The Autonomic Nervous System Differs from the Somatic Motor System in at Least Two Important Ways

The ANS differs from the somatic motor system in its target organs and in the number of neurons in its peripheral circuit. The somatic motor system innervates skeletal muscle, which is the muscle responsible for all movements of the body, as described in Chapters 5 and 6. In contrast, the ANS innervates smooth muscle, cardiac muscle, and some glands (Box 13-1). Cardiac muscle is the muscle of the heart (see Chapter 19). Smooth muscle is the muscle in blood vessels, in most of the gastrointestinal tract, in the bladder, and in other hollow visceral structures.

The ANS also differs in the number of neurons it has in the peripheral nervous system (Figure 13-1). The somatic nervous system has one neuron whose cell body is located in the central nervous system (CNS) and whose axon extends, uninterrupted, to the skeletal muscle, where the peripheral chemical synapse occurs. In contrast, the ANS has two peripheral neurons. The first, called a preganglionic neuron, also has its cell body in the CNS, but its axon innervates a second neuron in the chain, called the postganglionic neuron. Its cell body is in a peripheral structure called a ganglion, a collection of nerve cell bodies outside the CNS (recall the example of the dorsal root ganglia of sensory nerves leading to the spinal cord). There are chemically mediated synapses both between the preganglionic and postganglionic neurons and between the postganglionic neuron and its target organ.

FIGURE 13-1 The autonomic nervous system (ANS) differs from the somatic motor system in the number of neurons that it has in the peripheral nervous system. The somatic motor system has one neuron, whose cell body is located in the central nervous system (CNS) and whose axon extends, uninterrupted, to the skeletal muscle, where the peripheral chemical synapse occurs. In contrast, the ANS has two neurons in the path from the CNS to the target. The first, called a preganglionic neuron, also has its cell body in the CNS, but its axon innervates a second neuron in the chain, called the postganglionic neuron. Its cell body is in a peripheral structure called a ganglion.

The ANS also differs from the somatic motor system in the amount of myelin along the peripheral axons; postganglionic neurons are usually unmyelinated. In addition, somatic motor neurons always excite their skeletal muscle targets, whereas the autonomic postganglionic neurons can either excite or inhibit their targets.

The Autonomic Nervous System Has Two Major Subdivisions

The ANS is divided into two major subdivisions based on the anatomical origin of their preganglionic neurons and on their synaptic transmitters at the target organ. These two subdivisions are the sympathetic nervous system and the parasympathetic nervous system.

The Sympathetic Nervous System Arises from the Thoracolumbar Spinal Cord

The sympathetic nervous system generally has short preganglionic and long postganglionic axons. Preganglionic axons of the sympathetic nervous system leave the spinal cord by way of the ventral roots of the first thoracic through the third or fourth lumbar spinal nerves (Figure 13-2). For this reason, the sympathetic nervous system is often called the thoracolumbar system. The preganglionic axons pass through the ventral root and then a communicating branch (ramus) to enter the paravertebral sympathetic ganglion chain (also called the “sympathetic trunk”), where most synapse with a postganglionic neuron (Figure 13-3). The ganglion chain actually extends from cervical to sacral regions. The postganglionic axons then extend to one of the hollow visceral organs or reenter the spinal nerves to extend to more distal structures such as blood vessels, sweat glands, and hair erector muscles of the body wall. A few preganglionic axons pass through the paravertebral ganglia, forming splanchnic nerves that synapse with postganglionic neurons in prevertebral ganglia, usually named for neighboring blood vessels. Postganglionic neurons of the prevertebral ganglia also innervate hollow visceral organs.

FIGURE 13-2 The site of origin of preganglionic neurons in the central nervous system for both the sympathetic nervous system (left) and the parasympathetic nervous system (right). Several sites of projection of postganglionic axons are also shown.

(From Kandel ER, Schwartz JH: Principles of neural science, ed 2, New York, 1985, Elsevier Science Publishing.)

FIGURE 13-3 Synaptic organization of preganglionic (Pre) and postganglionic (Post) neurons of the sympathetic and parasympathetic divisions of the autonomic nervous system. RC, Ramus communicans (communicating branch).

(From Ganong WF: Review of medical physiology, ed 13, Norwalk, Conn, 1987, Appleton & Lange.)

The adrenal medulla is a special variation on this anatomical theme. A few sympathetic preganglionic axons extend all the way to the adrenal medulla, where they synapse with rudimentary postganglionic neurons that make up the adrenal medullary secretory cells. These vestigial postganglionic neurons secrete their transmitter substance directly into the circulating blood. The transmitter substance, acting as a true hormone, is carried by the blood to all tissues of the body.

The Parasympathetic Nervous System Arises from the Brainstem and Spinal Cord

The parasympathetic nervous system generally has long preganglionic and short postganglionic axons. Preganglionic axons of the parasympathetic system leave the CNS by way of cranial nerves III, VII, IX, and X and through several sacral spinal nerves. For this reason, it is called the craniosacral system (see Figure 13-2). The long preganglionic axons pass to parasympathetic ganglia in or near the target organ, where they synapse with short postganglionic neurons. (In the gastrointestinal system, postganglionic neurons participate in an extensive enteric neuron network called the intrinsic gastrointestinal nervous system, described in Chapter 27.)

Most viscera receive both sympathetic and parasympathetic innervation. Although the parasympathetic system originates in brainstem and sacral regions, it can provide parasympathetic innervation to organs in the thoracic and lumbar parts of the body by way of the vagus nerve (cranial nerve X), which wanders to regions of the body cavity. The sympathetic thoracolumbar system can influence organs in cranial and sacral regions by way of preganglionic sympathetic axons that travel to sympathetic postganglionic neurons in cervical and sacral regions of the sympathetic ganglion chain.

Most Autonomic Neurons Secrete Either Acetylcholine or Norepinephrine as a Neurotransmitter

As described in Chapter 5, acetylcholine is the neurotransmitter at the somatic neuromuscular synapse. Acetylcholine is also the neurotransmitter at all autonomic ganglia (Figure 13-4). The neurotransmitter secreted by parasympathetic postganglionic neurons is also acetylcholine. Acetylcholine-releasing synapses are often called cholinergic. Most anatomically sympathetic postganglionic neurons secrete norepinephrine. However, anatomically sympathetic postganglionic neurons traveling to blood vessels of skeletal muscle produce vasodilation and secrete acetylcholine, as do sympathetic, postganglionic neurons to sweat glands in some species. Norepinephrine-releasing synapses are often called adrenergic.

FIGURE 13-4 Classification of autonomic and somatic motor neurons with regard to their transmitter or mediator released, their postsynaptic receptors, and their general influence on the effector organ. Acetylcholine (ACh), released from the presynaptic membrane, can stimulate either a muscarinic (M) or a nicotinic (N) postsynaptic receptor, depending on the particular location of the synapse. Similarly, norepinephrine (Norepi) can stimulate either α or β receptors, again depending on the location of the synapse. Epi, epinephrine; GI, gastrointestinal.

In the case of the adrenal medulla, incoming preganglionic axons release acetylcholine, but the vestigial, postganglionic neurons of the adrenal medullary tissue release primarily epinephrine and some norepinephrine into the circulating blood.

It is important that, once released, the neurotransmitter not linger in the synaptic cleft. The neurotransmitter must be either destroyed in the cleft or dissipated so that the postsynaptic membrane can recover its resting potential and be ready for the next synaptic transmission. Because some synapses can transmit impulses up to several hundred times per second, neurotransmitter destruction must occur quickly. In the case of acetylcholine, acetylcholinesterase destroys the transmitter in the cleft. For norepinephrine, reuptake by the presynaptic neuron is the principal way in which its synaptic effect on the postsynaptic membrane is terminated. The hormonal actions of circulating epinephrine and norepinephrine released by the adrenal medulla, however, are primarily terminated by the enzyme catechol-O-methyltransferase (COMT), with a lesser contribution of the enzyme monoamine oxidase (MAO). These enzymes are widely distributed in the body, with highest concentrations in the liver and kidney.

Acetylcholine and Norepinephrine Have Different Postsynaptic Receptors

The neurotransmitters secreted by the ANS all stimulate their target organ by first binding with a postsynaptic receptor. These receptors are proteins in the cell membrane. When the transmitter binds with the postsynaptic receptor, the membrane’s permeability to selected ions is changed, and the postsynaptic membrane potential either increases or decreases, with a resulting change in the probability of action potentials in the postsynaptic cell.

Acetylcholine stimulates two different types of receptors (Figure 13-4). Muscarinic acetylcholine receptors are found on all the target cells stimulated by postganglionic parasympathetic neurons and cholinergic postganglionic neurons of the sympathetic nervous system. Nicotinic receptors are found at all synapses between autonomic preganglionic and postganglionic neurons and at the somatic neuromuscular junction.

Muscarinic receptors were named because they are stimulated by muscarine, a toadstool poison. Muscarine does not stimulate nicotinic receptors. Nicotine stimulates the nicotinic receptors but not muscarinic receptors. Acetylcholine stimulates both, and different drugs block each receptor. For example, atropine blocks muscarinic receptors, whereas curare blocks nicotinic receptors.

There are two major types of adrenergic receptors, called alpha (α) and beta (β) receptors. The β receptors have been further subdivided into β₁ and β₂ receptors, on the basis of the effect of adrenergic blocking and stimulating drugs. There is now evidence for a third class of β receptor (β₃) and for two classes of α receptor (α₁ and α₂).

There Are General Differences in Sympathetic and Parasympathetic Function

Although the sympathetic and parasympathetic systems are both important for homeostasis—maintaining the constancy of the internal environment—there are some important general differences in their function.

In physical and some emotional stress, the sympathetic (adrenergic) system discharges as a unit, resulting in widespread stimulation of the body. This causes an increase in heart rate and blood pressure, dilation of the pupil of the eye, an elevation in levels of blood glucose and free fatty acids, and an increased state of arousal. All these widespread effects are useful in responding to an emergency. Therefore the sympathetic system is sometimes referred to as the “fight or flight” system. The effect of sympathetic discharge not only is widespread but lasts longer than parasympathetic (cholinergic) discharge because of the prolonged circulation of epinephrine and norepinephrine. Indeed, the adrenal medulla’s secretion of epinephrine and norepinephrine into the circulating blood provides prolonged adrenergic stimulation to the entire body, even to some tissues that do not have direct sympathetic postganglionic stimulation.

Although the adrenergic system usually has a widespread effect, it is also capable of discrete control of particular organs. For example, the dilator smooth muscle in the iris causes enlargement of the pupil in low ambient light without more widespread effects on the body.

In contrast, the parasympathetic (cholinergic) system has more discrete effects on particular organs and is more concerned with the restorative aspects of daily living. For example, cholinergic stimulation assists digestion and absorption of food by increasing gastric secretion, increasing intestinal motility, and relaxing the pyloric sphincter. For this reason, the parasympathetic nervous system is sometimes called the anabolic or restorative nervous system, as well as the “rest and digest” system.

Many organs of the body have both sympathetic (adrenergic) and parasympathetic (cholinergic) innervation, each with a reciprocal effect. For example, adrenergic stimulation increases heart rate, whereas cholinergic stimulation decreases heart rate. Adrenergic stimulation enlarges pupillary diameter, whereas cholinergic stimulation causes pupillary constriction.

Table 13-1 gives a more complete listing of the response of various organs to adrenergic and cholinergic stimulation.

Table 13-1 Responses of Effector Organs to Autonomic Nerve Impulses and Circulating Catecholamines

The Autonomic Nervous System Participates in Many Homeostatic Reflexes

Many of the body’s visceral functions are regulated by autonomic reflexes. As with reflex arcs in the somatic nervous system (see Chapter 7), autonomic reflex arcs also include a sensory side to the arc, including a visceral receptor; a sensory neuron, often called a visceral afferent neuron; and one or more synapses in the CNS. The ANS is usually defined as the peripheral motor preganglionic and postganglionic neurons. Visceral afferent neurons are often not included in this definition but generally are essential parts of the autonomic reflex arc.

Autonomic reflexes are extremely common and are described in detail for each body system in later chapters. A few are described briefly here as examples.

Preganglionic Neurons Are Influenced by Many Regions of the Brain

Much as the lower motor neuron of the somatic system is influenced by the upper motor neuron (see Chapter 9), the preganglionic autonomic neuron is also influenced by CNS axons descending from the brainstem, hypothalamus, and even the cerebral cortex. Many brainstem nuclei are known to influence preganglionic neurons in order to control particular visceral functions; Figure 13-5 illustrates several such regions. In turn, each of these brainstem regions can be influenced by the hypothalamus and cerebral cortex of the forebrain, creating a complex system of upper motor neurons within the CNS that helps coordinate autonomic reflexes and directly influences action potential frequency within preganglionic neurons. As more is learned about these central systems controlling the ANS, their role in such conditions as hypertension and various gastrointestinal diseases may become clearer.

FIGURE 13-5 Autonomic control areas of the brainstem and diencephalon.

(From Guyton AC, Hall JE: Textbook of medical physiology, ed 11, Philadelphia, 2006, Saunders.)

Loss of Autonomic Neurons Results in the Hypersensitivity of the Target Organ to Transmitter

When the postganglionic neuron to a target organ is lost, the smooth muscle of that organ becomes hypersensitive to any transmitter circulating in the blood. For example, the arterioles of the skin are usually under some adrenergic stimulation, which results in some vasoconstriction. If the postganglionic adrenergic neuron to the skin is destroyed, vasodilation occurs. However, if norepinephrine is injected into the blood supply to this area of skin, a dramatic (hypersensitive) vasoconstriction occurs. This is thought to be caused, at least in part, by an increase in numbers of postsynaptic receptors at the denervated synapse.

Horner’s Syndrome