PHYSIOLOGY THE AUTO NERVOUS The nervous system is one of the most compli­ cated systems in the human body. Along with the endocrine system, it controls many bodily activ­ ities. The nervous system senses changes both in the internal and external environments, inter­ prets these changes, and then coordinates appro­ priate responses in order to maintain homeosta- sis.1 In response to changing conditions, the autonomic nervous system (ANS) shunts blood to more needy areas, speeds or slows heart and respiratory rates, adjusts blood pressure and body temperature, and increases or decreases stomach secretions.2 Most of this fine-tuning occurs without our conscious awareness or attention, implying a certain amount of func­ tional independence. Hence the term autonomic (auto=self; nom=govern).

ERIC J ZELINSKAS, CST, BA ONOMIC S SYSTEM 207 NOVEMBER 2001 CATEGORY 1

eneral organization Functionally, the PNS is subdivided into two The nervous system includes the central nervous specialized systems: the somatic nervous system system (CNS), consisting of the brain and spinal (SNS) and the autonomic nervous system cord, and the peripheral nervous system (PNS), (ANS).1 The SNS primarily innervates skeletal consisting of 12 pairs of cranial nerves (which muscle, producing consciously controlled, vol­ emerge from the base of the skull) and 31 pairs of untary movement (ie walking and talking). The spinal nerves (which emerge from the spinal ANS primarily innervates glands, smooth muscle cord). All of these nerves consist of fibers that and cardiac muscle. It’s responsible for control­ may be sensory or motor or a mixture of both.1 ling visceral functions and involuntary muscles in Nerves composed of both sensory and motor the respiratory, circulatory, digestive and urogen­ fibers are called mixed nerves. For example, the ital systems, and in the skin6 that are essential for facial nerve CN VII consists of motor fibers that the body to maintain homeostasis. The ANS oper­ control facial expressions (eg frowning and smil­ ates without conscious control. The autonomic ing) and sensory fibers, which transmit taste sen­ nervous system is activated mainly by centers sations from the tongue to the brain. located in the spinal cord, brain stem, and hypo- gReview of basic parts of the neuron

Dendrites

Cell body Nucleus

Axon Myelin sheath

Nerve impulse

Axon

Vesicle

Synaptic cleft Direction of impulse

Neurotransmitters

Receptor molecules Dendrite of receiving neuron Axon terminals thalamus.3 Often the autonomic nervous system autonomic (terminal) ganglia that lie near or FIGURE 1 operates by means of autonomic reflexes. Senso­ within the walls of the organs innervated (Figure ry signals from peripheral nerve receptors relay 1). Since the terminal ganglia are close to the The parasympa­ signals into the centers of the cord, brain stem, or innervated organs/structures, the axons of the hypothalamus, and these in turn transmit appro­ postganglionic fibers are short. PaNS pregan­ thetic nervous priate reflex responses back to the peripheral glionic neurons synapse with only a relatively few organs or tissues to control their activities.4 postganglionic neurons. For this reason they are system.3 much more precise and localized in their effects. Divisions of the autonomic nervous system Some effects of PaNS stimulation include: The autonomic nervous system (ANS) is further divided into two major subdivisions: the • constriction of pupils parasympathetic nervous system (PaNS) and • contraction of smooth muscle of alimentary sympathetic nervous system (SyNS). The two canal divisions are physiological antagonists and are in • constriction of bronchioles equilibrium with each other. Both divisions often • slowing of heart rate innervate the same organ (eg iris of the eye and the heart). Structurally, each division differs in the Ciliary ganglion location of their preganglionic neuron cell bodies Ciliary muscles of the eye within the CNS, the location of their autonomic III Pupillary sphincter ganglion, the relative lengths of their preganglion­ ic and postganglionic axons, and the ratio of pre­ VII Sphenopalatine ganglion IX ganglionic and postganglionic neurons. They V Lacrimal glands both integrate and operate continuously with the X Nasal glands rest of the nervous system by responding in vary­ ing degrees to information provided by the senso­ Submaxillary ganglion ry component of the nervous system. Submaxillary gland The SyNS dominates during stressful or physically strenuous situations. It sends impuls­ Otic ganglion es that increase blood pressure, speed up rate and Parotid gland force of the heartbeat, dilate bronchioles, increase blood sugar concentration and reroute Heart blood flow to skeletal muscle (fight or flight). Conversely, the PaNS dominates during times of Stomach emotional calm and/or physical rest. It sends Pylorus impulses that decrease blood pressure, decrease heart rate and stimulate gastrointestinal motility Colon (digestion and rest). Small intestine Parasympathetic nervous system Sacral The PaNS is the craniosacral division of the ANS. Ileocecal valve Preganglionic fibers originate from nuclei in the 1 midbrain, medulla and sacral portion of the 2 spinal cord. Neurons of the PaNS emerge from 3 4 Anal sphincter the brainstem and pass through as part of the III, Bladder Inc.© Education Pearson 2001 VII, IX, and X cranial nerves, and 2nd, 3rd, and Detrusor 4th sacral nerves from the sacral region.4 They Trigone synapse with postganglionic neurons located in ILLUSTRATION REPRINTED BY PERMISSION OF receptor sites in the effector gland, organ or muscle causing the desired effect (eg release of Eye hormones, muscular contraction, etc). (Table 1) The action of acetylcholine is relatively brief and usually lasts for only a fraction of a second. It Pilo-erector muscle B is rapidly broken down by the enzyme Heart cholinesterase, which is present both in the ter­ T-1 minal nerve ending and on the surface of the Sweat gland receptor organ. Acetylcholine (cholinergic) receptor sites are classified as either nicotinic or muscarinic. Nicotinic receptor sites for ACh occur at the junction between the preganglionic I2 Bronchi fibers and postganglionic fibers in both the SyNS Blood vessel and the PaNS divisions of the ANS. Muscarinic Celiac ganglion receptor sites for ACh occur at the junction between the postganglionic fibers and effector Pylorus sites in the PaNS division of the ANS. L-1 Adrenal Sympathetic nervous system 5 medulla The SyNS is the thoracolumbar division of the ANS (Figure 2). Preganglionic fibers originate Kidney from cell bodies in the lateral gray horn of all Ureter thoracic and the first two or three lumbar seg­ 5 ments of the spinal cord and leave the cord by way of the of the anterior (ventral) spinal nerve Intestine roots (Figure 3). They pass through the inter­ Ileocecal valve vertebral foramina, enter a white rami commu­

Pearson Education Inc. © 2001 Inc. Education Pearson nicans and connect with the ganglia of the par­ avertebral sympathetic chain, which are situated Anal sphincter Hypogastric plexus anterolaterally to the spinal cord. Each of these Detrusor Trigone ILLUSTRATION REPRINTED BY PERMISSION OF paired chains is a series of 22 ganglia spanning the length of the vertebral column.4 All pregan­ Transmission of an impulse between pregan­ glionic neurons in the SyNS are myelinated, glionic and postganglionic fibers takes place at which gives them a white appearance. Most pre­ an electrochemical junction called a synapse. ganglionic neurons end within the paraverte­ FIGURE 2 Both pre- and postganglionic neurons of the bral sympathetic ganglia and synapse with post­ PaNS are cholinergic and utilize the neurotrans- ganglionic efferent neurons. Some of the The sympathetic mitter acetylcholine (ACh).4 When a nerve postganglionic neurons re-enter the spinal impulse reaches the terminus of a preganglionic nerves via the grey rami communicans. They nervous system.3 fiber, it causes the release of ACh, which migrates appear grey because postganglionic neurons are across the synapse. The ACh combines with nonmyelinated. These neurons extend with Dashed lines represent post- receptors on the synaptic membrane of the post­ other neurons in the spinal nerves, eventually ganglionic fibers in the gray ganglionic fiber, causing depolarization and branch off and form visceral nerves that inner­ rami leading into the spinal continuing the impulse down the postganglion­ vate smooth muscle and sweat glands. Other nerves for distribution to ic fiber. Once the impulse reaches the postgan­ preganglionic neurons pass through the paraver­ blood vessels, sweat glands, glionic terminus and depolarizes it, ACh tebral sympathetic ganglia to a second set of gan­ and pilo-erector muscles. migrates across the synapse and binds to specific glia called collateral ganglia located mainly in the Table 1 ANS parasympathetic

preganglionic postganglionic effector organs, fibers fibers glands or muscles

Table 2 ANS sympathetic

sweat glands, preganglionic postganglionic blood vessels (some), fibers fibers external genitalia

heart, preganglionic postganglionic blood vessels (most), fibers fibers smooth muscle in GI tract

preganglionic adrenal blood fibers medulla stream

Table 3 Cholinergic and adrenergic receptors2

Neurotransmitter Receptor Type Major locations Effect of binding Acetylcholine Cholinergic Nicotinic All postganglionic neurons; adrenal Excitation medullary cells (also neuromuscular junc­ tions of skeletal muscle) Muscarinic All parasympathetic target organs Excitation in most cases; inhibition of car­ diac muscle Selected sympathetic targets: • Sweat glands Activation • Blood vessels in skeletal muscles Inhibition (causes vasodilation)

Norepinephrine (and Adrenergic epinephrine released β1 Heart, adipose tissue Increases heart rate and strength; stimu­ by adrenal medulla) lates lipolysis

β2 Kidneys, lungs, and most other sympathetic Stimulates secretion of renin; other effects target organs; abundant on blood vessels mostly inhibitory; dilation of blood vessels serving skeletal muscles and the heart and bronchioles; relaxes smooth muscle walls of digestive and urinary visceral organs

α1 Most important blood vessels serving the Activation: constricts blood vessels and vis­ skin, mucosae, abdominal viscera, kidneys, ceral organ sphincters and salivary glands; but virtually all sympa­ thetic target organs except heart

α2 Membrane of adrenergic axon terminals; Mediates inhibition of NE release from adren­ blood platelets ergic terminals; promotes blood clotting Paravertebral (sympathetic chain) ganglion Dorsal root and dorsal root ganglion

Lateral horn of gray matter (visceral motor zone) Sympathetic trunk Ventral ramus of spinal nerve Gray ramus communicans White ramus communicans Ventral root

Dorsal ramus of spinal nerve Blood vessels To effector To {

Skin (arrector pili mus- Splanchnic nerve cles and sweat glands)

Prevertebral (collateral) ganglion such as the celiac

Target organ (in abdomen) WB Saunders 2001 Co.© Publishing ILLUSTRATION REPRINTED BY PERMISSION OF FIGURE 3 abdomen close to the aorta and its major branch­ acetylcholine (ACh). They innervate the sweat Sympathetic es (eg celiac, superior mesenteric and inferior glands of the skin, some blood vessels within the mesenteric arteries). These bundles of collateral skeletal muscles and the external genitalia. But pathways.2 ganglia are often called plexus.4 The pregan­ by far, the majority of the sympathetic postgan­ glionic neurons synapse with nonmyelinated glionic nerves are adrenergic and utilize the neu­ synapse in a paraver­ neurons in the collateral ganglion. The postgan­ rotransmitter norepinephrine (NE).3 The affect tebral (chain) gan­ glionic neurons branch off and innervate the of NE released at the effector site produces diff- glion at the same level smooth muscles of the abdominal and pelvic vis­ erent results (excitation or inhibition) depend­ synapse in a paraver­ cera and the endocrine glands in that area. The ing on the receptor(s) to which it binds. (Table 2) tebral ganglion at a effects of the SyNS are extremely widespread There are two major classes of adrenergic different level rather than specific to one organ or muscle. (NE-binding) receptors: alpha (α) and beta (β). synapse in a preverte­ Preganglionic neurons of the SyNS are Organs that respond to NE (or epinephrine, bral (collateral) gan­ cholinergic and utilize the neurotransmitter EPI) display one or both types of receptors. In glion anterior to the acetylcholine (ACh). A few of the postganglionic general, NE or epinephrine binding to alpha vertebral column neurons of the SyNS are cholinergic and secrete receptors is stimulatory, while their binding to beta receptors is inhibitory. However, there are Surgical removal of the parasympathetic supply notable exceptions. For example, binding of NE to the gut by cutting the vagi can cause serious to the beta receptors of cardiac muscle prods the and prolonged gastric and intestinal atony, thus heart into more vigorous activity. These differ­ illustrating that in normal function the parasym­ ences reflect that both alpha and beta receptors pathetic tone to the gut is strong. This tone can be have two receptor subclasses (alpha 1 and alpha decreased by the brain, thereby inhibiting gas­ 2, beta 1 and beta 2). Each receptor type tends to trointestinal motility, or it can be increased, predominate in certain target organs (Table 3). thereby promoting increased gastrointestinal activity.5 The presence of dual innervation and Adrenal medulla the possibility of either increasing or decreasing Some preganglionic sympathetic (thoracic the tone permit a wide range of control.5 splanchnic) nerve fibers pass through the celiac ganglion without synapsing and terminate by Conclusion synapsing with hormone-producing medullary The art and science of medicine has changed very cells (chromaffin cells) of the adrenal gland. rapidly over the last 10 years. The high cost of When stimulated by the preganglionic fibers, the hospital care has created an impetus for surgical chromaffin cells release large quantities of epi­ technologists to master the knowledge and nephrine and norepinephrine directly into the advanced procedural skills necessary to meet the blood stream. These hormones are then carried growing demands of an ever more complex sur­ to tissues throughout the body where they rein­ gical environment. It is my hope this article has force the effects of the SyNS.4 provided a useful framework upon which surgi­ The epinephrine and norepinephrine released cal technologists can advance their knowledge by the combined efforts of the SyNS and the and understanding of human physiology as it adrenal glands is eventually dissipated either by relates to patient care, thus being better prepared being taken back into the synaptic nerve endings to move into the realm of advanced practice. or by action of the enzyme monoamine oxidase.4 References Sympathetic and parasympathetic tone 1. Gylys BA, Wedding, ME. Medical Terminolo­ The autonomic system generally maintains a gy, A Systems Approach. 3ed. Philadelphia: FA ‘tone,’ a basal level of activity, which then may be Davis Company; 1995: 318. either increased or decreased by central control.5 2. Marieb EN. Human Anatomy and Physiology. The sympathetic and parasympathetic systems 2ed. Redwood City, CA: The Benjamin/Cum- are continually active and the basal rates of stim­ mings Publishing Co, Inc; 1992: 457. ulation are known, respectively, as sympathetic 3. Guyton AC. Human Physiology and Mecha­ tone and parasympathetic tone.5 The value of nisms of Disease. 3ed. Philadelphia: WB Saun­ tone is that it allows a single nervous system to ders Publishing Co; 1982:439. increase or decrease the activity of an organ. For 4. The parasympathetic nervous system. example, sympathetic tone normally keeps greenfield.fortunecity.com Accessed 8/00 almost all the blood vessels of the body con­ 5. Regulation of Visceral Function. In: Selkurt EE, stricted to approximately half their maximum et al. Physiology 3ed., Selkurt EE, ed. Boston: diameter. By increasing the degree of sympathet­ Little, Brown and Company; 1971: 179-180. ic stimulation, the vessels can be constricted even 6. Autonomic Nervous System. Microsoft Encar­ more; but, on the other hand, by decreasing the ta Online Encyclopedia 2000. Microsoft Cor­ level of sympathetic stimulation, the vessels can poration. encarta.msn.com Accessed 8/24/00 be dilated.5 7. Dorland’s Pocket Medical Dictionary, 24th ed. Another example of tone is that of the Anderson DM, ed. Philadelphia: WB Saun­ parasympathetics in the gastrointestinal tract. ders; 1989. 207 NOVEMBER 2001 CATEGORY 1 1. The nervous system along with the ____ 6. Both pre- and postganglionic neurons of the system controls many bodily activities. parasympathetic nervous system (PaNS) A. cardiovascular utilize the neurotransmitter ____. B. respiratory A. epinephrine B. norepinephrine CECONTINUINGExam EDUCATION EXAMINATION CEExam C. endocrine C. cholinesterase D. acetylcholine D. urogenital 7. When stimulated by preganglionic sympa­ 2. The peripheral nervous system consists thetic (thoracic splanchnic) nerve fibers, the of ____. chromaffin cells of the adrenal glands A. 10 pairs of cranial nerves; 28 pairs of spinal nerves release large quantities of ____ directly B. 11 pairs of cranial nerves; 29 pairs of spinal nerves into the blood stream. C. 12 pairs of cranial nerves; 31 pairs of spinal nerves A. acetylcholine B. epinephrine Autonomic D. 13 pairs of cranial nerves;32 pairs of cranial nerves C. norepinephrine D. both B and C

nervous system 3. The autonomic nervous system (ANS) pri- 8. Which of the following branches of the marily innervates all of the following except aorta does not have a collateral ganglion ____. (plexus) located next to it? Earn CE credit at home A. glands A. celiac You will be awarded one continuing education (CE) credit for B. skeletal muscle B. renal recertification after reading the designated article and complet- C. smooth muscle C. superior mesenteric ing the exam with a score of 70% or better. D. cardiac muscle D. inferior mesenteric If you are a current AST member and are certified, credit earned through completion of the CE exam will automatically 4. The autonomic nervous system (ANS) is acti- 9. Preganglionic fibers originate from cell bod­ be recordedin your file—you do not have to submit a CE report- vated mainly by centers located in all of the ies in the ____ gray horn of all the thoracic ing form. A printout of all the CE credits you have earned, includ- following except ____. and first two or three lumbar segments of ing Journal CE credits, will be mailed to you in the first quarter A. Cerebellum the spinal cord. following the end of the calendar year. You may check the status B. Hypothalamus A. anterior B. lateral of your CE record with AST at any time. C. Brain stem C. medial D. posterior If you are not an AST member or not certified, you will be D. Spinal cord notified by mail when Journal credits are submitted, but your 10. ____ receptor sites for acetylcholine credits will not be recorded in AST’s files. 5. Which of the following is not a response to (cholinergic) occur at the junction between Detach or photocopy the answer block, include your check or sympathetic nervous system (SyNS) impulses? preganglionic and postganglionic fibers of money order made payable to AST and send it to the Accounting A. increase blood pressure both the sympathetic and parasympathetic Department, AST, 6 West Dry Creek Circle, Suite 200, Littleton, CO B. speed up force/rate of heart beat divisions of the ANS. 80120-8031. C. increase blood sugar concentration A. nicotinic B. muscarinic Members: $6 per CE, nonmembers: $10 per CE D. constrict bronchioles C. adrenergic D. oxidase

207 NOVEMBER 2001 CATEGORY 1

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