In addition to the central nervous system components (brain and spinal cord), the nervous system has a peripheral component, made up of the cranial nerves, spinal nerves, and autonomic nervous system. The cranial nerves (12 pairs) (figure 10.6) and the autonomic nervous system (figure 10.7) have their origins in the cranium and are therefore closely associated with the central nervous system. The spinal nerves (cervical, thoracic, lumbar, sacral, coccygeal) are discussed in chapter 1 and other chapters relevant to the anatomical areas they innervate.
The autonomic nervous system has two specific systems within it: the sympathetic (also known as the thoracolumbar system) and parasympathetic (also known as the craniosacral system). These systems, when stimulated, reflexively control the activity of all structures of the body that are not voluntarily controlled. Nerves conducting impulses to voluntary muscles are referred to as motor nerves; nerves conducting impulses to the heart, smooth muscle tissue, and various glands of the body are considered part of the autonomic nervous system. The nerves consist of efferent, afferent, and mixed fibers that conduct nerve impulses both to (efferent) and from (afferent) the central nervous system.
As can be observed from their alternative names, the cells of these nerve fibers originate from various levels of the central nervous system. The nerves of the sympathetic system originate from the gray matter of the spinal cord from the first thoracic to the third lumbar level.
The 12 pairs of cranial nerves (table 10.1) contain both efferent (motor) and afferent (sensory) nerve fibers and also function in some aspects of the parasympathetic system.
The first pair of cranial nerves is the olfactory, running from the upper aspect of the nose to the temporal lobe where the center of smell is located. The second pair of cranial nerves is the optic. Impulses are conducted between the retina and the center for vision located in the occipital lobe. Both the olfactory and optic cranial nerves are considered sensory nerves; the oculomotor (3rd cranial), trochlear (4th cranial), and abducens (6th cranial) are motor nerves that innervate the voluntary muscles of the eye. The trigeminal (5th cranial) nerve has sensory fibers that sense warmth, cold, pressure, and pain in the teeth and skin of the face and motor fibers that innervate the muscles of mastication. The facial (7th cranial) and the glossopharyngeal (9th cranial) nerves have sensory fibers connecting the brain with the taste buds of the tongue and the pharynx. The motor fibers of the glossopharyngeal innervate the muscles of the tongue and pharynx, and the facial innervates the muscles of the face. The auditory (8th cranial) nerve is sensory in nature and has two parts: one for hearing (located in the cochlea) connecting to the temporal lobe, and one for equilibrium (located in the semicircular canals) connecting with the cerebellum. The vagus or pneumogastric (10th cranial) nerve contains both sensory and motor fibers. The sensory fibers involve the heart, lungs, trachea, pharynx, larynx, esophagus, and gastrointestinal tract. The motor fibers of the vagus nerve innervate the muscles of the soft palate, pharynx, larynx, trachea, esophagus, stomach, intestines, other abdominal viscera, and the heart. The spinal accessory (11th cranial) and the hypoglossal (12th cranial) are motor nerves originating in the brain stem and innervating the tongue, face, and neck.