Cranial Nerves

Twelve pairs of cranial nerves arise from the underside of the brain. Except for the first pair, which begins within the cerebrum, these nerves originate from the brain stem. They pass from their sites of origin through foramina of the skull and lead to areas of the head, neck, and trunk.

Although most cranial nerves are mixed nerves, some of those associated with special senses, such as smell and vision, contain only sensory fibers. Others that innervate muscles and glands are primarily composed of motor fibers and have only limited sensory functions. These are neurons associated with certain receptors (proprioceptors) that respond to the rate or degree of contraction of skeletal muscles. Because these fibers contribute directly to motor control, cranial nerves whose only sensory component is from such proprioceptors are usually considered motor nerves. This pertains to cranial nerves III, IV, VI, XI and XII.

Neuron cell bodies to which the sensory fibers in the cranial nerves attach are located outside the brain and are usually in groups called ganglia (sing., ganglion). On the other hand, motor neuron cell bodies are typically located within the gray matter of the brain.

Cranial nerves are designated by numbers or name. The numbers indicate the order in which the nerves arise from the brain, from anterior to posterior. The names

Cranial Blood Vessels

Figure 11.25

Scanning electron micrograph of a peripheral nerve in cross section (350x). Note the bundles or fascicles of nerve fibers. Fibers include axons of motor neurons as well as peripheral processes of sensory neurons.

Figure 11.25

Scanning electron micrograph of a peripheral nerve in cross section (350x). Note the bundles or fascicles of nerve fibers. Fibers include axons of motor neurons as well as peripheral processes of sensory neurons.

Copyright by R.G. Kessel and R.H. Kardon, Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy. 1979 (W.H. Freeman & Co.).

Olfactory bulb -Olfactory tract

Optic tract

Vestibulocochlear (VIII) —

Hypoglossal (XII)-

Oculomotor (III) Trochlear (IV) Trigeminal (V)

- Facial (VII) ■ Glossopharyngeal (IX) - Accessory (XI)

Figure 11.26

The cranial nerves, except for the first pair, arise from the brain stem. They are identified either by numbers indicating their order, their function, or the distribution of their fibers.

describe primary functions or the general distribution of their fibers (fig. 11.26).

The first pair of cranial nerves, the olfactory nerves (I), are associated with the sense of smell and contain only sensory neurons. These neurons synapse with bipolar neurons, located in the lining of the upper nasal cavity, that serve as olfactory receptor cells. Axons from these receptors pass upward through the cribriform plates of the ethmoid bone. The synapses occur in the olfactory bulbs, which are extensions of the cerebral cortex, located just beneath the frontal lobes. Sensory impulses travel from the olfactory bulbs along olfactory tracts to cerebral centers where they are interpreted. The result of this interpretation is the sensation of smell.

The second pair, the optic nerves (II), are sensory and lead from the eyes to the brain and are associated

Lacrimal nerve

Ophthalmic division

Maxillary division

Mandibular division

Lingual nerve

Inferior alveolar nerve

Lacrimal nerve

Ophthalmic division

Maxillary division

Mandibular division

Lingual nerve

Inferior alveolar nerve

Nerf Lacrymal

Lacrimal gland Eye

Infraorbital nerve Maxilla

Tongue

Mental nerve Mandible

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.

Lacrimal gland Eye

Infraorbital nerve Maxilla

Tongue

Mental nerve Mandible

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