sign in sign up
<<
Home , Autonomic ganglion, Autonomic nerve, Autonomic nervous system, Esophageal plexus, Nerve plexus, Pelvic splanchnic nerves

OUTLINE

18.1 Comparison of the Somatic and Autonomic Nervous Systems 540 18.2 Overview of the 542 18 18.3 Parasympathetic Division 545 18.3a Cranial 545 18.3b Sacral Spinal Nerves 545 18.3c Effects and General Functions of the Parasympathetic Division 545 Autonomic 18.4 Sympathetic Division 547 18.4a Organization and of the Sympathetic Division 547 18.4b Sympathetic Pathways 550 Nervous 18.4c Effects and General Functions of the Sympathetic Division 550 18.5 Other Features of the Autonomic Nervous System 552 System 18.5a Autonomic 552 18.5b and Receptors 553 18.5c Dual Innervation 554 18.5d Autonomic 555 18.6 CNS Control of Autonomic Function 556 18.7 Development of the Autonomic Nervous System 557

MODULE 7: NERVOUS SYSTEM

mck78097_ch18_539-560.indd 539 2/14/11 3:46 PM 540 Chapter Eighteen Autonomic Nervous System

n a twisting downhill slope, an Olympic skier is concentrat- Recall from figure 14.2 (page 417) that the somatic nervous O ing on controlling his body to negotiate the course faster than system and the autonomic nervous system are part of both the anyone else in the world. Compared to the spectators in the viewing and the peripheral nervous system. The areas, his are more dilated, and his is beating faster SNS operates under our conscious control, as exemplified by vol- and pumping more blood to his skeletal muscles. At the same time, untary activities such as getting out of a chair, picking up a ball, system functions not needed in the race are practically shut walking outside, and throwing the ball for the dog to chase. (We down. , , and defecation can wait until the race have already seen that some SNS activities, such as swinging the is over. The skier exhibits a state of heightened readiness, called the arms while walking, occur at the subconscious level.) By contrast, “fight-or-flight” response, because the sympathetic division of the ANS functions are involuntary, and we are usually unaware of autonomic nervous system is dominant. them. For example, we are oblivious to the muscular actions of the The autonomic (aw-tō -nom ́ik; auto = self, nomos = law) during digestion or changes in diameter to nervous system (ANS) is a complex system of nerves that govern adjust . involuntary actions. The ANS works constantly with the somatic Both the SNS and the ANS use sensory and motor nervous system (SNS) to regulate body organs and maintain (figure 18.1). In the SNS, somatic sensory neurons conduct normal internal functions. We begin this chapter by comparing stimulus information from a sensory receptor, such as a tactile the SNS and the ANS. receptor in the , while somatic motor neurons innervate skel- etal muscle fibers. The ANS, by contrast, is activated by visceral sensory neurons. For example, some of these sensory neurons 18.1 Comparison of the Somatic detect pressure by monitoring stretch in blood vessels and organ and Autonomic Nervous Systems walls, while others measure carbon dioxide concentration in the blood. Some somatosensory receptors, such as those that detect Learning Objectives: temperature and light, also activate specific ANS responses (e.g., 1. Compare and contrast the anatomy and functions of the constriction in response to bright light). In addition, auto- SNS and the ANS. nomic motor neurons innervate cells, cardiac 2. Explain how the two- chain facilitates muscle cells, or glands. These motor neurons can either excite or communication and control in the ANS. inhibit cells in the viscera.

Somatic Nervous System Autonomic Nervous System

Autonomic Posterior root ganglion

Anterior root

Somatic Preganglionic autonomic receives sensory sends impulses to a ganglionic information from skin, motor neuron , joints, and special senses Ganglionic autonomic motor neuron (vision, hearing, etc.) sends nerve impulses to smooth muscle, , and glands Somatic motor neuron Visceral sensory neuron sends nerve impulses to receives sensory information from skeletal muscle blood vessels and smooth muscle in the viscera

Smooth muscle in trachea Sensory receptor in skin Sensory receptor in viscera

Skeletal muscle

Figure 18.1 Comparison of Somatic and Autonomic Motor Nervous Systems. The extends a single motor neuron to its effector, while the autonomic nervous system uses two motor neurons, which meet in an , to reach its effector. However, both systems use a single sensory neuron to convey impulses to the CNS.

mck78097_ch18_539-560.indd 540 2/14/11 3:46 PM Chapter Eighteen Autonomic Nervous System 541

Autonomic ganglion

Postganglionic Preganglionic axon

Preganglionic neuron cell body Ganglionic neuron cell body Effector organ Figure 18.2 Components of the Autonomic Nervous System. The autonomic nervous system employs a preganglionic neuron, which is housed in the CNS ( or spinal cord). The preganglionic axon with a ganglionic neuron in an autonomic ganglion. The postganglionic axon (from the ganglionic neuron) travels to the effector.

The motor neurons of the SNS innervate skeletal muscle within the ganglion, it is also known as a presynaptic neuron. The fibers, typically causing conscious, voluntary movements. A single axon of a ganglionic neuron, which extends to the effector, is also axon extends uninterrupted from the spinal known as the postsynaptic axon. cord to one or more muscle fibers (figure 18.1). The impulses con- ducted by these motor neurons stimulate skeletal muscle fibers, WHAT DO YOU THINK? causing them to contract. Contraction continues until neuron ●1 Why does the autonomic motor nervous system use two neurons impulses cease to stimulate the muscle fiber. By contrast, the ANS (preganglionic and ganglionic) in a chain to an effector? (For the uses a pathway that includes a two-neuron chain to innervate answer, read the next section.) muscles and glands (figure 18.2). The first of the two ANS motor neurons is the preganglionic (prē gang-l ́ ē -on ́ik) neuron. Its cell The two-neuron chain vastly increases communication and body lies within the or the spinal cord. A preganglionic control in the ANS. Neuronal convergence (kon-ver ́jens; con = axon extends from this cell body and exits the CNS in either a with, vergere = to incline) occurs when from numerous cranial nerve or a . This axon projects to the cell body preganglionic cells (converge) on a single ganglionic cell. of the second neuron, which is housed within an autonomic gan- In contrast, neuronal divergence (di-ver ́jens; di = apart) occurs glion in the peripheral nervous system. The second neuron in this when axons from one preganglionic cell synapse on numerous pathway is called a ganglionic neuron, and a postganglionic axon ganglionic cells. extends from its cell body to effector cells or an effector organ. Table 18.1 summarizes the characteristics of the somatic Since a preganglionic neuron synapses with a ganglionic neuron and autonomic nervous systems.

Table 18.1 Comparison of Somatic and Autonomic Motor Nervous Systems Feature Somatic Nervous System Autonomic Nervous System Type of Control Voluntary control (from cerebral cortex; input from basal Involuntary control (from brainstem, , nuclei, brainstem, , and spinal cord) , and spinal cord) Number of Neurons in Pathway One neuron in pathway; somatic motor neuron axon Two neurons in pathway; preganglionic neuron in extends from CNS to effector CNS projects preganglionic axon to ganglionic neuron; ganglionic neuron projects postganglionic axon to effector Ganglia Associated with Motor None Autonomic ganglia: ganglia; Neurons prevertebral ganglia; terminal or intramural ganglia Sensory Input General somatic senses, proprioceptors; special senses Some somatic and visceral senses Ganglia Associated with Sensory Posterior root ganglia; sensory ganglia of Posterior root ganglia; sensory ganglia of cranial nerves Input Effector Organs Skeletal muscle fi bers Cardiac muscle fi bers, smooth muscle fi bers, glands Response of Effector Excitation only Either excitation or inhibition of effectors Released (ACh) ACh from all preganglionic axons and parasympathetic postganglionic axons, and a few sympathetic postganglionic axons; (NE) from most sympathetic postganglionic axons Axon Properties Myelinated, thick; fast conduction Preganglionic axons are thin, myelinated; postganglionic axons are thinner, unmyelinated, have slow conduction

mck78097_ch18_539-560.indd 541 2/14/11 3:46 PM 542 Chapter Eighteen Autonomic Nervous System

Study Tip! A good way to understand the two-neuron ANS chain is to compare it to the U.S. airline system, which uses connecting flights and “airport hubs” to transport the maximum number of people in the most cost-effective way. 1 “Preganglionic” flight from Indianapolis Imagine that you are flying from Indianapolis to Miami for spring break: to Chicago (autonomic ganglion). Your first flight from Indianapolis to Chicago is the preganglionic neuron. Although flying north to Chicago is out of your way, the airline wants you to go to an airport hub because it is more efficient to send all

Indianapolis passengers to this main location before they take different Chicago flights throughout the United States. The airport hub in Chicago is the autonomic ganglion, the point where Indianapolis preganglionic and postganglionic flights meet up. Other preganglionic flights are meeting up at the airport hub, and here all these passengers will connect with other flights. Your connecting flight from Chicago to Miami is your postgangli- onic neuron. This flight will take you to your final destination, just as a postganglionic neuron sends a nerve impulse to an effector organ. On the Miami plane with you are people from other preganglionic flights who all want 2 “Postganglionic” flight to go to Miami as well. from Chicago to Miami Is using two different flights the most direct way for you to get from (effector organ). Indianapolis to Florida? Of course not. But it is the most cost-efficient way The autonomic nervous system is similar to connecting airline flights and for the airlines to transport many passengers with a limited number of “airport hubs” in that both try to group and disperse as many different planes. Keep this analogy in mind as you learn about the workings of the structures (neuronal impulses and passengers) with a limited number of autonomic nervous system. neurons or flights.

WHATW DID YOU LEARN? conserving energy and replenishing nutrient stores. Thus, it is most active when the body is at rest or digesting a meal, and has ●1 How are motor neurons organizationally different in the ANS versus the SNS? been nicknamed the “rest-and-digest” division. The parasympa- thetic division also helps maintain , a constant internal ●2 What organs are innervated by the ANS? environment. ●3 Where is a ganglionic neuron cell body located? The sympathetic (sim-pă-thet ́ik) division is primarily con- cerned with preparing the body for emergencies. It is often referred to as the “fight-or-flight” division because increased sympathetic 18.2 Overview of the Autonomic activity results in the increased alertness and metabolic activity needed in stressful or frightening situations. During these fight-or- Nervous System flight events, the sympathetic division exhibits a mass activation Learning Objective: response, whereby all components receiving sympathetic innerva- tion get stimulated. (In contrast, the parasympathetic division is 1. Compare and contrast the functions and components of the discrete and localized, meaning only one or a few structures are parasympathetic and sympathetic divisions. innervated at the same time.) The ANS is subdivided into the parasympathetic division and The parasympathetic and sympathetic divisions are similar the sympathetic division. These two divisions are similar in that in that their preganglionic axons are myelinated, while the post- they both use a preganglionic neuron and a ganglionic neuron to ganglionic axons are unmyelinated. These two divisions are also innervate muscles or glands. Both divisions contain the autonomic distinguished by several anatomic differences. The major differ- ganglia that house the ganglionic neurons. Both divisions are ence is that their preganglionic neuron cell bodies are housed in involuntary and are concerned with the body’s internal environ- different regions of the CNS. Parasympathetic preganglionic neu- ment in general. However, these two divisions perform dramati- rons originate in either the brainstem or the lateral gray matter of cally different functions (figure 18.3). the S2–S4 spinal cord segments, while sympathetic preganglionic The parasympathetic (par-ă-sim-pa-thet ́ik; para = alongside, neurons originate in the lateral horns of the T1–L2 spinal cord sympatheo = to feel with) division is primarily concerned with segments (figure 18.3).

mck78097_ch18_539-560.indd 542 2/14/11 3:46 PM Chapter Eighteen Autonomic Nervous System 543

Components of Autonomic Nervous System

Parasympathetic Division Sympathetic Division

Origin: Origin: Preganglionic neurons located CN III (oculomotor) Preganglionic neurons in brainstem nuclei and S2–S4 located in lateral horns of segments of spinal cord CN VII (facial) T1–L2 segments of spinal cord Functions: Functions: CN IX (glossopharyngeal) • “Rest and digest” response Sympathetic trunk • Activated in emergency • Brings body to homeostasis CN X (vagus) situations • “Fight-or-flight” response

• Also involved with homeostasis

T1–L2 segments of spinal cord

S2–S4 segments of spinal cord

Pelvic

Figure 18.3 Comparison of Parasympathetic and Sympathetic Divisions. The parasympathetic and sympathetic divisions of the ANS have the same basic components, but they differ in their origins, locations of the preganglionic cell bodies, axon lengths, and amount of branching.

Figure 18.4 depicts additional anatomic differences: between the divisions. Parasympathetic preganglionic axons tend (1) Parasympathetic preganglionic axons are longer, and postgan- to have few (less than 4) branches, while sympathetic pregangli- glionic axons are shorter when compared to their counterparts onic axons tend to have many branches (more than 20). in the sympathetic division. In the sympathetic division, pregan- Table 18.2 summarizes the comparison of the parasympa- glionic axons are shorter and postganglionic axons are longer. thetic and sympathetic divisions of the autonomic nervous system. (2) Parasympathetic autonomic ganglia are close to or within the WHATW DID YOU LEARN? wall of the effector organ, while sympathetic autonomic ganglia are relatively close to the . (3) The amount ●4 Describe the anatomic differences between the postganglionic of preganglionic axon branching to ganglionic neurons differs axons in the parasympathetic and sympathetic divisions.

mck78097_ch18_539-560.indd 543 2/14/11 3:46 PM 544 Chapter Eighteen Autonomic Nervous System

Parasympathetic Division

Short Ganglionic postganglionic neuron axon Preganglionic Long preganglionic axon neuron

Figure 18.4 Anatomic Differences Between Autonomic ganglion Parasympathetic and Sympathetic (close to or within effector organ wall) Neurons. In both the parasympathetic and sympathetic divisions, preganglionic axons are myelinated and relatively larger in diameter, and postganglionic axons Sympathetic Division are unmyelinated and relatively smaller in diameter. (Top) The parasympathetic Short, branching division has longer preganglionic axons preganglionic axon and shorter postganglionic axons; its preganglionic axons exhibit very little branching. (Bottom) The sympathetic Long postganglionic axon division has shorter preganglionic axons and longer postganglionic axons; the Preganglionic preganglionic axons show much branching. neuron

Ganglionic neuron Autonomic ganglion (close to the vertebral column)

Table 18.2 Comparison of Parasympathetic and Sympathetic Divisions Feature Parasympathetic Division Sympathetic Division Function Conserves energy and replenishes energy stores; Prepares body to cope with emergencies and intensive maintains homeostasis; “rest-and-digest” division muscle activity; “fi ght-or-fl ight” division Location of Preganglionic Neuron Brainstem and lateral gray matter in S2–S4 segments of Lateral horns in T1–L2 segments of spinal cord Cell Bodies spinal cord Location of Ganglionic Neuron Terminal or intramural ganglion Sympathetic trunk ganglion or prevertebral ganglion Cell Bodies Divergence of Axons Few (1 axon innervates < 4 ganglionic cell bodies) Extensive (1 axon innervates > 20 ganglionic cell bodies) Length of Preganglionic Axon Long Short Length of Postganglionic Axon Short Long Location of Ganglia Terminal ganglia located close to the target organ; Sympathetic trunk (paravertebral) ganglia located on intramural ganglia located within wall of the target organ either side of vertebral column; prevertebral (collateral) ganglia located anterior to vertebral column and descending Rami Communicantes None White rami attach to T1–L2 spinal nerves; gray rami attach to all spinal nerves

mck78097_ch18_539-560.indd 544 2/14/11 3:46 PM Chapter Eighteen Autonomic Nervous System 545

The glossopharyngeal nerve (CN IX) innervates the parotid 18.3 Parasympathetic Division . Parasympathetic stimulation exits the brainstem in the glossopharyngeal nerve. From this nerve, the preganglionic para- Learning Objectives: sympathetic axons branch and synapse on ganglionic neurons in the 1. Describe the anatomy of the parasympathetic division. otic (ō tik; ́ ous = ear) ganglion, which is positioned anterior to the ear 2. Explain the relationship of the parasympathetic division to near the foramen ovale. Postganglionic axons from the the brain, the cranial nerves, and the sacral spinal cord. cause an increase in secretion from the parotid salivary glands. 3. Identify the effects of parasympathetic innervation on Each (CN X) is responsible for supplying para- effectors. sympathetic innervation to the thoracic organs and most of the abdominal organs, as well as the gonads (ovaries and testes).1 The parasympathetic division of the ANS is structurally sim- Almost 80% of all parasympathetic preganglionic axons are trans- pler than the sympathetic division. The parasympathetic division mitted through the vagus nerve. The term vagus means “wan- is also termed the craniosacral (krā n ́ ē -ō -sā ́kr ăl) division because derer,” which describes the wandering pathway of the vagus nerve its preganglionic neurons are housed within nuclei in the brain- as it projects inferiorly through the and travels throughout stem and within the lateral gray matter of the S2–S4 spinal cord the trunk. Left and right vagus nerves extend multiple branches segments. The ganglionic neurons in the parasympathetic division to the thoracic organs. As these nerves travel inferiorly, their position are found in either terminal (ter mi-n ́ ăl; terminus = a boundary) changes slightly, and they are referred to as the anterior and poste- ganglia, which are located close to the target organ, or intramural rior vagal trunks. In the , parasympathetic innerva- (in tr ́ ă-mū r ́ ăl; intra = within, murus = wall) ganglia, which are tion causes increased mucous production and decreased diameter located within the wall of the target organ. in the airways, as well as decreases in the and the force 18.3a Cranial Nerves of heart contractions. These trunks pass through the diaphragm and associate with the descending abdominal aorta within the abdomi- The cranial nerves associated with the parasympathetic division nal cavity, where they project to their ganglia located immediately are the oculomotor (CN III), facial (CN VII), glossopharyngeal adjacent to or within the wall of their target organs. This parasym- (CN IX), and vagus (CN X) (see figure 18.3). The first three of these pathetic innervation also causes increased smooth muscle motility nerves convey parasympathetic innervation to the head, while and secretory activity in digestive tract organs. the vagus nerve is the source of parasympathetic stimulation for the thoracic and most abdominal organs (figure 18.5). 18.3b Sacral Spinal Nerves Review table 15.8 for illustrations of the cranial nerve pathways The remaining preganglionic parasympathetic axons originate and the locations of their associated . from preganglionic neuron cell bodies housed within the lateral The (CN III) is formed by axons extending gray matter of the S2–S4 spinal cord segments (figure 18.5). These from some cell bodies housed in nuclei in the mesencephalon. The preganglionic parasympathetic axons branch to form the pelvic preganglionic axons extend from CN III to the ciliary (sil ē ́ -ar-ē ; splanchnic (splangk nik; ́ visceral) nerves, which contribute to ciliaris = eyelash) ganglion within the orbit. Postganglionic axons the superior and inferior hypogastric . The preganglionic project from this ganglion to the and parasympathetic axons that emanate from each plexus project to pupillae muscle of the iris of the eye. Parasympathetic innervation the ganglionic neurons within either the terminal or intramural to the ciliary muscle results in lens accommodation, which makes ganglia. The target organs innervated include the distal portion of the lens more rounded so that we can see close-up objects. The the , the , most of the reproductive organs, postganglionic axons that travel to the pupillary constrictor muscle the , and the distal part of the . This para- result in pupil constriction when the eye is exposed to bright light. sympathetic innervation causes increased smooth muscle motility The (CN VII) contains parasympathetic pre- () and secretory activity in the digestive organs, ganglionic axons that exit the and control the production mentioned above, contraction of smooth muscle in the bladder and secretion of , nasal secretions, and saliva. Two branches wall, and of the female clitoris and the male penis. of parasympathetic preganglionic axons exit the facial nerve and terminate at one of two ganglia. The greater petrosal nerve termi- 18.3c Effects and General Functions nates at the pterygopalatine (ter ́i-gō -pal a-t ́ ı̄n) ganglion near the of the Parasympathetic Division junction of the maxilla and palatine bones. Postganglionic axons project to the lacrimal glands and small glands of the nasal cavity, The parasympathetic division is most active during times when the oral cavity, and palate to increase secretion by these glands. The body must process nutrients, conserve energy, and attempt to return chorda tympani terminates on ganglionic neurons in the sub- to homeostasis. The lack of extensive divergence in preganglionic mandibular (sŭb-man-dib ́ū -lăr; sub = under) ganglion near the axons prevents the mass activation seen in the sympathetic divi- angle of the mandible. Postganglionic axons projecting from this sion. Thus, the effects of the parasympathetic nervous system tend ganglion supply the submandibular and sublingual salivary glands to be discrete and localized. In other words, parasympathetic activity in the floor of the mouth, causing an increase in salivary gland can affect one group of organs without necessarily having to “turn secretions. Thus, your mouth waters when you smell an aromatic on” all other organs. Table 18.3 summarizes the effects of para- meal due in part to these parasympathetic axons. sympathetic innervation.

WHAT DO YOU THINK?

●2 The is sometimes nicknamed the “hay 1It is unclear what function, if any, these parasympathetic fibers have on ganglion.” Why is this nickname appropriate? the gonads.

mck78097_ch18_539-560.indd 545 2/14/11 3:46 PM 546 Chapter Eighteen Autonomic Nervous System

Preganglionic Postganglionic

Pterygopalatine Parotid salivary gland CN III ganglion Submandibular salivary gland Sublingual salivary gland

CN VII

CN IX Otic ganglion Pons

Heart CN X Cardiac plexus

Trachea

Esophageal plexus

Lung Abdominal aortic plexus

Spleen Spinal cord Ureter

Pancreas

Small intestine Hypogastric plexus Testis Ovary Descending colon

Rectum S2 S3 S4

Pelvic splanchnic nerves Bladder

Penis

Uterus Vagina

Figure 18.5 Overview of Parasympathetic Pathways. Preganglionic axons from the brain and spinal cord innervate the viscera in the head, neck, and trunk.

mck78097_ch18_539-560.indd 546 2/14/11 3:46 PM Chapter Eighteen Autonomic Nervous System 547

Table 18.3 Parasympathetic Division Outfl ow Nerve(s) CNS Origin of Preganglionic Autonomic Ganglion Effector Organs Innervated Neuron CN III (Oculomotor) Mesencephalon Ciliary ganglion Ciliary muscles to control lens for accommodation; sphincter pupillae muscle of eye to constrict pupil

CN VII (Facial) Pons Pterygopalatine ganglion Lacrimal glands; glands of nasal cavity, palate, oral cavity Submandibular ganglion Submandibular and sublingual salivary glands

CN IX (Glossopharyngeal) Otic ganglion Parotid salivary glands CN X (Vagus) Medulla oblongata Multiple terminal and intramural Thoracic viscera and most abdominal ganglia viscera

Pelvic Splanchnic Nerves S2–S4 segments of spinal cord Terminal and intramural ganglia Some abdominal viscera and most pelvic viscera

WHATW DID YOU LEARN? house sympathetic ganglionic neuron cell bodies. One sympathetic trunk ganglion is approximately associated with each spinal nerve. ●5 What are the differences between the terminal and intramural ganglia? However, the cervical portion of each sympathetic trunk is par- titioned into only three sympathetic trunk ganglia—the superior, ●6 Identify the cranial nerves involved in the parasympathetic division middle, and inferior —as opposed to the eight of the ANS. cervical spinal nerves. The superior cervical ganglion contains postganglionic sympathetic neuron cell bodies whose axons are distributed to structures within the head and neck. These sym- 18.4 Sympathetic Division pathetic postganglionic axons innervate the sweat glands in the Learning Objectives: head and neck, the smooth muscle in blood vessels of the head and neck, the dilator pupillae muscle of the eye, and the superior 1. Describe the anatomy of the sympathetic division. tarsal muscle of the eye (which elevates the eyelid). The middle 2. Explain the relationship of the sympathetic division to the and inferior cervical ganglia house neuron cell bodies that extend spinal cord and the spinal nerves. postganglionic axons to the thoracic viscera. 3. Describe the sympathetic function of the . Connecting the spinal nerves to each sympathetic trunk are 4. Identify the effects of sympathetic innervation on effectors. rami communicantes (rā m ́ ı̄ kŏ-mū -ni-kan t ́ ēz; communico = to The sympathetic division is also called the thoracolumbar share with someone). White rami communicantes (or white rami) (thō r ́ă-kō -lŭm ́bar) division because the preganglionic neuron cell carry preganglionic sympathetic axons from the T1–L2 spinal bodies originate and are housed between the first thoracic (T1) and nerves to the sympathetic trunk. Thus, white rami are associated the second lumbar (L2) spinal segments. only with the T1–L2 spinal nerves. Since preganglionic axons are myelinated, the white ramus has a whitish appearance (hence, its 18.4a Organization and Anatomy of the name). White rami are similar to “entrance ramps” onto a highway. Sympathetic Division Gray rami communicantes (or gray rami) carry postganglionic sympathetic axons from the sympathetic trunk to the spinal nerve. The sympathetic division is much more complex than the Since the postganglionic axons are unmyelinated, the gray rami parasympathetic division, both anatomically and functionally have a grayish appearance. Gray rami are similar to “exit ramps” (figure 18.6; see figure 18.4). The sympathetic preganglionic off a highway. Gray rami connect to all spinal nerves: the cervical, neuron cell bodies are housed in the lateral horn of the T1–L2 thoracic, lumbar, sacral, and coccygeal spinal nerves. By these segments of the spinal cord. From there, the preganglionic sym- routes, the sympathetic information that started out in the thora- pathetic axons travel with somatic motor neuron axons to exit columbar region can be dispersed to all parts of the body. the spinal cord and enter first the anterior roots and then the Splanchnic nerves are composed of preganglionic sympathetic T1–L2 spinal nerves. However, these preganglionic sympathetic axons that did not synapse in a sympathetic trunk ganglion. They axons remain with the spinal nerve for only a short distance extend anteriorly from each sympathetic trunk to most of the viscera. before they leave the spinal nerve. (These splanchnic nerves should not be confused with the pelvic Immediately anterior to the paired spinal nerves are the left splanchnic nerves associated with the parasympathetic division.) and right sympathetic trunks, each of which is located immedi- Some of the larger splanchnic nerves have specific names: ately lateral to the vertebral column (figure 18.7). A sympathetic trunk looks much like a pearl necklace. The “string” of the “neck- ■ The greater thoracic splanchnic nerve forms from lace” is composed of bundles of axons, while the “pearls” are the preganglionic axons extending from the T5–T9 sympathetic sympathetic trunk ganglia (paravertebral or chain ganglia), which trunk ganglia.

mck78097_ch18_539-560.indd 547 2/14/11 3:46 PM 548 Chapter Eighteen Autonomic Nervous System

Preganglionic Postganglionic Eye

Blood vessels and sweat glands of head

Salivary glands Blood vessels

Heart

Cardiac and Right Left pulmonary plexuses Superior cervical ganglion Inferior cervical ganglion

T1 T1T1 Celiac ganglion Greater thoracic splanchnic nerve T2 T2 Liver and Lesser thoracic splanchnic nerve gallbladder T3 T3 Stomach T4 T4 Adrenal medulla T5 T5 Superior Kidney mesenteric T6 T6 ganglion Ureter (proximal) T7 T7 Large intestine T8 T8 Small intestine T9 T9

T10 T10

T11 T11 Inferior Postganglionic fibers mesenteric to skin, blood vessels T12 T12 ganglion Rectum L1 L1 Ureter (distal) L2 L2L2 Least thoracic splanchnic nerve L3 Hypogastric plexus Bladder Lumbar splanchnic nerves Spinal cord L4 L5 Sacral splanchnic nerves Vas deferens S1 Seminal vesicle Sympathetic chain ganglia S2 Prostate

Ovary Uterus Testis

Figure 18.6 Overview of Sympathetic Pathways. The right sympathetic trunk shows the outflow of preganglionic axons and the distribution of postganglionic axons innervating the skin. The left sympathetic trunk illustrates sympathetic postganglionic axon pathways through the gray rami, spinal nerves, and splanchnic nerves. (Note, however, that in reality each sympathetic trunk innervates both the skin and the viscera.)

mck78097_ch18_539-560.indd 548 2/14/11 3:46 PM Chapter Eighteen Autonomic Nervous System 549

Figure 18.7 Sympathetic Trunk. An anterolateral cadaver photo of the right side of the thoracic cavity shows the sympathetic trunk, the gray and white rami communicantes, their attachment to Intercostal nerve the , and the greater thoracic Sympathetic trunk splanchnic nerve. Gray ramus White ramus

Descending Sympathetic trunk thoracic aorta ganglia

Azygos vein

Greater thoracic splanchnic nerve

Diaphragm

■ The lesser thoracic splanchnic nerve forms from name prevertebral) on the anterolateral surface of the aorta; and preganglionic axons extending from the T10–T11 (3) they are located only in the abdominopelvic cavity. Prevertebral sympathetic trunk ganglia. ganglia include the celiac, superior mesenteric, and inferior mes- ■ The least thoracic splanchnic nerve forms from enteric ganglia. preganglionic axons extending from the T12 sympathetic The celiac ganglion is adjacent to the origin of the celiac trunk ganglia. artery. Its appearance often varies in individuals; thus, it is usually ■ The lumbar splanchnic nerves originate from the L1 and composed of two connected masses, but may also form a single L2 sympathetic trunk ganglia. mass. The left and right greater thoracic splanchnic nerves (com- posed of axons from the T5–T9 segments of the spinal cord) synapse In addition to these, there also are small sacral splanch- on ganglionic neurons within the celiac ganglion. Postganglionic nic nerves that originate from the sacral . axons from the celiac ganglion innervate the stomach, spleen, liver, Splanchnic nerves typically terminate in prevertebral (or col- gallbladder, and proximal part of the and part of the lateral) ganglia. These ganglia are called “prevertebral” because pancreas. they are immediately anterior to the vertebral column on the The superior mesenteric (mez-en-ter ́ik; mesos = middle, anterolateral wall of the abdominal aorta. Prevertebral gang- enteron = intestine) ganglion is adjacent to the origin of the superior lia typically cluster around the origins of the major abdominal mesenteric artery. The lesser and least thoracic splanchnic nerves arteries and are named for these arteries. For example, the celiac project to and terminate in the superior mesenteric ganglion. Thus, ganglion is located around the origin of the celiac trunk (an artery). this ganglion receives preganglionic sympathetic neurons from the Sympathetic postganglionic axons extend away from the ganglionic T10–T12 segments of the spinal cord. Postganglionic axons extend- neuron cell bodies in these ganglia and innervate many of the ing from the superior mesenteric ganglion innervate the distal half abdominal organs. of the duodenum, part of the pancreas, the remainder of the small intestine, the proximal part of the large intestine, the kidneys, and Types of Prevertebral Ganglia the proximal parts of the . The prevertebral ganglia differ from the sympathetic trunk The inferior mesenteric ganglion is adjacent to the origin ganglia segments in that (1) they are single structures, rather than of the inferior mesenteric artery. It receives sympathetic pregang - paired; (2) they are anterior to the vertebral column (hence, the lionic axons via the lumbar splanchnic nerves, which originate

mck78097_ch18_539-560.indd 549 2/14/11 3:46 PM 550 Chapter Eighteen Autonomic Nervous System

in the L1–L2 segments of the spinal cord. Its postganglionic axons flight response. These hormones are epinephrine (ep ́i-nef rin; ́ project to and innervate the distal colon, rectum, urinary bladder, epi = upon, nephros = kidney) and, to a lesser degree, norepi- distal parts of the ureters, and most of the reproductive organs. nephrine (nō r-ep-i-nef rin) ́ (discussed in chapter 20). Both of these hormones potentiate (prolong) the effects of the sympathetic WHATW DID YOU LEARN? stimulation. For example, if you narrowly miss getting into a car accident, your heart continues to beat quickly, you breathe rapidly, ●7 The sympathetic division originates in what area and segments of the spinal cord? and you feel tense and alert well after the event. In this case, the epinephrine and norepinephrine circulating in your bloodstream ●8 Distinguish between the sympathetic trunk ganglia and the are prolonging the effects of the sympathetic stimulation. prevertebral ganglia. ●9 Describe the structural and functional differences between the 18.4c Effects and General Functions white and gray rami communicantes. Do these structures contain of the Sympathetic Division myelinated or unmyelinated axons? Which carry preganglionic axons, and which carry postganglionic axons? The sympathetic division may innervate a single effector or many effectors. For example, a single effector is involved when smooth 18.4b Sympathetic Pathways muscle controls the diameter of the pupil of the eye, while many All sympathetic preganglionic neurons originate in the lateral gray effectors respond together, a phenomenon termed mass activation, horns of the T1–L2 segments of the spinal cord. However, the sym- during an emergency or crisis situation. In mass activation, numer- pathetic pathways of the axons of these neurons vary, depending ous collateral branches of preganglionic sympathetic axons synapse upon the location and the type of effector organ being innervated. with a large number of ganglionic neurons to stimulate many gangli- Recall that preganglionic axons extend from the preganglionic onic sympathetic neurons and simultaneously activate many effector neuron cell bodies via the anterior roots and travel with the T1–L2 organs. Mass activation of the sympathetic division causes a height- spinal nerves. The preganglionic axons immediately leave the spi- ened sense of alertness due to stimulation of the reticular activation nal nerve and travel through white rami to enter the sympathetic system. Table 18.4 shows how specific structures are affected by the trunk. Once inside the sympathetic trunk, the preganglionic axons sympathetic division. may remain at the level of entry, or travel superiorly or inferiorly Mass activation often occurs simultaneously with an increase within the sympathetic trunk. in tonus in skeletal muscle. However, this increased skeletal muscle Axons exit the sympathetic trunk ganglia by one of four tension is not due to activation of the ANS, but merely to changes pathways (figure 18.8). An axon takes the spinal nerve path- in muscle tone. In addition, the affected individual experiences a way if a preganglionic neuron synapses with a ganglionic neuron feeling of excess energy, which is usually caused by mobilization in a sympathetic trunk ganglion. In this case, the postganglionic of energy reserves in the liver. Some obvious systemic changes axon travels through a gray ramus that is at the same “level” as accompany sympathetic stimulation, including increases in heart the ganglionic neuron. For example, if the preganglionic and gang- rate and blood pressure and parallel increases in depth of respira- lionic neurons synapse in the L4 sympathetic trunk ganglion, tion and breathing rate. Finally, the pupils dilate due to innervation the postganglionic axon travels through the gray ramus at the of the dilator pupillae muscle in the iris of the eye. level of the L4 spinal nerve. After the postganglionic axon travels through the gray ramus, it may enter the spinal nerve and extend WHAT DO YOU THINK? to its target organ. The structures in the skin (such as arrector pili ●3 When a person is very stressed and tense, his or her blood pressure muscles and blood vessels) receive their sympathetic innervation typically rises. What aspect of the sympathetic nervous system via this pathway. causes this rise in blood pressure? In the postganglionic sympathetic nerve pathway, the preganglionic neuron synapses with a ganglionic neuron in a sympathetic trunk ganglion, but the postganglionic axon does not CLINICAL VIEW leave the trunk via a gray ramus. Instead, the postganglionic axon extends away from the sympathetic trunk ganglion (in the form of a postganglionic sympathetic axon) and goes directly to the effec- Raynaud Syndrome tor organ. The esophagus, heart, , and thoracic blood vessels Raynaud syndrome, or Raynaud phenomenon, is a sudden spasm typically receive their sympathetic innervation from this pathway. or constriction of the small arteries of the digits. The immedi- The splanchnic nerve pathway uses splanchnic nerves, ate decrease in blood flow results in blanching (loss of the red which are preganglionic axons that pass through the sympathetic hue) of the skin distal to the area of vascular constriction. The trunk ganglia without synapsing. These splanchnic nerves extend vascular constriction is accompanied by , which may even from the anterior side of the sympathetic trunk ganglia to the continue for a while after the vessels have dilated and restored prevertebral ganglia. There, the preganglionic axon synapses with the local blood flow. Episodes are typically triggered by exposure a ganglionic neuron. The postganglionic axon then travels to the to cold, although emotional has been known to precipitate effector organs. The abdominal and pelvic organs receive their a Raynaud attack. Only a few people experience this condition, sympathetic innervation via this pathway. which is believed to result from an exaggerated local sympathetic The final pathway is the adrenal medulla pathway. In response. The severity of this medical condition depends on the this pathway, the internal region of the adrenal gland, called frequency and the length of time of each occurrence. Most people the adrenal (ă-drē n ́ ăl) medulla, receives preganglionic sympa- affected with Raynaud syndrome must avoid the cold and other thetic axons. When these preganglionic axons synapse on cells triggering circumstances. within the adrenal medulla, those cells release hormones that are circulated within the bloodstream and help prolong the fight-or-

mck78097_ch18_539-560.indd 550 2/14/11 3:46 PM Chapter Eighteen Autonomic Nervous System 551

Preganglionic axon Postganglionic axon

Posterior root ganglion Blood Posterior root vessel

Hair

Gray ramus Anterior root Lateral horn Posterior White ramus ramus

Anterior Cardiac plexus ramus Arrector pili and (parasympathetic sweat glands axons of plexus Spinal nerve not shown)

Gray ramus White ramus

Heart

Sympathetic trunk ganglion Sympathetic trunk

(a) Spinal nerve pathway (b) Postganglionic sympathetic nerve pathway

Sympathetic trunk ganglion

Gray ramus Gray ramus White ramus White ramus

Splanchnic nerve

Adrenal medulla

Preganglionic axon Splanchnic nerves Prevertebral ganglion Prevertebral ganglion (no synapse occurs)

Intestine

(c) Splanchnic nerve pathway (d) Adrenal medulla pathway

Figure 18.8 Types of Sympathetic Pathways. Pathways of ( a) a spinal nerve, (b) a postganglionic sympathetic nerve, (c) a splanchnic nerve, and (d) the adrenal medulla.

mck78097_ch18_539-560.indd 551 2/14/11 3:46 PM 552 Chapter Eighteen Autonomic Nervous System

Table 18.4 Sympathetic Division Outfl ow Destination Spinal Cord Segment Origin Postganglionic Axon Pathway Organs Innervated1 from Sympathetic Trunk Head and neck T1–T2 (almost all sympathetic Via superior cervical ganglion and Blood vessels, sweat glands, and innervation to the head comes travel with blood vessels to the head arrector pili muscles of head and from T1) neck; dilator pupillae muscle of eye, tarsal glands of eye, superior tarsal muscle of eye Integumentary structures T1–L2 Via gray rami to all spinal nerves Sweat glands and arrector pili muscles, blood vessels in skin Thoracic organs T1–T5 Via cervical and Esophagus, heart, lungs, blood to plexuses near vessels within thoracic cavity organs Most abdominal organs T5–T12 Via thoracic splanchnic nerves to Abdominal portion of esophagus; prevertebral ganglia (e.g., celiac, stomach, liver, gallbladder, spleen, superior mesenteric, and inferior pancreas, small intestine, most of mesenteric ganglia) large intestine, kidneys, ureters, adrenal glands, blood vessels within abdominopelvic cavity Pelvic organs T10–L2 Via lumbar and sacral splanchnic Distal part of large intestine, anal nerves to autonomic nerve plexuses canal, and rectum; distal part of that travel to target organ ureters; urinary bladder, reproductive organs

1 Sympathetic axons innervate the smooth muscle, cardiac muscle, and glands associated with the organs listed.

WHATW DID YOU LEARN? 18.5 Other Features of the ●10 How can the sympathetic axons stimulate so many effector organs Autonomic Nervous System simultaneously? Learning Objectives: ●11 What is the function of splanchnic nerves in the sympathetic division? 1. Identify the structure and location of autonomic plexuses. 2. Compare and contrast the types of neurotransmitters. ●12 From what structure are epinephrine and norepinephrine released following sympathetic stimulation? 3. Explain dual innervation by the parasympathetic and sympathetic divisions of the ANS. 4. Describe how autonomic reflexes help maintain homeostasis. Both divisions of the autonomic nervous system innervate organs through specific axon bundles called autonomic plexuses. CLINICAL VIEW Communication between neurons and effectors in the autonomic nervous system is by chemical messengers, called neurotrans- Horner Syndrome mitters. These chemical messengers and the receptors on body organs to which they bind are specific in each division of the Horner syndrome is a condition caused by damage to the sympa- autonomic nervous system. Most organs are innervated by both thetic innervation to the head. This damage results from impinge- divisions of the autonomic nervous system in what is called dual ment, injury, or severing of the cervical sympathetic trunk or the innervation. Autonomic reflexes help us maintain homeostasis. T1 sympathetic trunk ganglion, where postganglionic sympathetic We discuss autonomic plexuses first. axons traveling to the head originate. The absence of sympathetic innervation on one side of the head leads to certain clinical signs 18.5a Autonomic Plexuses on that side. The patient presents with ptosis (to¯ sis; ́ a falling), Autonomic plexuses are collections of sympathetic postganglionic in which the superior eyelid droops because the superior tarsal axons and parasympathetic preganglionic axons, as well as some muscle is paralyzed. Paralysis of the dilator pupillae muscle of visceral sensory axons. These sympathetic and parasympathetic the eye results in miosis (mı¯-o¯ sis; ́ meiosis = lessening), which axons are close to one another, but they do not interact or synapse is a constricted pupil. Anhydrosis (an-hı˘-dro¯ sis; ́ an = without, with one another. Although these plexuses look like disorganized hidros = sweat) occurs because the sweat glands no longer receive masses of axons, they provide a complex innervation pattern to sympathetic innervation. A fourth symptom is distinct flushing their target organs (figure 18.9). due to lack of sympathetic innervation to blood vessel walls that In the of the thoracic cavity, the cardiac plex- results in . us consists of postganglionic sympathetic axons that extend from the cer vical and thoracic sympathetic trunk ganglia, as well as

mck78097_ch18_539-560.indd 552 2/14/11 3:46 PM Chapter Eighteen Autonomic Nervous System 553

Trachea

Sympathetic trunk ganglion Left vagus nerve (X)

Right vagus nerve (X)

Cardiac plexus

Pulmonary plexus Greater thoracic splanchnic nerve Lesser thoracic splanchnic nerve Aorta

Inferior vena cava Esophagus Diaphragm

Celiac trunk Celiac ganglia and plexus Superior mesenteric artery

Superior mesenteric ganglia and plexus Abdominal aortic plexus

Inferior mesenteric artery Inferior mesenteric ganglia and plexus

Hypogastric plexus

Figure 18.9 Autonomic Plexuses. Autonomic plexuses are located in both the thoracic and abdominopelvic cavities. This anterior view shows the cardiac, pulmonary, and esophageal plexuses in the thoracic cavity and the abdominal aortic plexus (celiac, superior mesenteric, inferior mesenteric plexuses) in the abdominopelvic cavity.

preganglionic axons from the vagus nerve. Increased sympathetic projecting from the prevertebral ganglia and preganglionic axons activity increases heart rate and blood pressure, while increased from the vagus nerve that enter the abdominopelvic cavity with parasympathetic activity decreases heart rate. the esophagus. The pulmonary plexus consists of postganglionic sympa- The hypogastric plexus consists of a complex meshwork thetic axons from the thoracic sympathetic trunk ganglia and of postganglionic sympathetic axons (from the aortic plexus and preganglionic axons from the vagus nerve. The axons project to the lumbar region of the sympathetic trunk) and preganglionic the bronchi and blood vessels of the lungs. Stimulation of this parasympathetic axons from the pelvic splanchnic nerve. Its axons parasympathetic pathway causes a reduction in the diameter of innervate viscera within the pelvic region. the bronchi (called bronchoconstriction ) and increased secretion from mucous glands of the bronchial tree. Sympathetic inner- 18.5b Neurotransmitters and Receptors vation causes bronchodilation (increase in the diameter of the Two neurotransmitters are used in the ANS: acetylcholine (ACh) and bronchi). norepinephrine (NE) (figure 18.10). All preganglionic axons release The esophageal plexus consists of preganglionic axons ACh, which binds specific receptors in the ganglionic plasma mem- from the vagus nerve. Smooth muscle activity in the inferior brane and has an excitatory effect on the ganglionic cell. All postgang- esophageal wall is coordinated by parasympathetic axons that lionic parasympathetic axons and a few postganglionic sympathetic control the in the inferior region of the esopha- axons release ACh onto the effector. The ACh released from para- gus by innervating smooth muscle in the inferior esophageal wall sympathetic axons has either an excitatory or inhibitory effect on the and the cardiac sphincter, a valve through which swallowed food effector, depending on the receptor on the effector plasma membrane. and drink must pass. In contrast, the ACh released from sympathetic axons is excitatory The abdominal aortic plexus consists of the celiac plexus, only. Most postganglionic sympathetic axons release NE onto the superior mesenteric plexus, and inferior mesenteric plexus. The effector, which has either an excitatory or an inhibitory effect on the abdominal aortic plexus is composed of postganglionic axons effector, depending on the receptor on the effector plasma membrane.

mck78097_ch18_539-560.indd 553 2/14/11 3:46 PM 554 Chapter Eighteen Autonomic Nervous System

Parasympathetic Pathway Sympathetic Pathways

Preganglionic axon releases ACh

ACh ACh ACh

Ganglionic neuron cell body and ACh ACh ACh always contain receptors receptors receptors receptors for ACh

Postganglionic axon releases ACh or NE

ACh ACh NE

ACh ACh NE receptors receptors receptors Target cells contain either ACh receptors (bind ACh) or NE receptors (bind NE) Target cell Target cell Target cell

Figure 18.10 Neurotransmitters Used in the Autonomic Nervous System. In the parasympathetic pathway, both the preganglionic and postganglionic axons release acetylcholine (ACh). In the sympathetic pathway, all preganglionic axons and a few specific postganglionic axons release ACh. Most postganglionic sympathetic axons release norepinephrine (NE).

The axons that release acetylcholine are called cholinergic. ■ Control of digestive system activities. Sympathetic The axons that release norepinephrine are called adrenergic. stimulation reduces blood flow to the GI tract; parasympathetic innervation increases activities related to 18.5c Dual Innervation the digestion and processing of ingested food. Many visceral effectors have dual innervation, meaning that they ■ Control of heart rate. Sympathetic stimulation increases are innervated by postganglionic axons from both ANS divisions. the heart rate; parasympathetic stimulation decreases the The actions of the divisions usually oppose each other, and so they heart rate. are said to exert antagonistic effects on the same organ. Examples In some ANS effectors, opposing effects are achieved of dual innervation include the following: without dual innervation. For example, many blood vessels are ■ Control of pupillary diameter. Sympathetic innervation innervated by sympathetic axons only. Maintaining sympathetic causes pupil dilation; parasympathetic innervation causes stimulation holds smooth muscle contraction constant, resulting pupil constriction. in blood pressure stability. Increased sympathetic stimulation

mck78097_ch18_539-560.indd 554 2/14/11 3:46 PM Chapter Eighteen Autonomic Nervous System 555

causes and results in increased blood pressure, while decreased stimulation causes vasodilation and results in CLINICAL VIEW decreased blood pressure. Thus, opposing effects are achieved by increasing or decreasing activity in one division.

Autonomic dysreflexia is a potentially dangerous vascular con- 18.5d Autonomic Reflexes dition that causes blood pressure to rise profoundly, sometimes The autonomic nervous system helps maintain homeostasis so high that blood vessels rupture. At greatest risk are the thin- through the involuntary activity of autonomic reflexes, also walled cerebral vessels; stroke is a common fatal complication of termed visceral reflexes. Autonomic reflexes consist of smooth this condition. Autonomic dysreflexia is caused by hyperactivity muscle contractions, cardiac muscle contractions, or secre- of the autonomic nervous system in the weeks and months after tion by glands that are mediated by autonomic reflex arcs in a . The majority of patients are either quad- response to a specific stimulus. One common autonomic reflex riplegic or have some form of spinal cord lesion superior to the is the micturition reflex, which partly controls the release of sixth thoracic segment. urine (figure 18.11). Other reflexes include alteration of heart rate, changes in and depth, regulation of diges- Of ten, the initial reaction to spinal cord trauma or injur y is spinal tive system activities, and alteration of pupil diameter. A clas- shock, which is characterized by the loss of autonomic reflexes. sic autonomic reflex involves the reduction of blood pressure. However, this decrease in reflex activities may suddenly be When an individual has elevated blood pressure, stretch recep- replaced by autonomic reflex activities that cause certain viscera tors in the walls of large blood vessels are stimulated. Impulses to respond abnormally to the lack of nerve supply, a phenomenon from these stretch receptors then travel through visceral sen- called denervation hypersensitivity. For example, when a person sory neurons to the cardiac center in the medulla oblongata. loses the ability to voluntarily evacuate the bladder, the bladder This leads to parasympathetic input to the pacemaker of the may continue to fill with urine to the point of overdistension. This heart, resulting in a decrease in heart rate and a concomitant induces a spinal cord reflex resulting in the involuntary relaxation decrease in blood pressure. Autonomic reflexes are comparable of the internal urethral sphincter, thus allowing the bladder to to spinal reflexes because they involve a sensory receptor, empty. Essentially, this is an “override” mechanism designed to sensory neurons, in the CNS, motor neurons, and prevent rupture of the urinary bladder. Unfortunately, activation effector cells. of this override mechanism can also stimulate a sympathetic ner- vous system reflex that causes transient, though marked, blood vessel narrowing due to vasoconstriction. The area of vascular constriction is inferior to the level of the spinal cord injury or WHATW DID YOU LEARN? lesion. This vasoconstriction produces the profound elevation in ●13 What neurotransmitters are used in the ANS? blood pressure characteristic of autonomic dysreflexia. ●14 What is meant by dual innervation?

Ureters

Urinary 2 Nerve impulse travels through sensory bladder neuron to integration center in the spinal cord 3 Nerve impulse is processed stretches in the integration center as it fills with urine 1 Stimulus activates Spinal cord receptor Pelvic splanchnic nerve

4 Motor impulses are conducted through motor neurons

Postganglionic axon Ureter Figure 18.11 Autonomic Reflexes. An autonomic reflex receives a visceral sensory stimulus (in the form of a nerve impulse) from an Urinary bladder organ; in this case, urine fills the bladder and causes the bladder wall to stretch. The nerve impulse is processed by an contracts interneuron in the CNS, and autonomic motor neurons then 5 Effector responds to send a nerve impulse to the muscles or glands within that impulse from motor neuron Internal urethral sphincter organ. The effector responds—in this case, by contracting the (smooth muscle contraction relaxes detrusor muscle and relaxing the internal urethral sphincter so occurs in the bladder wall that urination can occur. and relaxation in the internal urethral sphincter)

mck78097_ch18_539-560.indd 555 2/14/11 3:46 PM 556 Chapter Eighteen Autonomic Nervous System

and pupil size. The centers for cardiac, digestive, and 18.6 CNS Control of Autonomic functions are housed within the brainstem. Some autonomic responses, notably the parasympathetic Function activities associated with defecation and urination, are processed Learning Objective: and controlled at the level of the spinal cord without the involve- ment of the brain. However, the higher centers in the brain may 1. Compare and contrast the CNS hierarchy that controls the consciously inhibit these reflex activities. autonomic nervous system. Several levels of CNS complexity are required to coordinate and regulate ANS function. Thus, despite the name “autonomic,” the ANS is a regulated nervous system, not an independent one. Autonomic function is influenced by four CNS regions: the cere- Study Tip! brum, hypothalamus, brainstem, and spinal cord (figure 18.12). The analogy of a corporation can help you understand the ANS activities are affected by conscious activities in the hierarchy of control of the ANS: cerebral cortex and subconscious communications between asso- ■ The hypothalamus is the president of the Autonomic Nervous ciation areas in the cortex with the centers of sympathetic and System Corporation. It oversees all activity in this system. parasympathetic control in the hypothalamus. Additionally, sen- ■ The autonomic reflex centers in the brainstem and spinal sory processing in the and emotional states controlled in cord are the vice presidents of the corporation. They have the limbic system directly affect the hypothalamus. a lot of control and power in this corporation. Ultimately, The hypothalamus is the integration and command center though, they must answer to the president (hypothalamus). for autonomic functions. It contains nuclei that control visceral ■ The preganglionic and ganglionic neurons are the workers functions in both divisions of the ANS, and it communicates in the corporation. They are ultimately under the control with other CNS regions, including the cerebral cortex, thalamus, of both the president and vice presidents of the corporation. brainstem, cerebellum, and spinal cord. The hypothalamus is the Also, these workers tend to do most of the real work in central brain structure involved in and drives that act the company! through the ANS. For example, the sympathetic nervous system fight-or-flight response originates in the sympathetic nucleus in this brain region. The brainstem nuclei in the mesencephalon, pons, and medulla oblongata mediate visceral reflexes. These reflex centers WHATW DID YOU LEARN? control accommodation of the lens, blood pressure changes, blood ●15 What CNS structure is the integration and command center for vessel diameter changes, digestive activities, heart rate changes, autonomic function?

Cerebrum Conscious activities in the affect hypothalamus control of the ANS

Hypothalamus Integration and command center for autonomic functions; involved in emotions

Brainstem Contains major ANS reflex centers Figure 18.12 Control of Autonomic Functions by Higher Brain Centers. ANS functions are influenced by activities within the cerebrum and hypothalamus, which in turn control ANS centers in the brainstem and spinal cord. Spinal cord Contains ANS reflex centers for defecation and urination

mck78097_ch18_539-560.indd 556 2/14/11 3:46 PM Chapter Eighteen Autonomic Nervous System 557

Posterior

Future posterior root ganglion Neural tube

Notochord Sympathetic trunk ganglion cells Aorta

Adrenal medulla cells Cortex of developing adrenal gland

Prevertebral ganglion cells

Digestive tube

Anterior

5 weeks Figure 18.13 Neural Crest Cell Derivatives. A transverse section through a 5-week embryo shows structures that develop from migrating neural crest cells, including posterior root ganglia, many of the ANS structures, and the cells of the adrenal medulla.

hypothalamus (the master control center of the ANS), the auto- 18.7 Development of the Autonomic nomic nervous system centers in the brainstem, the white rami, and the autonomic reflex centers within the spinal cord. In general, Nervous System the neural crest cells form all autonomic ganglia, ganglionic neu- Learning Objective: rons and their postganglionic axons, gray rami communicantes, the sympathetic chain ganglia, and the adrenal medulla. 1. Explain how the autonomic nervous system develops in an The neural crest cells begin to migrate during the fourth embryo. week of development. Those slated to form ANS structures dif- Recall from previous chapters that the embryonic neural ferentiate soon thereafter. Preganglionic neurons begin to extend tube forms the central nervous system structures, while the neural axons anteriorly from the neural tube during the fifth week of crest cells form most of the peripheral nervous system structures. development (figure 18.13). These axons encounter the ganglionic Because the autonomic nervous system has both CNS and PNS neurons, and the sympathetic trunk begins to form during week 6. By components, it forms from both neural tube and neural crest cells. the end of the eighth week, the rami communicantes have formed; In general, the neural tube forms the cell bodies of preganglionic the developing heart and lungs begin to receive autonomic inner- neurons (since these structures are housed within the CNS), the vation in the tenth week of development.

Clinical Terms

Hirschsprung disease (congenital ) Dilation and vagotomy (vā -got ́ō -mē ; tome = incision) Surgical separation or hypertrophy of the colon due to absence (aganglionosis) splitting of the vagus nerve, usually performed to reduce or marked reduction (hypoganglionosis) in the number of secretion in ulcer patients when medications ganglion cells within the colon. have failed.

mck78097_ch18_539-560.indd 557 2/14/11 3:46 PM 558 Chapter Eighteen Autonomic Nervous System

Chapter Summary

18.1 Comparison ■ The SNS innervates skeletal muscle. The ANS innervates smooth muscle, cardiac muscle, and glands, and controls of the Somatic involuntary motor activities. and Autonomic ■ A single motor neuron axon innervates skeletal muscle fibers in the SNS, while the ANS has a two-neuron pathway Nervous consisting of preganglionic neurons in the CNS and ganglionic neurons in the PNS. Systems 540

18.2 Overview of ■ The ANS is composed of a parasympathetic division and a sympathetic division. the Autonomic Nervous System 542

18.3 ■ The parasympathetic preganglionic neurons are housed either within the brainstem or within the sacral region of the Parasympathetic spinal cord. Division 545 ■ The ganglionic neurons in the parasympathetic division are located within either terminal ganglia or intramural ganglia. 18.3a Cranial Nerves 545 ■ Parasympathetic preganglionic axons extend from cell bodies in brainstem nuclei through the oculomotor, facial, glossopharyngeal, and vagus cranial nerves. 18.3b Sacral Spinal Nerves 545 ■ The remaining preganglionic parasympathetic cell bodies are housed within the S2–S4 segments of the spinal cord and form pelvic splanchnic nerves. 18.3c Effects and General Functions of the Parasympathetic Division 545 ■ The parasympathetic division of the ANS alters activities of effector organs to manage and control food processing, energy absorption, and relaxation activities.

18.4 Sympathetic ■ The sympathetic division outflow is from the T1–L2 lateral horn segments. Division 547 18.4a Organization and Anatomy of the Sympathetic Division 547 ■ Preganglionic neuron cell bodies are housed within the lateral gray horn of the spinal gray matter. ■ Myelinated, preganglionic sympathetic axons exit the spinal cord through the anterior root of a spinal nerve and travel through the white rami communicantes to the sympathetic trunk ganglia. 18.4b Sympathetic Pathways 550 ■ In the spinal nerve pathway, the postganglionic axon enters the spinal nerve through the gray ramus and travels to the blood vessels and glands distributed throughout the limbs and body wall of the trunk. ■ In the postganglionic sympathetic nerve pathway, the postganglionic axon leaves the sympathetic trunk and extends directly to the target organ. ■ In the splanchnic nerve pathway, the preganglionic axon passes through the sympathetic trunk without synapsing and travels to the prevertebral ganglia. ■ In the adrenal medulla pathway, the preganglionic axons extend through the autonomic ganglia without synapsing. They synapse on secretory cells in the adrenal medulla that release epinephrine and norepinephrine. 18.4c Effects and General Functions of the Sympathetic Division 550 ■ Sympathetic division pathways prepare the body for fight or flight.

18.5 Other ■ Both divisions of the autonomic nervous system innervate organs through specific axon bundles. Features of the Autonomic Nervous 18.5a Autonomic Plexuses 552 System 552 ■ Autonomic plexuses are meshworks of postganglionic sympathetic axons, preganglionic parasympathetic axons, and visceral sensory neuron axons in the anterior body cavities that merge and intermingle but do not synapse with each other. 18.5b Neurotransmitters and Receptors 553 ■ Two neurotransmitters are used in the ANS: acetylcholine (ACh) and norepinephrine (NE). ■ Both the preganglionic and postganglionic axons in the parasympathetic division release acetylcholine; the preganglionic axon and a few postganglionic axons in the sympathetic division release acetylcholine; however, most of the postganglionic axons of the sympathetic division release norepinephrine. 18.5c Dual Innervation 554 ■ Many visceral effectors have dual innervation, meaning they are innervated by axons from both ANS divisions. The actions of the divisions often oppose each other, and thus they exert antagonistic effects on the same organ. 18.5d Autonomic Refl exes 555 ■ Homeostasis in the body is maintained through the activity of autonomic reflexes. These reflexes result in smooth muscle contractions, cardiac muscle contractions, or secretion by glands.

mck78097_ch18_539-560.indd 558 2/14/11 3:46 PM Chapter Eighteen Autonomic Nervous System 559

18.6 CNS Control ■ Autonomic function is influenced by four CNS regions: cerebrum, hypothalamus, brainstem, and spinal cord. of Autonomic Function 556

18.7 Development ■ The neural tube gives rise to most of the CNS structures of the ANS. of the Autonomic ■ The neural crest cells give rise to most of the PNS structures of the ANS. Nervous System 557

Challenge Yourself

Matching ______3. Which of the following is not a function of the Match each numbered item with the most closely related lettered sympathetic division of the ANS? item. a. increases heart rate and breathing rate b. prepares for emergency ______1. norepinephrine a. contains sympathetic c. increases digestive system motility and activity postganglionic axons only ______2. autonomic plexus d. dilates pupils b. controls entire ANS ______3. ganglionic neuron ______4. Postganglionic axons from the celiac ganglion function innervate which of the following? ______4. hypothalamus c. hormone secreted by a. stomach ______5. sympathetic adrenal medulla b. urinary bladder division c. lung d. second ANS neuron d. adrenal medulla ______6. gray ramus e. neurotransmitter for all ______5. Sympathetic division splanchnic nerves end in the ______7. splanchnic nerve preganglionic axons ______ganglia, which are anterior to the vertebral ______8. sympathetic trunk f. craniosacral division column and aorta. ganglia a. intramural g. preganglionic axons to b. sympathetic trunk ______9. parasympathetic prevertebral ganglia c. prevertebral division h. network of pre- and d. terminal ______10. acetylcholine postganglionic axons ______6. All parasympathetic division synapses use ______i. fight-or-flight division as a neurotransmitter. a. dopamine j. lateral to spinal cord b. norepinephrine c. acetylcholine d. epinephrine Multiple Choice ______7. Which autonomic innervates the Select the best answer from the four choices provided. pelvic organs? a. cardiac plexus ______1. A splanchnic nerve in the sympathetic division of b. esophageal plexus the ANS c. hypogastric plexus a. connects neighboring sympathetic trunk ganglia. d. inferior mesenteric plexus b. controls parasympathetic functions in the thoracic cavity. ______8. Which of the following describes a sympathetic c. is formed by preganglionic axons that travel to postganglionic axon? prevertebral ganglia. a. long, unmyelinated axon d. travels through parasympathetic pathways in the b. short, myelinated axon head. c. short, unmyelinated axon d. long, myelinated axon ______2. Some parasympathetic preganglionic neuron cell bodies are housed within the ______9. Neural crest cells form a. hypothalamus. a. the hypothalamus. b. sacral region of the spinal cord. b. white rami communicantes. c. cerebral cortex. c. autonomic ganglia. d. thoracolumbar region of the spinal cord. d. autonomic reflex centers.

mck78097_ch18_539-560.indd 559 2/14/11 3:46 PM 560 Chapter Eighteen Autonomic Nervous System

______10. All of the following cranial nerves carry 7. Identify the types of axons that compose the gray and white parasympathetic preganglionic nerve axons except rami communicantes, describe their anatomic arrangement a. CN III (oculomotor). and location, and discuss the reason for the differences in b. CN V (trigeminal). their color. c. CN IX (glossopharyngeal). 8. Describe how the general functions of the sympathetic and d. CN X (vagus). parasympathetic divisions of the ANS differ. Content Review 9. What may occur with the mass activation of the sympathetic division of the ANS? 1. What four CNS regions control the autonomic nervous 10. Describe the embryonic components that form ANS system? structures. 2. For the following ganglia, identify the location and the division of the ANS each is part of: sympathetic trunk Developing Critical Reasoning ganglia, prevertebral ganglia, terminal and intramural 1. Holly takes night classes at the local community college. ganglia. After her lecture, she walks alone to her car and suddenly 3. Compare and contrast the postganglionic axons of the hears several dozen screeching birds fly away from the tree parasympathetic and sympathetic divisions. Examine she is walking under. Holly immediately feels her heart the axon length, myelination (or lack thereof), and the pounding and notices that her breathing rate has increased. neurotransmitter used. Minutes later, she still feels tense and “on edge.” What 4. Explain how adrenal medulla stimulation potentiates happened internally to cause Holly’s initial response? Why (prolongs) the effects of the sympathetic division of the did Holly still feel tense minutes later? autonomic nervous system. 2. Some faculty dislike teaching lecture classes after lunch, 5. Identify and describe the four basic pathways used in the complaining that the students do not pay attention at this sympathetic division. time. From an anatomic viewpoint, what is happening to 6. Are the cell bodies of sympathetic and parasympathetic these students? neurons located in the central nervous system, in the peripheral nervous system, or in both? Explain your answer.

Answers to “What Do You Think?”

1. Compared to the somatic motor system, the autonomic 2. The pterygopalatine ganglion is nicknamed the “hay fever motor system has a limited number of resources (nerves) ganglion” because when it is overstimulated, it causes some to transmit the motor information throughout the body. of the classic allergic reactions, including watery eyes, By using a two-neuron chain, nerve impulses are able to runny and itchy nose, sneezing, and scratchy throat. diverge to a larger number of resources. (The study tip 3. Sympathetic innervation causes vasoconstriction of most comparing the ANS to the airline industry [see page 542] blood vessels. When blood vessels are constricted, it takes also helps answer this question.) more force and pressure to pump blood through the vessels, so blood pressure rises.

www.mhhe.com/mckinley3 Enhance your study with practice tests and activities to assess your understanding. Your instructor may also recommend the interactive eBook, individualized learning tools, and more.

mck78097_ch18_539-560.indd 560 2/14/11 3:46 PM