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15.1COMPARISON OF SOMATIC AND AUTONOMIC NERVOUS SYSTEMS OBJECTIVE •Compare the structural and functional differences between the somatic and autonomic parts of the nervous system. Somatic Nervous System The somatic nervous system includes both sensory and motor neurons. Sensory neurons convey input from receptors for somatic senses (tactile, thermal, pain, and proprioceptive sensations; see Chapter16) and from receptors for the special senses (sight, hearing, taste, smell, and equilibrium; see Chapter17). All these sensations normally are consciously perceived. In turn, somatic motor neurons innervate skeletal muscles—the effectors of the somatic nervous system—and produce both reflexive and voluntary movements. When a somatic motor neuron stimulates the muscle, it contracts; the effect always is excitation. If somatic motor neurons cease to stimulate a muscle, the result is a paralyzed, limp muscle that has no muscle tone. Although we are generally not conscious of breathing, the muscles that generate respiratory movements are also skeletal muscles controlled by somatic motor neurons. If the respiratory motor neurons become inactive, breathing stops. A few skeletal muscles, such as those in the middle ear, are controlled by reflexes and cannot be contracted voluntarily. Autonomic Nervous System The main input to the ANS comes fromautonomic (visceral) sensory neurons.Mostly, these neurons are associated withinteroceptors(IN ter ō sep′ tors), which are sensory receptors located in blood vessels,‐‐ ‐‐ visceral organs, muscles, and the nervous system that monitor conditions in theinternalenvironment. Examples of interoceptors are chemoreceptors that monitor blood CO2level and mechanoreceptors that detect the degree of stretch in the walls of organs or blood vessels. Unlike those triggered by a flower's perfume, a beautiful painting, or a delicious meal, these sensory signals are not consciously perceived most of the time, although intense activation of interoceptors may produce conscious sensations. Two examples of perceived visceral sensations are pain sensations from damaged viscera and angina pectoris (chest pain) from inadequate blood flow to the heart. Input that influences the ANS also includes some sensations monitored by somatic sensory and special sensory neurons. For example, pain can produce dramatic changes in some autonomic activities. Autonomic motor neuronsregulate visceral activities by either increasing (exciting) or decreasing (inhibiting) ongoing activities in their effector tissues (cardiac muscle, smooth muscle, and glands). Changes in the diameter of the pupils, dilation and constriction of blood vessels, and adjustment of the rate and force of the heartbeat are examples of autonomic motor responses. Unlike skeletal muscle, tissues innervated by the ANS often function to some extent even if their nerve supply is damaged. For example, the heart continues to beat when it is removed for transplantation into another person, smooth muscle in the lining of the gastrointestinal tract contracts rhythmically on its own, and glands produce some secretions in the absence of ANS control. Most autonomic responses cannot be consciously altered to any great degree. You probably cannot voluntarily slow your heartbeat to half its normal rate. For this reason, some autonomic responses are the basis for polygraph (“lie detector”) tests. However, practitioners of yoga or other techniques of meditation may learn
how to regulate at least some of their autonomic activities through long practice.Biofeedback,in which monitoring devices display information about a body function such as heart rate or blood pressure, enhances the ability to learn such conscious control. Signals from the general somatic and special senses, acting via the limbic system, also influence responses of autonomic motor neurons. Seeing a bike about to hit you, hearing squealing brakes of a nearby car, or being grabbed by an attacker would all increase the rate and force of your heartbeat. Examples Anatomy Overview: Nervous System: Organization of the ANS Comparison of Somatic and Autonomic Motor Neurons Recall from Chapter10that the axon of a single, myelinated somatic motor neuron extends from the central nervous system (CNS) all the way to the skeletal muscle fibers in its motor unit (Figure15.1a). By contrast, most autonomic motor pathways consist of two motor neurons in series, that is, one following the other (Figure15.1b). The first neuron (preganglionic neuron) has its cell body in the CNS; its myelinated axon extends from the CNS to anautonomic ganglion. (Recall that aganglionis a collection of neuronal cell bodies in the PNS.) The cell body of the second neuron (postganglionic neuron) is also in that same autonomic ganglion; its unmyelinated axon extends directly from the ganglion to the effector (smooth muscle, cardiac muscle, or a gland). Alternatively, in some autonomic pathways, the first motor neuron extends to specialized cells calledchromaffin cellsin the adrenal medullae (inner portions of the adrenal glands) rather than an autonomic ganglion. In addition, all somatic motor neurons release only acetylcholine (ACh) as their neurotransmitter, but autonomic motor neurons release either ACh or norepinephrine (NE). Figure15.1 Motor neuron pathways in the (a) somatic nervous system and (b) autonomic nervous system (ANS). Note that autonomic motor neurons release either acetylcholine (ACh) or norepinephrine (NE); somatic motor neurons release ACh. Somatic nervous system stimulation always excites its effectors (skeletal muscle fibers); stimulation by the autonomic nervous system either excites or inhibits visceral effectors. Kevin Somerville/Imagineering What does dual innervation mean? Unlike somatic output (motor), the output part of the ANS has two divisions: thesympathetic divisionand theparasympathetic division. Most organs havedual innervation;that is, they receive impulses from both sympathetic and parasympathetic neurons. In some organs, nerve impulses from one division of the ANS stimulate the organ to increase its activity (excitation), and impulses from the other division decrease the organ's activity (inhibition). For example, an increased rate of nerve impulses from thesympatheticdivision increases heart rate, and an increased rate of nerve impulses from theparasympatheticdivision decreases heart rate. The sympathetic division is often called thefight or flight division‐‐. Sympathetic activities result in increased alertness and metabolic activities in order to prepare the body for an emergency situation. Responses to such situations, which may occur during physical activity or emotional stress, include a rapid heart rate, faster breathing rate, dilation of the pupils, dry mouth, sweaty but cool skin, dilation of blood vessels to organs
involved in combating stress (such as the heart and skeletal muscles), constriction of blood vessels to organs not involved in combating stress (for example, the gastrointestinal tract and kidneys), and release of glucose from the liver. The parasympathetic division is often referred to as therest and digest division‐‐because its activities conserve and restore body energy during times of rest or digesting a meal; the majority of its output is directed to the smooth muscle and glandular tissue of the gastrointestinal and respiratory tracts. The parasympathetic division conserves energy and replenishes nutrient stores. Although both the sympathetic and parasympathetic divisions are concerned with maintaining homeostasis, they do so in dramatically different ways. Table15.1compares the somatic and autonomic nervous systems. Table15.1Comparison of the Somatic and Autonomic Nervous Systems Somatic Nervous SystemAutonomic Nervous System Sensory inputFrom somatic senses and special senses. Mainly from interoceptors; some from somatic senses and special senses. Control of motor output Voluntary control from cerebral cortex, with contributions from basal ganglia, cerebellum, brain stem, and spinal cord. Involuntary control from hypothalamus, limbic system, brain stem, and spinal cord; limited control from cerebral cortex. Motor neuron pathway One neuron pathway:‐ Somatic motor neurons extending from CNS synapse directly with effector. Usually two neuron pathway: Preganglionic neurons‐ extending from CNS synapse with postganglionic neurons in autonomic ganglion, and postganglionic neurons extending from ganglion synapse with visceral effector. Alternatively, preganglionic neurons may extend from CNS to synapse with chromaffin cells of adrenal medullae. Neurotransmitters and hormones All somatic motor neurons release ACh. All sympathetic and parasympathetic preganglionic neurons release ACh. Most sympathetic postganglionic neurons release NE; those to most sweat glands release ACh. All parasympathetic postganglionic neurons release ACh. Chromaffin cells of adrenal medullae release epinephrine and norepinephrine. EffectorsSkeletal muscle.Smooth muscle, cardiac muscle, and glands. ResponsesContraction of skeletal muscle. Contraction or relaxation of smooth muscle; increased or decreased rate and force of contraction of cardiac muscle; increased or decreased secretions of glands. 15.2ANATOMY OF AUTONOMIC MOTOR PATHWAYS
OBJECTIVES •Describe preganglionic and postganglionic neurons of the autonomic nervous system. •Compare the anatomical components of the sympathetic and parasympathetic divisions of the autonomic nervous system. Anatomical Components Each division of the ANS has two motor neurons. The first of the two motor neurons in any autonomic motor pathway is called apreganglionic neuron(Figure15.1b). Its cell body is in the brain or spinal cord; its axon exits the CNS as part of a cranial or spinal nerve. The axon of a preganglionic neuron is a small diameter,‐ myelinated type B fiber that usually extends to an autonomic ganglion, where it synapses with apostganglionic neuron, the second neuron in the autonomic motor pathway. Note that the postganglionic neuron lies entirely outside the CNS in the PNS. Its cell body and dendrites are located in anautonomic ganglion, where it forms synapses with one or more preganglionic axons. The axon of a postganglionic neuron is a small diameter, unmyelinated type C fiber that terminates in a visceral effector. Thus, preganglionic‐ neurons convey nerve impulses from the CNS to autonomic ganglia, and postganglionic neurons relay the impulses from autonomic ganglia to visceral effectors. Preganglionic Neurons In the sympathetic division, the preganglionic neurons have their cell bodies in the lateral horns of the gray matter in the 12 thoracic segments and the first two (and sometimes three) lumbar segments of the spinal cord (Figure15.2). For this reason, the sympathetic division is also called thethoracolumbar division(thōr′ a kō‐ ‐‐ LUM bar), and the axons of the sympathetic preganglionic neurons are known as the‐thoracolumbar outflow. Figure15.2Structure of the sympathetic division of the autonomic nervous system. Solid lines represent preganglionic axons; dashed lines represent postganglionic axons. Although the innervated structures are shown for only one side of the body for diagrammatic purposes, the sympathetic division actually innervates tissues and organs on both sides. Cell bodies of sympathetic preganglionic neurons are located in the lateral horns of gray matter in the 12 thoracic and first two lumbar segments of the spinal cord. Imagineering Which division, sympathetic or parasympathetic, has longer preganglionic axons? Why? Cell bodies of preganglionic neurons of the parasympathetic division are located in the nuclei of four cranial nerves in the brain stem (III, VII, IX, and X) and in the lateral gray matter of the second through fourth sacral segments of the spinal cord (Figure15.3). Hence, the parasympathetic division is also known as thecraniosacral division(krā′ nē ō SĀK ral), and the axons of the parasympathetic preganglionic neurons‐‐ ‐‐ are referred to as thecraniosacral outflow.
Figure15.3Structure of the parasympathetic division of the autonomic nervous system. Solid lines represent preganglionic axons; dashed lines represent postganglionic axons. Although the innervated structures are shown only for one side of the body for diagrammatic purposes, the parasympathetic division actually innervates tissues and organs on both sides. Cell bodies of parasympathetic preganglionic neurons are located in brain stem nuclei and in the lateral gray matter in the second through fourth sacral segments of the spinal cord. Imagineering Which ganglia are associated with the parasympathetic division? Sympathetic division? Autonomic Ganglia There are two major groups of autonomic ganglia: (1) sympathetic ganglia, which are components of the sympathetic division of the ANS, and (2) parasympathetic ganglia, which are components of the parasympathetic division of the ANS. Sympathetic Ganglia The sympathetic ganglia are the sites of synapses between sympathetic preganglionic and postganglionic neurons. There are two major types of sympathetic ganglia: sympathetic trunk ganglia and prevertebral ganglia.Sympathetic trunk ganglia(also calledvertebral chain gangliaorparavertebral ganglia) lie in a vertical row on either side of the vertebral column. These ganglia extend from the base of the skull to the coccyx (Figure15.2). Postganglionic axons from sympathetic trunk ganglia primarily innervate organs above the diaphragm, such as the head, neck, shoulders, and heart. Sympathetic trunk ganglia in the neck have specific names. They are thesuperior, middle,andinferior cervical ganglia.The remaining sympathetic trunk ganglia do not have individual names. Because the sympathetic trunk ganglia are near the spinal cord, most sympathetic preganglionic axons are short and most sympathetic postganglionic axons are long. The second group of sympathetic ganglia, theprevertebral (collateral) ganglia,lies anterior to the vertebral column and close to the large abdominal arteries. In general, postganglionic axons from prevertebral ganglia innervate organs below the diaphragm. There are five major prevertebral ganglia (Figure15.2; see also Figure15.5): (1) Theceliac ganglion(SĒ lē ak) is on either side of the celiac trunk, an artery that is just‐‐ inferior to the diaphragm. (2) Thesuperior mesenteric ganglion(MEZ en ter′ ik) is near the beginning of the‐‐‐ superior mesenteric artery in the upper abdomen. (3) Theinferior mesenteric ganglionis near the beginning of the inferior mesenteric artery in the middle of the abdomen. (4) Theaorticorenal ganglion(ā or′ ti kō RĒ‐‐ ‐‐‐ nal) and (5) therenal ganglionare near the renal artery of each kidney. Parasympathetic Ganglia Preganglionic axons of the parasympathetic division synapse with postganglionic neurons interminal (intramural) ganglia. Most of these ganglia are located close to or actually within the wall of a visceral organ. Terminal ganglia in the head have specific names. They are theciliary ganglion, pterygopalatine ganglion(ter′ i gō PAL a tīn),‐ ‐‐‐ ‐submandibular ganglion,andotic ganglion(Figure15.3). The remaining terminal ganglia do not have specific names. Because terminal ganglia are located either close to or in the wall of the visceral organ, parasympathetic preganglionic axons are long, in contrast to parasympathetic postganglionic axons, which are short. Postganglionic Neurons Once axons of sympathetic preganglionic neurons pass to sympathetic trunk ganglia, they may connect with postganglionic neurons in one of the following ways (Figure15.4):
1.An axon may synapse with postganglionic neurons in the ganglion it first reaches. 2.An axon may ascend or descend to a higher or lower ganglion before synapsing with postganglionic neurons. The axons of incoming sympathetic preganglionic neurons that pass up or down the sympathetic trunk collectively form thesympathetic chains,the fibers on which the ganglia are strung. 3.An axon may continue, without synapsing, through the sympathetic trunk ganglion to end at a prevertebral ganglion and synapse with postganglionic neurons there. 4.An axon may also pass, without synapsing, through the sympathetic trunk ganglion and a prevertebral ganglion and then extend to chromaffin cells of the adrenal medullae that are functionally similar to sympathetic postganglionic neurons. Figure15.4Types of connections between ganglia and postganglionic neurons in the sympathetic division of the ANS. Numbers correspond to descriptions in the text. Also illustrated are the gray and white rami communicantes. Sympathetic ganglia lie in two chains on either side of the vertebral column (sympathetic trunk ganglia) and near large abdominal arteries anterior to the vertebral column (prevertebral ganglia). Kevin Somerville/Imagineering What is the significance of the sympathetic trunk ganglia? A single sympathetic preganglionic fiber has many axon collaterals (branches) and may synapse with 20 or more postganglionic neurons. This pattern of projection is an example ofdivergenceand helps explain why many sympathetic responses affect almost the entire body simultaneously. After exiting their ganglia, the postganglionic axons typically terminate in several visceral effectors (see Figure15.2). Axons of preganglionic neurons of the parasympathetic division pass to terminal ganglia near or within a visceral effector (see Figure15.3). In the ganglion, the presynaptic neuron usually synapses with only four or five postsynaptic neurons, all of which supply a single visceral effector, allowing parasympathetic responses to be localized to a single effector. Autonomic Plexuses In the thorax, abdomen, and pelvis, axons of both sympathetic and parasympathetic neurons form tangled networks calledautonomic plexuses, many of which lie along major arteries. The autonomic plexuses also may contain sympathetic ganglia and axons of autonomic sensory neurons. The major plexuses in the thorax are the cardiac plexus, which supplies the heart, and the pulmonary plexus, which supplies the bronchial tree (Figure15.5). Figure15.5Autonomic plexuses in the thorax, abdomen, and pelvis. An autonomic plexus is a network of sympathetic and parasympathetic axons that sometimes also includes autonomic sensory axons and sympathetic ganglia.Kevin Somerville
Which is the largest autonomic plexus? The abdomen and pelvis also contain major autonomic plexuses (Figure15.5), and often the plexuses are named after the artery along which they are distributed. Theceliac (solar) plexusis the largest autonomic plexus and surrounds the celiac trunk. It contains two large celiac ganglia, two aorticorenal ganglia, and a dense network of autonomic axons and is distributed to the stomach, spleen, pancreas, liver, gallbladder, kidneys, adrenal medullae, testes, and ovaries. Thesuperior mesenteric plexuscontains the superior mesenteric ganglion and supplies the small and large intestines. Theinferior mesenteric plexuscontains the inferior mesenteric ganglion, which innervates the large intestine. Axons of some sympathetic postganglionic neurons from the inferior mesenteric ganglion also extend through thehypogastric plexus,which is anterior to the fifth lumbar vertebra, to supply the pelvic viscera. Therenal plexuscontains the renal ganglion and supplies the renal arteries within the kidneys and ureters. With this background in mind, we can now examine some of the specific structural features of the sympathetic and parasympathetic divisions of the ANS in more detail. Examples Animation: ANS: Motor Pathways Anatomy Overview: Visceral Receptors Anatomy Overview: Visceral Effectors Structure of the Sympathetic Division Pathway from Spinal Cord to Sympathetic Trunk Ganglia Cell bodies of sympathetic preganglionic neurons are part of the lateral gray horns of all thoracic segments and of the first two lumbar segments of the spinal cord (see Figure15.2). The preganglionic axons leave the spinal cord along with the somatic motor neurons at the same segmental level. After exiting through the intervertebral foramina, the myelinated preganglionic sympathetic axons pass into the anterior root of a spinal nerve and enter a short pathway called awhite ramus(RĀ mus) before passing to the nearest sympathetic trunk‐ ganglion on the same side (see Figure15.4). Collectively, the white rami are called the whiterami communicantes(kō mū ni KAN tēz; singular is ramus communicans). Thus, white rami communicantes are‐‐‐‐ structures containing sympathetic preganglionic axons that connect the anterior ramus of the spinal nerve with the ganglia of the sympathetic trunk. The “white” in their name indicates that they contain myelinated axons. Only the thoracic and first two or three lumbar nerves have white rami communicantes. Organization of Sympathetic Trunk Ganglia The pairedsympathetic trunk gangliaare arranged anterior and lateral to the vertebral column, one on either side. Typically, there are 3 cervical, 11 or 12 thoracic, 4 or 5 lumbar, 4 or 5 sacral sympathetic trunk ganglia, and 1 coccygeal ganglion. The right and left coccygeal ganglia are fused together and usually lie at the midline. Although the sympathetic trunk ganglia extend inferiorly from the neck, chest, and abdomen to the coccyx, they receive preganglionic axons only from the thoracic and lumbar segments of the spinal cord (see Figure15.2). The cervical portion of each sympathetic trunk is located in the neck and is subdivided into superior, middle, and inferior ganglia (see Figure15.2). Postganglionic neurons leaving thesuperior cervical ganglionserve the head and heart. They are distributed to the sweat glands, smooth muscle of the eye, blood vessels of the face, lacrimal glands, pineal gland, nasal mucosa, salivary glands (which include the submandibular,
sublingual, and parotid glands), and heart. Postganglionic neurons leaving themiddle cervical ganglionand theinferior cervical ganglioninnervate the heart. The thoracic portion of each sympathetic trunk lies anterior to the necks of the corresponding ribs. This region of the sympathetic trunk receives most of the sympathetic preganglionic axons. Postganglionic neurons from the thoracic sympathetic trunk innervate the heart, lungs, bronchi, and other thoracic viscera. In the skin, these neurons also innervate sweat glands, blood vessels, and arrector pili muscles of hair follicles. The lumbar portion of each sympathetic trunk lies lateral to the corresponding lumbar vertebrae. The sacral region of the sympathetic trunk lies in the pelvic cavity on the medial side of the anterior sacral foramina. Pathways from Sympathetic Trunk Ganglia to Visceral Effectors Axons leave the sympathetic trunk in four possible ways: (1) They can enter spinal nerves; (2) they can form cephalic periarterial nerves; (3) they can form sympathetic nerves; and (4) they can form splanchnic nerves. Spinal Nerves Recall that some of the incoming sympathetic preganglionic neurons synapse with postganglionic neurons in the sympathetic trunk, either in the ganglion at the level of entry or in a ganglion farther up or down the sympathetic trunk. The axons of some of these postganglionic neurons leave the sympathetic trunk by entering a short pathway called agray ramusand then merge with the anterior ramus of a spinal nerve. Therefore,gray rami communicantesare structures containing sympathetic postganglionic axons that connect the ganglia of the sympathetic trunk to spinal nerves (see Figure15.4). The “gray” in their name indicates that they contain unmyelinated axons. Gray rami communicantes outnumber the white rami because there is a gray ramus leading to each of the 31 pairs of spinal nerves. The axons of the postganglionic neurons that leave the sympathetic trunk to enter spinal nerves provide sympathetic innervation to the visceral effectors in the skin of the neck, trunk, and limbs, including sweat glands, smooth muscle in blood vessels, and arrector pili muscles of hair follicles. Cephalic Periarterial Nerves Some sympathetic preganglionic neurons that enter the sympathetic trunk ascend to the superiorcervical ganglion, where they synapse with postganglionic neurons. The axons of some of these postganglionic neurons leave the sympathetic trunk by formingcephalic periarterial nerves(per′ ē ar TĒ rē al), nerves that‐ ‐‐‐‐ extend to the head by wrapping around and following the course of various arteries (such as the carotid arteries) that pass from the neck to the head (see Figure15.4). Cephalic periarterial nerves provide sympathetic innervation to visceral effectors in the skin of the face (sweat glands, smooth muscle of blood vessels, and arrector pili muscles of hair follicles), as well as other visceral effectors of the head (smooth muscle of the eye, lacrimal glands, pineal gland, nasal mucosa, and salivary glands). Sympathetic Nerves Some of the incoming sympathetic preganglionic neurons synapse with postganglionic neurons in one or more ganglia of the sympathetic trunk. Then, the axons of the postganglionic neurons leave the trunk by formingsympathetic nervesthat extend to visceral effectors in the thoracic cavity (Figure15.4). Sympathetic nerves provide sympathetic innervation to the heart and lungs. •Sympathetic nerves to the heart.Sympathetic innervation of the heart consists of axons of preganglionic neurons that enter the sympathetic trunk and then form synapses with postganglionic neurons in the superior, middle, and inferior cervical ganglia and first through fourth thoracic ganglia (T1–T4). From these ganglia, axons of postganglionic neurons exit the sympathetic trunk by forming sympathetic nerves that enter the cardiac plexus to supply the heart (see Figure15.2). •Sympathetic nerves to the lungs.Sympathetic innervation of the lungs consists of axons of preganglionic neurons that enter the sympathetic trunk and then form synapses with postganglionic
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