20A). A very 2 small evoked response was observed in two out of six experiments and had conduction times which correlated with the first evoked response obtained at the ipsilateral DLS. Thus stimulation of the contralateral

DLS does evoke responses in the T white rarni. 2 75 FIGURE 20

IPSILATERAL T2 STIMULATION OF THE DLS BEFORE AND AF'l1ER LESION

I

Average evoked responses recorded from the T2 white rami while stimulating the DLS of the ipsilateral T2 segment. Evoked responses were obtained before (A) and after (B) a lesion was made at the Ca to Ti segment. Note the evoked response which was obtained after (B) the lesion was made. Calibration markers: 10 uV and 10 msec. 76 The role of the descending sympathetic pathway in transmitting the evoked res:ponses obtained by stimulation of the DLS was examined by placing discrete lesions in the ipsilateral descending pathway at the to T segment of the spinal cord. After a lesion was made in Ca 1 the descending pathway, the ipsilateral and contralateral DLS were stimulated againM An early response was observed from stimulation of the ipsilateral DLS as illustrated in figure 20B, however, this early response was not observed consistently. The second and third evoked responses were abolished or significantly attenuated. No responses were usually observed, however, there is a smaller evoked response when the contralateral DJ,S was stimulated (fig. 21B). These observa­ tions indicated that lesions in the ipsilateral dorsolateral funiculi could abolish the evoked responses with long conduction latencies.

Attenuating or abolishing the second and third evoked responses by placing the lesion at the Ca to T segment did not determine whether 1 ascending or descending pathways were interrupted. Interrupted path­ ways were determined by stimulating the ipsilateral DLS at the Ca segment, rostral to the lesion. The effect of this stimulation is illustrated in figure 22. Three evoked responses were observed from the T preganglionic nerve recordings when the DLS was stimulated. 2 The first evoked responses had an average conduction time of 13.3 msec

(range of 11.5 to 15.7 msec) for the initial rise and 17.5 msec

(range of 17.0 to 19.0 msec) for the peak of the response. The first evoked response was observed in four out of five experiments. The second evoked response had an average conduction time 25.s msec 77 FIGURE 21

CONTRALATERAL T STIMULATION OF THE DLS 2 BEFORE AND AFTER LESION

B

Average evoked responses recorded from the T2 white rami while stimulating the DLS of the contralateral T2 segment. Evoked responses were obtained before (A) and after (B) a lesion was made at the Cs to T1 segment. Calibration markers: 10 uV and 10 msec. 78 FIGURE 22

IPSILATERAL ca DLS STIMULATION :BEFORE AND AFTER LESION

A I

I-

8 I

Averaoc-e evoked responses recorded from the T2 white rami while stimulating the DLS at the ipsilateral Cs segment. Evoked re­ sponses were obtained be.fore (A) and after (B) a lesion was placed in the Ca to T1 segment. Note the small response after (B) the lesion was made. Calibration markers: 10 uV and 10 msec. 19 {range of 20.0 to 29.3 msec) for the initial rise and 32.3 msec (range of 27.0 to 35.4 msec) for the peak of the responses. The average con­ duction time of the third evoked response was 43.4 msec {range of

36.5 to 56.o msec) for the initial rise and 52.a msec {range of 45.0 to 6a msec) for the peak. After the lesion was made at the Ca to T1 segment the three evoked responses were significantly attentuated or abolished {fig. 22B). On some occasions both the first and third e­ voked responses would be abolished but there would be a small response observed which corresponded to the second evoked response.

In two experiments, the evoked responses were activated from the DLS of the Ca segment while the lesion was being made at the Ca to T segment. No significant change was observed when the lesion 1 was made from the DLS to approximately 1 mm ventrolateral to it. How­ ever, the responses were significantly attenuated when the lesion had interrupted fibers between 1 mm ventrolateral to the DLS and the dentate ligament. This indicated that the evoked responses were affected by interrupting descending fibers of the sympathetic tract.

The DLS of the Ca segment on the contralateral side of the spinal cord was stimulated and the evoked responses were recorded from the T preganglionic nerve. The responses observed from this stimu­ 2 lation are shown in figure 23A. The first evoked response was not observed in four out of five experiments, but a very small response was recorded in one experiment. The second response had an average conduction time of 23.7 msec {range of 21.5 to 26.7 msec) for the initial rise and 32.4 msec (ra!lcr:-e of 27;5 to 37.1 msec) for the peak. 80 FIGURE 23

CONTRALATERAL DLS STHIDLATION c8 BEFORE AND .AFTER LESION

A I

B I

Average evoked responses recorded from the T2 white rami while stimulating the DLS of the contralateral Cs segment. Evoked responses were obtained before (A) and after (B) a lesion was made at the Ce segment. Note the residual response after (B) the lesion was made. Calibration markers: 10 uV and 10 msec. 81

The average conduction of the third evoked response was 44.6 msec

(range of 37.5 to 58.6 msec) for the initial rise and 53.6 msec (range of 43.7 to 69.5 rnsec) for the peak. After the lesion was placed in the

to T dorsolateral funiculus of the ipsilateral side of the spinal c8 1 cord the evoked responses were attenuated or abolished, (fig. 23B). There was a small peak after the lesion was made which correlated with the second evoked response.

Evoked responses were also elicited from the T segment to 8 determine whether any responses would be observed at the T pregang­ 2 lionic nerve without being transmitted to supraspinal levels of the central nervous system. Figure 24A illustrates the responses observed by stimulating the DLS at the T segment of the spinal cord. The 8 second evoked response had an average conduction time of 28.9 msec

(range of 25.5 to 32.5 msec) for the initial rise and 35.2 msec (range of 31.5 to )8.0 msec) for the peak. Average conduction time for the third evoked response was 41.7 msec (rano"'e of 36.5 to 52.0 msec) for the initial rise and 49.6 msec (range of 44.0 to 63.0 msec) for the peak. A ver:y small evoked response was seen in two out of six ex­ periments and corresponds with the first response. Lesion placement in the to T dorsolateral funiculus abolished the evoked responses c8 1 (fig. 24B). 82 FIGURE 24

IPSILATERAL '1.1 DLS STIMULATION 8 :BEFORE AND .AP.rER LESION

B I

Average evoked responses recorded from the T2 white rami while stimulating the DLS at the ipsilateral T8 segment. Evoked responses were recorded before {A) and after {B) a lesion was made at C9. Calibration markers: 10 uV and 10 msec. CHAPTER V

DISCUSSION

Electrical stimulation of the dorsolateral funiculus of the spinal cord shows that sympathetic vasopressor fibers are located on the surface between the DLS and the dentate ligament in the cervical and uvper thoracic segments. The largest concentration of vasopressor fibers was located 1.5 to 2.0 mm ventrolateral to the DLS, with fewer fibers being distributed near the DLS. Location of these blood pressure responses was sinilar to that reported by Kerr and Alexander (76) ex­ cept, in their study, the sympathetic fibers were apparently localized in the anterolateral, as well as the dorsolateral funiculus. Increases, decreases or no changes in blood pressure were observed when the dentate level of the spinal cord was stimulated, but the average value for blood pressure in nine animals showed no significant change from control blood pressure levels. Therefore, the lower limit of the sympathetic tract may vary in its location near the dentate ligament.

Superficial spinal tracts on the surface of the lateral funiculus have been shown to transmit impulses to the cerebellum. The dorsospinocerebellar tract is described as an uncrossed ascending tract which is located along the dorsolateral periphery of the spinal cord

83 84 and arises from the large cells of Clarke's Column (13, 145). These

fibers become incorporated into the inferior cerebellar peduncle which

enter the cerebellum. Fibers of the dorsospinocerebellar tract have

conduction velocities ranging from 30 to 100 m/sec (93, 101). This

tract relays impulses to the cerebellum from stretch, touch and

pressure receptors in the periphery (101). There is a possibility

that the descending sympathetic pathway may be intermingled with the dorsospinocerebellar tracts.

The descending sympathetic pathways could be intermingled with other descending pathways of the spinal cord, such as the rubrospinal or corticospinal pathway. The corticospinal tract descends in the dorsal part of the lateral funiculus between the dorsospinocerebellur tract and the lateral fasciculus proprius (33, 109). Landau (88) has

shown that presser responses could be activated by stimulating the corticospinal tract in the medullary region. Kerr and Alexander (76) have shown that presser responses disappear after stimulating one millimeter or less into the dorsolateral funiculus. The descending

sympathetic pathway could be intermingled in the dorsal and lateral aspects of the corticospinal tract.

Another pathway which could be involved with the descending sympathetic pathway is the rubrospinal tract. The rubrospinal tract lies just ventral to the corticospinal tract in the lateral portion of the dorsolateral funiculus (20, 33, 109). Busch (20) suggested that the rubrospinal tract is made up predominately of large fibers, however, he pointed out that the particular stain used in his study 85 would only show larger fibers. Fibers smaller than one to two microns

would not be observed with his technique. Small fiber size may be

critical since the conduction velocity of the descending sympathetic

fibers described in this study suggest that small fibers are involved.

Petras (109) noted that a significant number of fibers in the rubro­

spinal tract terminate in the lateral part of the zona intermedia, which would suggest that some of these fibers could possibly synapse with sympathetic neurons in the lateral horn; as well as with cells

in the motor nuclei.

Descending synpathetic pathways may be involved independently with either the rubrospinal or corticospinal tract. However, a more

likeJy possibility is that the sympathetic pathway is intermingled

in the lateral regions of the rubrospinal and corticospinal tract and is located throughout the dorsospinocerebellar tract.

Indirect evidence for the location of descending sympathetic pathways can be ascertained from the studies of Dahlstrom and Fuxe

(35). Using histochemical fluorescence techniques with transection and degeneration studies, they were able to determine the location of descending fibers in the lateral funiculus which had intimate contact with the sympathetic lateral horn. Sympathetic fibers were localized by transecting the thoracic spinal cord and looking for accumulations

of norepinephrine and 5-hydroxytryptamine rostral to the transection.

They reported that most of the fibers which contain 5-hydroxytryptamine descend in the superficial aspect of the dorsolateral funiculus and the fibers which contain noradrenaline descend predominately in the 86 anterior funiculus. Cell changes caused by retrograde degeneration, after spinal nerve transections, show that both 5-hydro:xytryptamine and norepinepbrine terminals make intimate contact with the cell bodies or cell processes of the intermediolateral or intermediomedial cell columns. The 5-hydroxytryptamine descending pathway correlates closely with the functional location of the pathway in this study.

Further evidence to suggest that the 5-hyd.ro:xytryptamine de­ scending pathway and the functional pathway described in this study may be related comes from the studies of De Groat and Ryall (36) and Ryall (124). Electrophoretic application of 5-hydroxytryptamine on preganglionic neurons increased spontaneous activity. Excitatory action of 5-hydroxytryptamine suggests that the nerve terminals con­ taining this transmitter in the intermediolateral region of the spinal cord may be the descending fibers described by Dahlstrom or Fuxe (35). However, before the proposal can be established, experiments must show that the 5-hydroxytryptamine is liberated by selective activation of these nerve fibers and is analagous to the action of the electrophoretic application of 5-hydro:xytryptamine. Indirect evidence does suggest that the vasopressor pathway reported in this study may be the nerve fibers containing 5-hydroxytryptamine. Evoked responses recorded from the thoracic white rami while stimulating the lateral funiculus by millimeter increments also localized the major concentration of sympathetic fibers. Stimulation

1.5 to 2.5 mm ventrolateral to the DLS usually evoked a response, but stimulation either 1.0 mm or 3.0 mm ventrolateral to the DLS evoked 87 small responses or, more commonly, no responses from these levels. This observation further indicated that the major concentration of sympa­ thetic fibers is located 1.5 to 2.5 mm ventrolateral to the DLS.

In comparing the blood pressure responses and evoked responses during electrical stimulation, frequencies of 50 to 60 Hz were used to increase blood pressure and frequencies of 0.5 to 1.0 Hz were used to produce evoked responses. One could argue that different pathways are possibly being stimulated by changing frequency, because it is known that louer frequencies cause depressor responses and high frequencies cause pressor responses. I<'edina (44) reported that these two blood pressure responses to low and high frequencies of stimulation do not require two different pathways. At low frequencies of stimulation there are evoked responses followed by inhibition of spontaneous activity. The inhibition of spontaneous activity permits vasodilata­ tion to occur because the slow frequency of evoked discharges can not compensate for the inhibition. As frequencies of stimulation increase, the evoked discharges shorten or overcome the period of inhibition and cause vasoconstriction. It was also reported by Koizumi et al.

(83) that Group II and III afferent fibers predominately cause the silent period or inhibition of spontaneous activity, but Group IV afferent fibers have much less effect on the silent period, therefore, as frequencies increase one may be stimulating more Group IV fibers.

From this evidence it is suggested that the same pathways are being stimulated at two different frequencies of stimulation.

Activation of sympathetic responses by stimulating the dorso- 88

lateral funiculus does not indicate whether these responses are evoked by afferent pathways relayed to supraspinal regions, or are actually descending pathways located in the spinal cord. Kerr and Alexander

(76) transected the spinal cord and then stimulated rostral and caudal to the transection. Blood pressure responses observed rostral to the lesion were abolished or attenuated but stimulation caudal to the lesion elevated blood pressure responses to levels observed before the lesion. Illert and Gabriel (65) recorded blood pressure responses, as well as renal and splanchnic sympathetic activity, while stimulating the lateral funiculus at the c to c segment in a spinal animal. 2 3 They also noted increased sympathetic discharge and blood pressure from their stimulations.

In this study, blood pressure responses and evoked responses, obtained by stimulating the dorsolateral funiculus, were abolished or attenuated by stimulating rostral to a superficial lesion, but did not change significantly when stimulating caudal to the lesion. These observations were obtained by stimulating approximately 2 mm ventro­ lateral to the DLS, but different results were obtained when the blood pressure responses were elicited by stimulating the DLS; these ob­ servations will be discussed with the afferent stimulation in a later section.

Stimulation of the spinal cord 2 mm ventrolateral to the DLS at the T segment did not activate an evoked response for the T pre­ 8 2 ganglionic recording. If afferent fibers were distributed in the dorsolateral funiculus, evoked responses could have been observed by 89 stimulation at this segment. Evoked responses were only activated when the DLS at the T segment was stimulated. 8 Other investigators have indirectly analyzed this descending pathway which relays impulses from the supraspinal central nervous

system to the peripheral nervous system. Kell and Hoff (73) reported increases in blood pressure when stimulating the anterior sigmoid gyri in the cortex of the cat. These blood pressure responses could be attenuated or abolished by placing discrete lesions in the dorso­ lateral funiculi of the cervical spinal cord. Stimulation of the hypothalamus (76, 150) also caused increases in blood pressure which could be abolished by lesions in the lateral funiculus. Achari et al.

(1) noted that increases in blood pressure by stimulating the fastigal nucleus of the cerebellum would be abolished by placing lesions in the dorsolateral funiculus. The experiments reported in this study, as well as results obtained by earlier investigators, give support to the view that the sympathetic pathway located in the dorsolateral funiculus is·descending and not ascending.

Control blood pressure levels decreased significantly when unilateral and bilateral lesions were made on the surface of the dorsolateral funiculi. This observation indicated that fibers regu­ lating tonic sympathetic activity were located near the surface of the dorsolateral funiculus. The tonic sympathetic region supporting cardiovascular activity has been located in the medulla. This location has been discussed in the literature review. Alexander (3) suggested that a bulbar pressor region in the floor of the fourth ventricle of 90 the medulla could contribute its part to the general excitatory state

and tonic function of the cardiovascular system.

Tonic or spontaneous discharges from the pre- or postganglionic

neurons of the sympathetic nervous system have been analyzed to de­

termine discharge characteristics from the spinal cord and central

nervous system. Bronk et al. (14, 15) reported direct evidence of tonic

discharge. Single unit recordings were made from the inferior cardiac

nerves and cervical sympathetic trunk. They noted that discharges

occurred with a frequency of less than one impulse per second to several

impulses per second with the aninals in a resting state. Folkow (47)

reported that frequencies of one to three impulses per second would

be s1lfficient to :naintain a normal vasoconstriction in blood vessels

of skeletal muscle. Other investigators (72, 129) observed firing

rates of one to two impulses per second from single unit recordings

of the white rami. Janig and Schmidt (68) reported that there was a

discharge rate of 1.7 per second for royelinated fibers and 2.9 per

second for unmyelinated fibers ih the cervical sympathetic trunk.

Koizumi and Sato (83) reported that spontaneous discharges had a mean

frequency of 1.9 impulses per second but there was a wide distribution

of discharges ranging from 0.2 to 4.0 impulses per second.

'11onic discharges could possibly originate from preganglionic neuroLs of the spinal cord. Polosa (113) noted that spontaneous pre­ ganglionic activity occurs at a much slower rate in an isolated de­ afferented spinal cord. Koizumi et al. (84) observed spontaneous activity from chronic spinal animals. These observations suggest 91 that tonic activity originates in supraspinal segments of the central nervous system, but the spinal cord is capable of displaying a reduced level of spontaneous activity.

Evidence from this study indicates that tonic activity, which originates from the medulla and possibly supramedulla.ry levels, may be transmitted to preganglionic neurons by these descending dorsolateral sympathetic path·.-:ays; because complete spinal transections did not significantly change control blood pressure levels when compared to control levels observed after making small bilateral lesions. Lack of decrease in control blood pressure levels after cosplete spinal cord transection does not eliminate the possibility that other pathways in the spinal cord may transnit supraspinal sympathetic activity; but they may not exhibit tonic control of blood pressure.

An interesting observation was made when the control heart rate levels were compared between the right and left unilateral lesions and bilateral lesions. If the lesion was made in the descending path­ way of the right dorsolateral funiculus, control heart rate decreased by 56 beats from the control rate. However, if a lesion was made in the left dorsolateral funiculus first, heart rate did not change significantly frotl the intact preparation; it only decreased after the second lesion was placed in the contralateral funiculus. Bilateral lesions, in the preparations with the lesion made in the right side first, had heart rates similar to tho rates after the right unilateral lesion.

Peiss ( 106) has noted that stin;uJ.ation of the right side of 92 the hypothalamus activated more rapid heart rates than did stimulation

of the left side. Chai and Wang (23) have shown similar responses from

unilateral stimulation in the medulla. Randall et al. (118, 119) has

also shown that stimulation of the right stellate ganglion and right

ventral roots elicit much faster heart rates than stimulation of the

left side. These reports coupled with the observations noted in this

study suggest that neural mechanisms controlling heart rate from supra­

spinal segments of the central nervous system have fibers which are

predominately located in the right descending sympathetic pathway.

Increases in heart rate were not observed when stimulating the

right descending sympathetic pathway or reflexly activating the heart

rate renponse by occluding the carotid arteries. T~ro major reasons

can be given for these observations. One, bilateral vagotomy

essentially removes the parasympathetic components to the heart. Con­

trol heart rate usually increased after the vagotomies were made.

Second, the experir::tents in these studies were done when the animals

were anesthetized with sodium pentobarbital. Since the heart rate

had a mean of 240 beats per minute for control levels, the heart was

beating at near maximal rates; therefore, it was not capable of in­

creasing significantly when the descending pathway was stimulated

or the carotid arteries were occluded.

In this study, only a very small reflex increase in blood

pressure could be obtained with bilateral carotid occlusions after bi­ latero.l lesions were ma.de at the c to c segments of the spinal cord. 7 8 Attenuated blood pressure respo::1Ses to bilateral carotid occlusions 93 after placeraent of the lesions in the dorsolateral sulcus indicated that these descending sympathetic pathways on the surface of the dorsolateral funiculus are transmitting impulses from the brainstem which are activated by baroceptor reflex raechanisr.is.

Hypertensive responses from either right or left carotid occlusions were analyzed before and after lesions were placed either on the right or left dorsolateral funiculus of the spinal cord. Uni­ lateral occlusion with an ipsilateral lesion was used to determine whether baroceptor reflexes of the sympathetic nervous system would descend bilaterally in the descending sympathetic pathways or would remain as a unilateral response. The data in this study suggested that unilateral carotid occlusions descend bilaterally, because sir:iilar blood pressure responses were observed during unilateral carotid occlusion when either the ipsilateral or contralateral descending path­ way was interrupted by the lesion.

The possibility of descending sympathetic pathways crossing the midline between the upper cervical segments and upper thoracic segments was also examined in this study. The results suggest that there is no evidence of midline crossover in descending pathways, because evoked responses were recorded from white rami when only the ipsilateral pathway was stimulated; however, similar increases in blood pressure were elicited when either funiculus was stimulated.

Evidence from previous investigations have suggested that autonomic pathways could be crossed and uncrossed in the spinal cord as they descend from supraspinal regions of the central nervous system. 94 Chen et al. (26) noted that if a lesion was made in the lateral funicu­ lus stimulation of the ipsilateral sympathetic region of the floor of the fourth ventricle in the medulla would abolish peripheral sympathe­ tic responses, but stimulation of the contralateral sympathetic region of the medulla would activate sympathetic responses which were similar to sympathetic responses before placement of the ipsilateral lesion.

Hagoun, Ranson.and Hetherington (94) made a hemisection at the cer­ vical segments and reported that lateral hypothalamic stimulation from the contralateral side showed no decrease in pressor responses; however, during ipsilateral stimulation pressor responses were :marked- ly decreased. From this evidence they suggested descending pathways fro~ the hy~othQlaz::us are ~redo~inately uncrossed.

Decussating patterns of synpathetic efferent pathways for both sides of the hypothalamus were also studied (57) by making a hemisec­ tion at the c to c or c to c segnents and by performing a contra­ 5 6 6 7 lateral sympathectomy. Subsequently, there was an increase in blood pressure when the hypothalamus was stimulated on the sympathectomized side, which may have been caused by activation of a descending pathway crossing below the hemisection. Residual responses could be activated by impulses being transmitted through another cross connection of the spinal cord caudal to the hemisection. Increases in blood pressure were less during stimulation of the hypothalamus on the hemisected side and could be explained by possible destruction of an uncrossed descending pathway. From these previous observations, as well as the results of this study, there is a strong possibility that major un- 95 crossed descending sympathetic pathways and crossed components descend in different tracts of the spinal cord.

Conduction velocity of the descending sympathetic pathway was determined by subtracting the thoracic conduction time from the cervi­ cal conduction time and dividing this value into the distance between the two points of stimulation. Conduction time of thoracic stimula­ tion was subtracted, because this would eliminate conduction in the white rami and the complex synaptic delay in the lateral horn of the gray matter. One important assumption ma.de in calculating the con­ duction velocity of the descending pathway was that no synapses were made in the descending pathway between the cervical and thoracic stimu­ lating electrodes. No direct evidence can be cited concerning synaptic characteristics of these descending pathways nor has anything been re­ ported concerning small fiber degeneration in the regions containing the descending pathways. The most significant indirect evidence of fiber characteristics of the descending pathway is that of Dahlstrom and Fuxe (35), and Carlsson, Falck, Fuxe and Hillarp (21). After tran­ section of the dorsolateral funiculus at the c or c segments, they ex­ 4 8 amined thoracic transverse sections utilizing the histochemical fluor­ escence technique. They reported that a majority of the 5-hydroxytryp­ tamine and norepinephrine neurons were not observed in the dorsolateral funiculus several days after the lesion was made in the spinal cord.

If there were a sienificant number of fibers having synaptic junctions between the segment transected and the segments analyzed, the number of fibers should not have diminished to the extent they reported in 96 their study. Another observation made by these investigators was that they could follow the descending fibers from the medulla to the thoracic, lumbar and sacral segments of the spinal cord. Inference made from the characteristics of long descending tracts such as the corticospinal, vestibulospinal, rubrospinal and reticulospinal tracts, also suggest that synaptic connections are unlikely after entering the spinal cord.

Thus, it is assumed that synaptic junctions are not located in the de­ scending sympathetic pathway.

The calculated conduction velocity of the descending sympathe­ tic pathway was 5.9 m/sec for the fastest conducting fibers and 4.4 m/sec for the greatest concentration of fibers. Fibers conducting at this velocity would possibly have small diameters, according to the Hursh (64) factor. Using that factor, the fibers could have a diameter of less than two microns. Dahlstrom and Fuxe (35) noted that the size of the 5-hydroxytryptamine and norepinephrine fibers were between one to two microns and they did not seem to have myelin. Their fiber size correlates with the conduction velocity calculated from the fibers in this pathway.

Coote and Downman (28) described central delay times for re­ cording evoked responses from the inferior cardiac nerve and renal nerve while stimulating the T dorsal root. Conduction time to the 10 recording site on the inferior cardiac nerve was subtracted from the conduction time to the renal nerve. The conduction time between the recording sites and their origin on the ventral surface of the spinal cord was subtracted from total conduction times for the inferior 91 cardiac and renal nerve. Stimulation of the T dorsal root evoked 10 a response in the inferior cardiac nerve 6 to 10 msec earlier than in

the renal nerve. Since they were only interested in the somato-sympa-

thetic reflexes, they ignored the possibility of a descending sympa-

thetic pathway which could conduct afferent impulses. Therefore, they did not estimate the conduction velocity of the descending pathway.

An estimated distance between the T spinal nerve (inferior cardiac 3 nerve) and T {renal nerve) spinal nerve is 60 mm. If one divides 10 the central delay time of 6 to 10 msec into the distance of 60 mm, the conduction velocity for this descending path•,,ray is between 6 to 10 m/ sec. The conduction velocity from this calculation correlates closely with the conduction velocity deter.n:i.ned in this study.

Average conduction tim~ from stimulating the T segment 2 mm 2 ventrolateral to the DLS to the recordings site on the T white rami 2 was 11.4 msec for the fastest fibers and 15.4 msec for the greatest number of fibers. Central delay time bet... 1een the preganglionic neuron and the descending sympathetic pathway can be indirectly determined by utilizing the experiments of Fernandez de :Molina and Perl (45) and

Hongo and Ryall (61). They exar.iined the conduction velocity of pre- ganglionic neurons by antidromically stimulating the preganglionic neurons and recording action potentials from the lateral horn of the gray matter in the spinal cord. They reported antidromic conduction velocities in the ranee of 1.5 to 9 n/sec with an averaee being ap­ proximately 4 m/sec. Using these velocities, preganglionic conduction time would be in the range of 2.2 to 14 msec with an averarse of 5 msec; 98 these calculations are for an estimated distance of 20 mm between stimulating and recordinc electrodes. If the value of 5 msec is sub­ tracted from the thoracic conduction time of 11.4 rnsec for the fastest fibers and 15.2 rnsec for the majority of fibers a calculated central delay time of 6.4 to 9.8 rnsec would be obtained. These observations and calculations suggest there is a long central latency between de­ scending sympathetic pathways and :preganglionic cell bodies.

Beachem and Perl (7) have suggested that there are polysynaptic connections between somatic afferent fibers and the preganglionic neuron. Conduction latencies reported in their study were sir.iilar to the latencies observed in this study. Polysynaptic connections can be postulated if one assumes that the s;yno;ptic delay between the pre- and postsynaptic site is similar to that calculated for known synapses. Synaptic delays have been reported to range from 0.3 rnsec to 1 to 2 msec for different synaptic junctions (40). There is also a possibility that the synaptic delay times or the membrane character­ istics of the preganglionic cell.body are different from the rnotoneuron.

No conclusions can be obtained from these results which could describe the synaptic characteristics between the descending pathway and the preganglionic neurons.

Two major components of somato-sympathetic reflexes have been described by several investigators as being supraspinal or late, and spinal or early reflexes. Extensive investigation of spinal reflexes was first conducted in spinal ani~als by Beachem and Perl (7, 8).

Spinal somato-sympathetic reflexes had conduction latencies of 10 to 99 35 msec and a central delay of 3 to 15 msec. Early responses had to be stimulated from afferent fibers in the same segment or adjacent segment to the white rami being recorded (79, 85, 132). This is an important characteristic of spinal reflexes.

The late response or supraspinal reflex had conduction latencies of 80 to 120 msec when recorded from the lumbar syr.ipathetic trunk (133) and 60 to 85 msec when recorded from the lumbar white rami (84, 130).

Coote and Downman (28) showed that the somatic afferent stimulation of the T dorsal root activated evoked discharges to msec later in 10 39 45 the inferior cardiac nerve and 45 to 54 nsec later in the renal nerve.

A characteristic of the late reflex is that evoked responses have sim­ ilar amplitudes and magnitudes when stimulated by afferent fibers sev-. eral segments from the recording site (132).

Similar observations for conduction time of the early and late responses were obtained from this study. All ,evoked responses in this study were recorded fron the T white rami. The early response was 2 always seen when stimulating the DLS of the ipsilateral T segment. 2 Very small early responses were observed in four out of five experi­ ments when stimulating the ipsilateral DLS of the segment. The c8 conduction latency and segmental characteristics of the early responses were similar to the studies made by previous investigators.

Late responses were activated by stimulating the DLS of the

, T , and T segments on the ipsilateral side of the spinal cord and c8 2 8 the and T segments of the contralateral segments. Conduction c8 2 latencies from all three segments were sir.iilar to the central delay 100

time of the recording from the inferior cardiac nerve (28). Conduction

time from the T segment was approximately 2 msec longer when measured 8 from the initial rise of the evoked response. This delay is probably due to the increase in conduction distance for afferent fibers from

the T segment. However, no difference in conduction latency was noted 8 when the peaks of the evoked responses from the T and T segments were 2 8 compared. This lack of difference could be due to the fact that the amplitude and magnitude of the late evoked responses varied from dif­ ferent animals, as well as from stimulation to stimulation. The average conduction latency from the CS segment was in fact 3 to 5 msec longer than the conduction latency from the T segment. No definite explana­ 2 tion can be obtained from this study c~ncernir.g this increased delay from the CS segnent when compared to the T segment, but it could also 2 be caused by the variation in evoked responses.

Comparison of conduction latencies of the evoked response with this study and earlier investigations was necessary, because in all previous studies the somato-sympathetic reflex was activated by stimulating dorsal roots, limb nerves, or spinal nerves; however in this study the soma.to-sympathetic responses were activated by directly stimulating the DLS. The DLS was stimulated directly to have consist­ ency of stimulation with blood pressure responses and evoked responses on the surface of the spinal cord. Therefore, the comparable conduc­ tion latencies between this study and earlier investigations sucgests similar fibers were activated by utilizing this method of stimulation.

One inportant difference noted in DLS stioulation, was the 101 appearance of an evoked response which appeared after the early response but before the late response. This response has not been reported in the literature. However, careful examination of the figures in the report by Sato and Schmidt (132) reveal the possibility of a second evoked response between the early and the late response. This evoked response was only noted in the tracings which had been averaged by the computer.

The appearance of this second evoked response, which was con­ sistently observed in the experiments, could be caused by the follow­ ing possibilities. One possibility is that averaging one hundred stimulations to determine the conduction latencies of evoked responses may have been able to ur.u-nask this second response. Two, directly stimulating the DLS at various segments may have activated this response which normally would not be elicited by stimulating the dorsal roots.

Conduction latency of the late response is explained by the fact that it first ascends to supraspinal levels of the central nervous system then descends to the preganglionic neurons. Transaction of the spinal cord at cervical segments abolishes the late response (28, 84,

131, 133). Coote and Dmrnman (28) cooled the floor of the fourth ventricle and abolished the late reflex; it reappeared after the fourth ventricle was rewarmed. Iwamura (67) placed a lesion in the presser region of the medulla which effectively abolished the late response.

Only one report by Khayutin and Lukoshkova (78) suggested the late re­ sponses could be observed in spinal animals, however, their report indicates that the spinal cord and the supraspinal regions were still 102 intact.

Data observed from this study suggested that the late responses which are transnitted to supraspinal recions, descend through the de­ scending Syt!pathetic pathway. This hypothesis is based on the fact that discrete lesions in the dorsolateral funiculus between the DLS and the dentate ligament at the to T seement effectively abolished the c8 1 late response. Stimulation at the T secraent usually elicited the early 2 responses, but the second or third response was not observed.

One could suggest that the lesion in the dorsolateral funicu­ lus interrupted afferent fibers which transmit the late response to the supraspinal levels and did not affect the descending sympathetic path1my. However, stir:mlation of the DJ,S at the sccr:ient and rostral c8 to the lesion did not evoke late responses; they should have appeared if the descending SJ1:1pathetic pathway was intact. The DLS at the TS segment was also stimulated before and after the lesion to make certain that evoked responses were not recorded from the T white rami without 2 first being transmitted to the supraspinal levels; no late responses were observed.

Stimulation of the DLS at the CS segment rostral to the lesion did evoke a small response which correlated with the conduction latency of the second response. The amplitude of the response was attenuated but the duration of the response was increased. This may suggest that this second evoked response could have some connections with the pre­ ganglionic neurons which do not require an intact descending sympathe­ tic pathway. 103 Observations from this study indicate that evoked responses

which were produced by stimulating the DLS at the and T segments c8 2 of the contralateral side of the spinal cord while recording from the

T white ramus have bilateral inputs to the preganglionic neurons. 2 Impulses from the contralateral DLS may ascend to supraspinal levels,

possibly the medulla, and make synaptic con..riections with efferent

fibers which descend to the preganglionic neurons. It is suggested

that the evoked responses ascend to the supraspinal regions, because

conduction latencies were similar to the second and late responses re­

corded fro3 the ipsilateral DLS.

Late responses of the contralateral stimulation of the DLS

were abolished by }1lacim_; lesions in the ipsilatera1 descending syr:rpa­

thetic pathway. The residual evol::ed response frorct the DLS of the

contralateral segnent again correlates with the second response. c8 A sr:m.11 response was also noted in the recordings fro.r:i the contra­

lateral DLS at the T segment. This response has a conduction latency 2 vhich cones between the first and second response. It is possible that

this response does represent an early or first response which became

unnasked after the lesion was made in the dorsolateral funiculus. An

early or first response was observed in only one of six experiments

before the lesion was made. No conclusion can be made from this study

concernirlb the small peak which was observed from the contralateral

T J)LS. 2 Blood pressure studies have also been done to determine whether stir:mlation of the DLS would activate the bilateral descending 104 pathways. After a discrete lesion was placed in the right dorsolateral

funiculus between the CS to T segment, stimulation of the DLS caudal 1 to the lesion was attenuated or abolished. However, stimulation of the

DLS at the c8 segment significantly elevated blood pressure. After these two stimulations were completed, the left dorsolateral funiculus was transected and the stimulations again repeated from the right DLS. Blood pressure was markedly reduced or abolished when the DLS was stimulated rostral to the lesion. The observation that blood pressure could be increased by stimulating the DLS rostral to a unilateral lesion and could be reduced or abolished with bilateral lesions, supports the hypothesis that afferent stimulation on one side of the spinal cord can descend bilaterally through SYJ'.:1pathetic pathways. This also correlates with the data obtained from analyzing evoked responses. Previous investigations have not emphasized the possibility of bilateral activation of the sympathetic nervous system when the somatic afferent stimulation is evoked from one side. Miyamoto and Alanis (99) did suggest the possibility that the late response could be produced by ipsi- or contralateral afferent stimulation. Sato (131, 133) reported that he stimulated the ipsilateral or contralateral radial nerves in his experiments but did not discuss the significance of these stimulations.

The figures illustrating evoked responses obtained by stimu­ lating the dorsolateral sulcus at , T and TS segments of the ipsi­ c8 2 and contralateral side of the spinal cord came from different animals to show the variation in the amplitudes and Ill3i::,~itudes of evoked 105 responses. Variation in magnitude and duration of the evoked response was even noted from stimulation to stimulation when averaging the responses. Sato and Schmidt (132) suggest that the variation of e­ voked responses is dependent on the condition of the animals. If the animals were depressed the evoked response, especially to the supra­ spinal regions, would becor.ie attenuated, but would not necessarily affect the spinal reflex. This suggests that the supraspinal regions of the central nervous system may fluctuate in exerting influences on the late responses and possibly also on the second response. The obser.'ations r:iade in this study would concur with these observations.

Several animals had to be excluded from the study because late response could not be evoked, even though the early responses were usually present but occasionally diminished in size. In the anir.ials which showed de::)ression of late evoked responses, the blood pressure was low, or there ;:ere usually problems with bleeding and surgery. These ob­ servations are another indication of the influences of the central nervous system on the sonato-syr.ipathetic reflex. CHAPTER VI

CONCLUSIONS

The following observations and conclusions can be obtained from the results concerning the descending sympathetic pathway:

1. A descending sympathetic pathway was located in the dorso­ lateral funiculus of the spinal cord by electrical stimulation. The greatest presser responses and largest evoked discharges were obtained by stimulating 1.5 to 2 rr.r:i ventrolateral to the DLS indicating that the greatest concentration of fibers was located at this level.

2. Discrete bilateral lesions placed on the surface of the dorsolateral funiculi decreased control blood pressure to levels characteristic of acute spinal animals, suggesting that tonic cardio­ vascular activity was interrupted.

3. Control heart rate decreased after a lesion was made in the right dorsolateral funiculus first, but did not decrease signifi­ cantly after a lesion was ma.de in the left dorsolateral funiculus first.

This observation suggests that control of heart rate was predominately located. in the right descending sympathetic pathway.

106 107 4. Pressor responses resulting from bilateral carotid occlusions were interrupted by placing lesions in the dorsolateral funiculi, indicating that these descending pathways transmit sympathe­ tic reflex activity.

5. Right or left unilateral carotid occlusions reflexly activate bilateral descending sympathetic pathways, because a right unilateral occlusion, after a lesion was placed in the right dorso­ lateral funiculus, increased blood pressure to levels which are similar to blood pressure during left unilateral occlusions.

6. Conduction velocity of the descending sympathetic path­ ways were approximately 6 m/sec for the fastest conducting fibers and approximately 4.5 m/sec for the greatest majority of fibers.

7. Uncrossed pathways exist between the second cervical and second thoracic segments of the spinal cord.

8. Conduction time between the descending sympathetic path­ way and the preganglionic neurons was calculated and found to be 6 to 9 msec for an intraspinal distance of 3 mm, but no conclusion can be made concerning the mechanisms of delay.

Stimulation of the DLS at the ipsilateral T and 9. 2 c8 segments activated three evoked responses. The early response could usually be produced when the T DLS was stimulated after a to T 2 c8 1 lesion of the dorsolateral funiculus. An evoked response occurred 108 between the early and late evoked response which was attenuated but not abolished by placing a lesion in the dorsolateral funiculus. No other conclusions can be made concerning this evoked response.

10. Late evoked responses are abolished by interrupting the ipsilateral descending sympathetic pathway, indicating that afferent impulses which are transmitted to the supraspinal levels of the cen­ tral nervous system, are transmitted to preganglionic neurons by the sympathetic pathway.

11. Unilateral stimulation of afferent fibers, bilaterally transmit impulses through the descending sympathetic pathways, because stimulation of DLS on the contralateral side of the spinal cord pro­ duces an evoked response in the T white ramus which has conduction 2 latencies similar to the late evoked responses. 109

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This dissertation submitted by Robert D. Foreman has been

read and approved by the following five members of the Dissertation Committee: Robert D. Wurster, Ph.D. (Chairman of the Committee) Walter c. Randall, Ph.D. Clarence N. Peiss, Ph.D. Syogoro .Nishi, M.D., Ph.D. Douglas Stuart, Ph.D.

The final copies have been examined by the director of the

dissertation and the signature which appears below verified the fact that necessary changes have been incorporated, and that the disserta-

tion is now given final approval with reference to content, form, and mechanical accuracy. This dissertation is therefore accepted in partial fulfill- ment of the requirements for the degree of Doctor of Philosophy.

)J/.,.,,v I t ,,/jJ_j I ' 1 Date Signature of Advisor