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Figure

Each trigeminal nerve has three large branches that supply various regions of the head and face: the ophthalmic division, the maxillary division, and the mandibular division.

with vision. The cell bodies of these neurons form ganglion cell layers within the eyes, and their axons pass through the optic foramina of the orbits and continue into the visual nerve pathways of the brain (see chapter 12, p. 495).

The third pair, the oculomotor (ok"u-lo-mo'tor) nerves (III), arise from the midbrain and pass into the orbits of the eyes. One component of each nerve connects to a number of voluntary muscles, including those that raise the eyelids and four of the six muscles that move the eye.

A second portion of each oculomotor nerve is part of the autonomic nervous system, supplying involuntary muscles inside the eyes. These muscles help adjust the amount of light that enters the eyes and help focus the lenses of the eyes. This nerve is considered motor, with some proprioceptive fibers.

The fourth pair, the trochlear (trok'le-ar) nerves

(IV), are the smallest cranial nerves. They arise from the midbrain and carry motor impulses to a fifth pair of external eye muscles, the superior oblique muscles, which are not supplied by the oculomotor nerves. This nerve is considered motor, with some proprioceptive fibers.

The fifth pair, the trigeminal (tri-jem'i-nal) nerves

(V), are the largest of the cranial nerves and arise from the pons. They are mixed nerves, with the sensory portions more extensive than the motor portions. Each sensory component includes three large branches, called the ophthalmic, maxillary, and mandibular divisions (fig. 11.27).

The ophthalmic division consists of sensory fibers that bring impulses to the brain from the surface of the eye, the tear gland, and the skin of the anterior scalp, forehead, and upper eyelid. The fibers of the maxillary division carry sensory impulses from the upper teeth, upper gum, and upper lip, as well as from the mucous lining of the palate and facial skin. The mandibular division includes both motor and sensory fibers. The sensory branches transmit impulses from the scalp behind the ears, the skin of the jaw, the lower teeth, the lower gum, and the lower lip. The motor branches supply the muscles of mastication and certain muscles in the floor of the mouth.

The sixth pair, the abducens (ab-du'senz) nerves (VI), are quite small and originate from the pons near the medulla oblongata. They enter the orbits of the eyes and supply motor impulses to the remaining pair of external eye muscles, the lateral rectus muscles. This nerve is considered motor, with some proprioceptive fibers.

Temporal nerve

Zygomatic nerve Buccal nerve Facial nerve

Posterior auricular nerve

Parotid salivary gland

Mandibular nerve

Cervical nerve

Salivary Gland Facial Nerve

Figure

The facial nerves are associated with taste receptors on the tongue and with muscles of facial expression.

Posterior auricular nerve

Parotid salivary gland

Mandibular nerve

Cervical nerve ure U.ZE

Figure

The facial nerves are associated with taste receptors on the tongue and with muscles of facial expression.

A disorder of the trigeminal nerve called trigeminal neuralgia (tic douloureux) causes severe recurring pain in the face and forehead on the affected side. If drugs cannot control the pain, surgery may be used to sever the sensory portion of the nerve. However, the patient loses sensations in other body regions that the sensory branch supplies. Consequently, after such surgery, care must be taken when eating or drinking hot foods or liquids, and the mouth must be inspected daily for food particles or damage to the cheeks from biting.

The seventh pair of cranial nerves, the facial (fa'shal) nerves (VII), are mixed nerves that arise from the lower part of the pons and emerge on the sides of the face. Their sensory branches are associated with taste receptors on the anterior two-thirds of the tongue, and some of their motor fibers transmit impulses to muscles of facial expression. Still other motor fibers of these nerves function in the autonomic nervous system by stimulating secretions from tear glands and certain salivary glands (submandibular and sublingual glands) (fig. 11.28).

The eighth pair, the vestibulocochlear (ves-tib"u-lo-kok'le-ar) nerves (VIII, acoustic, or auditory, nerves), are sensory nerves that arise from the medulla oblongata. Each of these nerves has two distinct parts—a vestibular branch and a cochlear branch.

The neuron cell bodies of the vestibular branch fibers are located in ganglia near the vestibule and semicircular canals of the inner ear. These structures contain receptors that sense changes in the position of the head and, in response, initiate and send impulses to the cerebellum, where they are used in reflexes that maintain equilibrium.

The neuron cell bodies of the cochlear branch fibers are located in a ganglion of the cochlea, a part of the inner ear that houses the hearing receptors. Impulses from this branch pass through the medulla oblongata and midbrain on their way to the temporal lobe, where they are interpreted.

The ninth pair, the glossopharyngeal (glos"o-fah-rin'je-al) nerves (IX), are associated with the tongue and pharynx. These nerves arise from the medulla oblongata, and, although they are mixed nerves, their predominant fibers are sensory. These fibers carry impulses from the lining of the pharynx, tonsils, and posterior third of the tongue to the brain. Fibers in the motor component of the glossopharyngeal nerves innervate constrictor muscles in the wall of the pharynx that function in swallowing.

The tenth pair, the vagus (va'gus) nerves (X), originate in the medulla oblongata and extend downward through the neck into the chest and abdomen. These nerves are mixed, containing both somatic and auto-nomic branches, with the autonomic fibers predominant.

Among the somatic components of the vagus nerves are motor fibers that carry impulses to muscles of the larynx. These fibers are associated with speech and swallowing reflexes that employ muscles in the soft palate and pharynx. Vagal sensory fibers carry impulses from the linings of the pharynx, larynx, and esophagus and from the viscera of the thorax and abdomen to the brain. Autonomic motor fibers of the vagus nerves supply the heart and many smooth muscles and glands in the viscera of the thorax and abdomen (fig. 11.29).

The eleventh pair, the accessory (ak-ses'o-re) nerves (XI, spinal accessory), originate in the medulla oblongata and the spinal cord; thus, they have both cranial and spinal branches. Each cranial branch of an accessory nerve joins a vagus nerve and carries impulses to muscles of the soft palate, pharynx, and larynx. The spinal branch descends into the neck and supplies motor fibers to the trapezius and sternocleidomastoid muscles. This nerve is considered motor, with some proprioceptive fibers.

The twelfth pair, the hypoglossal (hi"po-glos'al) nerves (XII), arise from the medulla oblongata and pass into the tongue. They primarily consist of fibers that carry impulses to muscles that move the tongue in speaking, chewing, and swallowing. This nerve is considered motor, with some proprioceptive fibers. Table 11.9 summarizes the functions of the cranial nerves.

Meningeal branch Auricular branch

Palate

Superior laryngeal nerve

Recurrent laryngeal nerve

Liver

Meningeal branch Auricular branch

Palate

Liver

Kidney

Peripheral Nervous System Definition

Spleen

Pancreas

Small intestine

The vagus nerves (only the left vagus is shown) extend from the medulla oblongata downward into the chest and abdomen to supply many organs.

Spleen

Pancreas

Kidney

Small intestine ure 11.29

Figure

The vagus nerves (only the left vagus is shown) extend from the medulla oblongata downward into the chest and abdomen to supply many organs.

U Define peripheral nervous system.

^9 Distinguish between somatic and autonomic nerve fibers.

^9 Describe the structure of a peripheral nerve.

^9 Distinguish among sensory, motor, and mixed nerves.

Q Name the cranial nerves, and list the major functions of each.

Essentials of Human Physiology

Essentials of Human Physiology

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