INVOLUNTARY EFFECTORS The autonomic nervous system helps regulate the activities of cardiac muscle, smooth muscles, and glands. In this regulation, impulses are conducted from the CNS by an axon that synapses with a second autonomic neuron. It is the axon of this second neuron in the pathway that innervates the involuntary effectors. L E A R N I N G O U T C O M E S After studying this section, you should be able to: 1. Describe the organization of autonomic motor neurons. 2. Describe how neural regulation of smooth and cardiac muscles differs from neural regulation of skeletal muscles. Figure 9.1 The autonomic system has preganglionic and post-ganglionic neurons. The preganglionic neurons of the autonomic system have cell bodies in the CNS, whereas the postganglionic neurons have cell bodies within autonomic ganglia. The sympathetic and parasympathetic divisions differ in the particular locations of their preganglionic neuron cell bodies within the CNS, and in the location of their ganglia. CNS Autonomic motor nerves innervate organs whose functions are not usually under voluntary control. The effectors that respond to autonomic regulation include cardiac muscle (the heart), smooth muscles, and glands. These effectors are part of the visceral organs (organs within the body cavities) and of blood vessels. The involuntary effects of autonomic innervation contrast with the voluntary control of skeletal muscles by way of somatic motor neurons. Autonomic Neurons Neurons of the peripheral nervous system (PNS) that conduct impulses away from the central nervous system (CNS) are known as motor, or efferent, neurons (chapter 7, section 7.1) . There are two major categories of motor neurons: somatic and autonomic. Somatic motor neurons have their cell bodies within the CNS and send axons to skeletal muscles, which are usually under voluntary control. This was briefly described in chapter 8 (see fig. 8.28), in the section on the reflex arc. The control of skeletal muscles by somatic motor neurons is discussed in depth in chapter 12, section 12.5. Unlike somatic motor neurons, which conduct impulses along a single axon from the spinal cord to the neuromuscular junction, autonomic motor control involves two neurons in the efferent pathway ( fig. 9.1 and table 9.1 ). The first of these neurons has its cell body in the gray matter of the brain or spinal cord. The axon of this neuron does not directly innervate the effector organ but instead synapses with a second neuron within an autonomic ganglion (a ganglion is a collection of cell bodies outside the CNS). The first neuron is thus called a preganglionic neuron. The second neuron in this pathway, called a postganglionic neuron, has an axon that extends from the autonomic ganglion to an effector organ, where it synapses with its target tissue ( fig. 9.1 ). Preganglionic autonomic fibers originate in the midbrain and hindbrain and in the upper thoracic to the fourth sacral levels of the spinal cord. Autonomic ganglia are located in the head, neck, and abdomen; chains of autonomic ganglia also parallel the right and left sides of the spinal cord. The origin of the preganglionic fibers and the location of the autonomic ganglia help to distinguish the sympathetic and parasympathetic divisions of the autonomic system, discussed in later sections of this chapter. Autonomic ganglion Involuntary effector Smooth muscle Preganglionic neuron Postganglionic neuron 244 The Autonomic Nervous System 245 Table 9.1 | Comparison of the Somatic Motor System and the Autonomic Motor System FeatureSomatic MotorAutonomic Motor Effector organs Skeletal muscles Cardiac muscle, smooth muscle, and glands Presence of ganglia No ganglia Cell bodies of postganglionic autonomic fibers located in paravertebral, prevertebral (collateral), and terminal ganglia Number of neurons from CNS to effector One Two Type of neuromuscular junction Specialized motor end plate No specialization of postsynaptic membrane; all areas of smooth muscle cells contain receptor proteins for neurotransmitters Effect of nerve impulse on muscle Excitatory only Either excitatory or inhibitory Type of nerve fibers Fast-conducting, thick (9–13 m m), and myelinated Slow-conducting; preganglionic fibers lightly myelinated but thin (3 m m); postganglionic fibers unmyelinated and very thin (about 1.0 m m) Effect of denervation Flaccid paralysis and atrophy Muscle tone and function persist; target cells show denervation hypersensitivity The sensory neurons that conduct information from the viscera for autonomic nerve reflexes can have the same anatomy as those sensory neurons involved in somatic motor reflexes (chapter 8, fig. 8.28). That is, the sensory information enters the spinal cord on the dorsal roots of the spinal nerves. However, some important visceral sensory information can instead enter the brain in cranial nerves. For example, information about blood pressure, plasma pH, and oxygen concentration is carried into the brain by sensory axons in cranial nerves IX and X. These are mixed nerves, containing both sensory and parasympathetic motor axons. Visceral Effector Organs Because the autonomic nervous system helps regulate the activities of glands, smooth muscles, and cardiac muscle, autonomic control is an integral aspect of the physiology of most of the body systems. Autonomic regulation, then, plays roles in endocrine regulation (chapter 11), smooth muscle function (chapter 12), the functions of the heart and circulation (chapters 13 and 14), and, in fact, all the remaining systems to be discussed. Although the functions of the target organs of autonomic innervation are described in subsequent chapters, at this point we will consider some of the common features of autonomic regulation. Unlike skeletal muscles, which enter a state of flaccid paralysis and atrophy when their motor nerves are severed, the involuntary effectors are somewhat independent of their innervation. Smooth muscles maintain a resting tone (tension) in the absence of nerve stimulation, for example. In fact, damage to an autonomic nerve makes its target tissue more sensitive than normal to stimulating agents. This phenomenon is called denervation hypersensitivity. Such compensatory changes can explain why, for example, the ability of the stomach mucosa to secrete acid may be restored after its neural supply from the vagus nerve has been severed. (This procedure is called vagotomy, and is sometimes performed as a treatment for ulcers.) In addition to their intrinsic (“built-in”) muscle tone, cardiac muscle and many smooth muscles take their autonomy a step further. These muscles can contract rhythmically, even in the absence of nerve stimulation, in response to electrical waves of depolarization initiated by the muscles themselves. Autonomic innervation simply increases or decreases this intrinsic activity. Autonomic nerves also maintain a resting tone, in the sense that they maintain a baseline firing rate that can be either increased or decreased. A decrease in the excitatory input to the heart, for example, will slow its rate of beat. The release of acetylcholine (ACh) from somatic motor neurons always stimulates the effector organ (skeletal muscles). By contrast, some autonomic nerves release transmitters that inhibit the activity of their effectors. An increase in the activity of the vagus, a nerve that supplies inhibitory fibers to the heart, for example, will slow the heart rate, whereas a decrease in this inhibitory input will increase the heart rate. | CHECKPOINT 1. Describe the preganglionic and postganglionic neurons in the autonomic system. Use a diagram to illustrate the difference in efferent outflow between somatic and autonomic nerves. 2. Compare the control of cardiac muscle and smooth muscles with that of skeletal muscles. How is each type of muscle tissue affected by cutting its innervation Explain it for oral

9.2 DIVISIONS OF THE AUTONOMIC NERVOUS SYSTEM Preganglionic neurons of the sympathetic division originate in the thoracic and lumbar levels of the spinal cord and send axons to sympathetic ganglia, which parallel the spinal cord. Preganglionic neurons of the parasympathetic division originate in the brain and in the sacral level of the spinal cord, and send axons to ganglia located in or near the effector organs. L E A R N I N G O U T C O M E S After studying this section, you should be able to: 3. Describe the structure of the sympathetic nervous system, locating the ganglia and the preganglionic and postganglionic neurons. 4. Explain the relationship between the sympathetic nervous system and the adrenal medulla. 5. Describe the structure and innervation pathways of the parasympathetic division of the autonomic system. The sympathetic and parasympathetic divisions of the autonomic system have some structural features in common. Both consist of preganglionic neurons that originate in the CNS and postganglionic neurons that originate outside of the CNS in ganglia. However, the specific origin of the preganglionic fibers and the location of the ganglia differ in the two divisions of the autonomic system. Sympathetic Division The sympathetic division is also called the thoracolumbar division of the autonomic system because its preganglionic fibers exit the spinal cord, in the ventral roots of spinal nerves, from the first thoracic (T1) to the second lumbar (L2) levels. Most sympathetic nerve fibers separate from the somatic motor fibers and synapse with postganglionic neurons within a double row of sympathetic ganglia, called paravertebral ganglia, located on either side of the spinal cord ( fig.  9.2 ). Ganglia within each row are interconnected, forming a sympathetic chain of ganglia that parallels the spinal cord on each lateral side. The myelinated preganglionic sympathetic axons exit the spinal cord in the ventral roots of spinal nerves, but they soon diverge from the spinal nerves within short pathways called white rami communicantes. The axons within each ramus enter the sympathetic chain of ganglia, where they can travel to ganglia at different levels and synapse with postganglionic sympathetic neurons. The axons of the postganglionic sympathetic neurons are unmyelinated and form the gray rami communicantes as they return to the spinal nerves and travel as part of the spinal nerves to their effector organs ( fig. 9.3 ). Because sympathetic axons form a component of spinal nerves, they are widely distributed to the skeletal muscles and skin of the body where they innervate blood vessels and other involuntary effectors. Divergence occurs within the sympathetic chain of ganglia as preganglionic fibers branch to synapse with numerous postganglionic neurons located in ganglia at different levels in the chain. Convergence also occurs here when a postganglionic neuron receives synaptic input from a large number of preganglionic fibers. The divergence of impulses from the spinal cord Spinal cord Posterior (dorsal) root Anterior (ventral) root Rami communicantes Spinal nerve Rib Sympathetic chain of paravertebral ganglia Sympathetic ganglion Vertebral body Figure 9.2 The sympathetic chain of paravertebral ganglia. This diagram shows the anatomical relationship between the sympathetic ganglia and the vertebral column and spinal cord. The Autonomic Nervous System 247 1. Preganglionic axons synapse with postganglionic neurons Dorsal root Dorsal root ganglion Spinal nerve Visceral effectors: Smooth muscle of blood vessels, arrector pili muscles, and sweat glands Sympathetic chain ganglion Sympathetic chain 2. Postganglionic axons innervate target organs White ramus Ventral root Splanchnic nerve Gray ramus Visceral effector: intestine Collateral ganglion (celiac ganglion) Spinal cord Preganglionic neuron Postganglionic neuron Figure 9.3 The pathway of sympathetic neurons. The preganglionic neurons enter the sympathetic chain of ganglia on the white ramus (one of the two rami communicantes). Some synapse there, and the postganglionic axon leaves on the gray ramus to rejoin a spinal nerve. Others pass through the ganglia without synapsing. These ultimately synapse in a collateral ganglion, such as the celiac ganglion. to the ganglia and the convergence of impulses within the ganglia can result in the mass activation of almost all of the postganglionic sympathetic neurons. This mass activation allows the entire sympathetic division to be tonically (constantly) active to a certain degree and to increase its activity in response to “fight-or-flight” situations (section 9.3). However, mass activation does not always occur. In response to particular visceral stimuli (such as changes in blood pressure, blood volume, and plasma osmolality), the CNS can direct appropriate increases or decreases in the activity of postganglionic sympathetic axons to the heart and kidneys that allows these organs to compensate for the changes and maintain homeostasis. Collateral Ganglia Many preganglionic fibers that exit the spinal cord below the level of the diaphragm pass through the sympathetic chain of ganglia without synapsing. Beyond the sympathetic chain, these preganglionic fibers form splanchnic nerves. Preganglionic fibers in the splanchnic nerves synapse in collateral, or prevertebral, ganglia. These include the celiac, superior mesenteric, and inferior mesenteric ganglia ( fig. 9.4 ). Postganglionic fibers that arise from the collateral ganglia innervate organs of the digestive, urinary, and reproductive systems. Adrenal Glands The paired adrenal glands are located above each kidney ( fig. 9.4 ). Each adrenal is composed of two parts: an outer cortex and an inner medulla. These two parts are really two functionally different glands with different embryonic origins, different hormones, and different regulatory mechanisms. The adrenal cortex secretes steroid hormones; the adrenal medulla secretes the hormone epinephrine (adrenaline) and, to a lesser degree, norepinephrine, when it is stimulated by the sympathetic system. The adrenal medulla can be likened to a modified sympathetic ganglion; its cells are derived from the same embryonic tissue (the neural crest, chapter 8) that forms postganglionic sympathetic neurons. Like a sympathetic ganglion, the cells of the adrenal medulla are innervated by preganglionic sympathetic fibers ( fig.  9.5 ). The adrenal medulla secretes epinephrine into the blood in response to this neural stimulation. The effects of epinephrine are complementary to those of the neurotransmitter norepinephrine, which is released from postganglionic sympathetic nerve endings. For this reason, and because the adrenal medulla is stimulated as part of the mass activation of the sympathetic system, the two are Explain