The Human Body

The Nerve Network

Cranial Nerves
Whereas most major nerves emerge from the spinal cord, the 12 pairs of cranial nerves project directly from the brain. All but 1 pair relay motor or sensory information (or both); the tenth, or vagus nerve, affects visceral functions such as heart rate, vasoconstriction, and contraction of the smooth muscle found in the walls of the trachea, stomach, and intestine.

The cranial nerves connect to the brain by passing through openings in the skull, or cranium. Nerves associated with the spinal cord pass through openings in the vertebral column and are called spinal nerves. Both cranial and spinal nerves consist of large numbers of processes that convey impulses to the central nervous system and also carry messages outward; the former processes are called afferent, the latter are called efferent. Afferent impulses are referred to as sensory; efferent impulses are referred to as either somatic or visceral motor, according to what part of the body they reach. Most nerves are mixed nerves made up of both sensory and motor elements.

The cranial and spinal nerves are paired; the number in humans are 12 and 31, respectively. Cranial nerves are distributed to the head and neck regions of the body, with one conspicuous exception: the tenth cranial nerve, called the vagus. In addition to supplying structures in the neck, the vagus is distributed to structures located in the chest and abdomen. Vision, auditory and vestibular sensation, and taste are mediated by the second, eighth, and seventh cranial nerves, respectively. Cranial nerves also mediate motor functions of the head, the eyes, the face, the tongue, and the larynx, as well as the muscles that function in chewing and swallowing. Spinal nerves, after they exit from the vertebrae, are distributed in a bandlike fashion to regions of the trunk and to the limbs. They interconnect extensively, thereby forming the brachial plexus, which runs to the upper extremities; and the lumbar plexus, which passes to the lower limbs.