The Peripheral Nervous System
The peripheral nervous system includes the cranial nerves, the spinal nerves, and the autonomic nervous system.
There are 12 pairs of cranial nerves that emerge from the lower surface of the brain and pass through the foramina in the skull. Three are entirely sensory (I, II, VIII), five are motor (III, IV, VI, XI, and XII), and four are mixed (V, VII, IX, and X) as they have both sensory and motor functions (Downey & Leigh, 1998; Hickey, 2003). The cranial nerves are numbered in the order in which they arise from the brain. For example, cranial nerves I andattach in the cerebral hemispheres, whereas cranial nerves IX, X, XI, and XII attach at the medulla (Fig. 60-9). Most cranial nerves innervate the head, neck, and special sense structures. Table 60-2 lists the names and primary functions of the cranial nerves.
The spinal cord is composed of 31 pairs of spinal nerves: 8 cervi-cal, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Each spinal nerve has a ventral root and a dorsal root (Fig. 60-10).
The dorsal roots are sensory and transmit sensory impulses from specific areas of the body known as dermatomes (Fig. 60-11) to the dorsal ganglia. The sensory fiber may be somatic, carrying information about pain, temperature, touch, and position sense (proprioception) from the tendons, joints, and body surfaces; or visceral, carrying information from the internal organs.
The ventral roots are motor and transmit impulses from the spinal cord to the body. These fibers are also either somatic or vis-ceral. The visceral fibers include autonomic fibers that control the cardiac muscles and glandular secretions.
The autonomic nervous system regulates the activities of inter-nal organs such as the heart, lungs, blood vessels, digestive organs, and glands. Maintenance and restoration of internal homeostasis is largely the responsibility of the autonomic nervous system. There are two major divisions: the sympathetic nervous system, with predominantly excitatory responses, most notably the “fight or flight” response, and the parasympathetic nervous system, which controls mostly visceral functions.
The autonomic nervous system innervates most body organs. Although usually considered part of the peripheral nervous sys-tem, it is regulated by centers in the spinal cord, brain stem, and hypothalamus.
The autonomic nervous system has two neurons in a series extending between the centers in the CNS and the or-gans innervated. The first neuron, the preganglionic neuron, is located in the brain or spinal cord, and its axon extends to the autonomic ganglia. There, it synapses with the second neuron, the postganglionic neuron, located in the autonomic ganglia, and its axon synapses with the target tissue and innervates the effector organ. Its regulatory effects are exerted not on individual cells but on large expanses of tissue and on entire organs. The responses elicited do not occur instantaneously but after a lag period. These responses are sustained far longer than other neurogenic re-sponses to ensure maximal functional efficiency on the part of receptor organs, such as blood vessels.
The quality of these responses is explained by the fact that the autonomic nervous system transmits its impulses by way of nerve pathways, enhanced by chemical mediators, resembling in this re-spect the endocrine system. Electrical impulses, conducted through nerve fibers, stimulate the formation of specific chemical agents at strategic locations within the muscle mass; the diffusion of these chemicals within the muscle is responsible for the contraction.
The hypothalamus is the major subcortical center for the regulation of visceral and somatic activities, with an inhibitory– excitatory role in the autonomic nervous system. The hypothal-amus has connections that link the autonomic system with the thalamus, the cortex, the olfactory apparatus, and the pituitary gland. Located here are the mechanisms for the control of visceral and somatic reactions that were originally important for defense or attack, and are associated with emotional states (eg, fear, anger, anxiety); for the control of metabolic processes, including fat, carbohydrate, and water metabolism; for the regulation of body temperature, arterial pressure, and all muscular and glandular ac-tivities of the gastrointestinal tract; for control of genital functions; and for the sleep cycle.
The autonomic nervous system is separated into the anatom-ically and functionally distinct sympathetic and parasympathetic divisions. Most of the tissues and the organs under autonomic control are innervated by both systems. Sympathetic stimuli are mediated by norepinephrine and parasympathetic impulses are mediated by acetylcholine. These chemicals produce opposing and mutually antagonistic effects. Both divisions produce stimu-latory and inhibitory effects. For example, the parasympathetic division causes contraction (stimulation) of the urinary bladder muscles and a decrease (inhibition) in heart rate, whereas the sympathetic division produces relaxation (inhibition) of the uri-nary bladder and an increase (stimulation) in the rate and force of the heartbeat. Table 60-3 compares the sympathetic and the parasympathetic effects on the different systems of the body.
The sympathetic division of theautonomic nervous system is best known for its role in the body’s “fight-or-flight” response. Under stress conditions from either physical or emotional causes, sympathetic impulses increase greatly. As a result, the bronchioles dilate for easier gas exchange; the heart’s contractions are stronger and faster; the arteries to the heart and voluntary muscles dilate, carrying more blood to these organs; peripheral blood vessels constrict, making the skin feel cool but shunting blood to essential organs; the pupils dilate; the liver releases glucose for quick energy; peristalsis slows; hair stands on end; and perspiration increases. The sympathetic neurotrans-mitter is norepinephrine (noradrenaline), and this increase in sympathetic discharge is the same as if the body has been given an injection of adrenalin—hence, the term adrenergic is often used to refer to this division.
Sympathetic neurons are located in the thoracic and the lum-bar segments of the spinal cord; their axons, or the preganglionic fibers, emerge by way of anterior nerve roots from the eighth cer-vical or first thoracic segment to the second or third lumbar seg-ment. A short distance from the cord, these fibers diverge to join a chain, composed of 22 linked ganglia, that extends the entire length of the spinal column, adjacent to the vertebral bodies on both sides. Some form multiple synapses with nerve cells within the chain. Others traverse the chain without making connections or losing continuity to join large “prevertebral” ganglia in the tho-rax, the abdomen, or the pelvis or one of the “terminal” ganglia in the vicinity of an organ, such as the bladder or the rectum (Fig. 60-12). Postganglionic nerve fibers originating in the sym-pathetic chain rejoin the spinal nerves that supply the extremities and are distributed to blood vessels, sweat glands, and smooth muscle tissue in the skin. Postganglionic fibers from the prever-tebral plexuses (eg, the cardiac, pulmonary, splanchnic, and pelvic plexuses) supply structures in the head and neck, thorax, abdomen, and pelvis, respectively, having been joined in these plexuses by fibers from the parasympathetic division.
The adrenal glands, kidneys, liver, spleen, stomach, and duo-denum are under the control of the giant celiac plexus, commonly known as the solar plexus. This receives its sympathetic nerve components by way of the three splanchnic nerves, composed of preganglionic fibers from nine segments of the spinal cord (T4 to L1), and is joined by the vagus nerve, representing the parasym-pathetic division. From the celiac plexus, fibers of both divisions travel along the course of blood vessels to their target organs.
Certain syndromes are distinctive todiseases of the sympathetic nerve trunks. Among these are dila-tion of the pupil of the eye on the same side as a penetrating wound of the neck (evidence of disturbance of the cervical sym-pathetic cord); temporary paralysis of the bowel (indicated by the absence of peristaltic waves and the distention of the intestine by gas) after fracture of any one of the lower dorsal or upper lumbar vertebrae with hemorrhage into the base of the mesentery; and the marked variations in pulse rate and rhythm that often follow compression fractures of the upper six thoracic vertebrae.
The parasympathetic nervoussystem functions as the dominant controller for most visceral ef-fectors. During quiet, nonstressful conditions, impulses from parasympathetic fibers (cholinergic) predominate. The fibers of the parasympathetic system are located in two sections, one in the brain stem and the other from spinal segments below L2. Be-cause of the location of these fibers, the parasympathetic system is referred to as the craniosacral division, as distinct from the tho-racolumbar (sympathetic) division of the autonomic nervous system.
The parasympathetic nerves arise from the midbrain and the medulla oblongata. Fibers from cells in the midbrain travel with the third oculomotor nerve to the ciliary ganglia, where post-ganglionic fibers of this division are joined by those of the sym-pathetic system, creating controlled opposition, with a delicate balance maintained between the two at all times.