Xii

Hypoglossal

Three purely motor nerves that innervate the muscles of the eye are collectively called oculomotor nerves and comprise the oculomotor nerve (III), the trochlear nerve (IV), and the abducens nerve (VI). The 4 nerves with both sensory and motor components that innervate the jaws, face, throat, and the thoracic and abdominal viscera are the trigeminal nerve (V), the facial nerve (VII), the glossopharyngeal nerve (IX), and the vagus nerve (X). Cranial nerve VIII, the vestibulocochlear (or statoacoustic) nerve, innervates the inner ear organs for the auditory and vestibular senses. Cranial nerves XI, the spinal accessory nerve, and XII, the hypoglossal nerve, are purely motor and innervate the muscles of the neck that are used to turn the head (the sternocleidomastoid and upper part of the trapezius) and the muscles of the tongue, respectively.

Most of the sensory cranial nerves are formed by the processes of bipolar (or pseudounipolar) neurons whose cell bodies lie within one or two sensory ganglia located on the nerve in the peripheral part of the nervous system. The sensory neurons each have a distal portion that either innervates a separate receptor cell or has a modified ending that itself is the receptor. Sensory transduction—the translation of the sensory stimulus into neuronal activity—involves a variety of physical and chemical mechanisms. The proximal portions of the bipolar sensory neurons project to a group of multipolar neurons that lie within nuclei in the central nervous system and in turn project to other groups of multipolar neurons in the sensory pathway. For each sensory system pathway, the bipolar-receptive, multipolar neurons are referred to here as firstorder multipolar neurons since they are the first of several groups of multipolar neurons in the pathway. The sensory nuclei that contain the first-order multipolar neurons are named for the name of their major cranial nerve input (vestibular, cochlear, and trigem-inal nuclei), the particular sensory system (gustatory nucleus), or for their appearance (solitary nucleus).

The motor cranial nerves innervate either muscles or glands. Most of the nerves that innervate muscles have cell bodies within their respective nuclei in the brain stem; their axons exit the brain in the cranial nerve and terminate directly on the particular muscle. Most of the nuclei of these nerve components have the same name as the nerve (oculomotor, trochlear, trigeminal motor, abducens, facial motor, and hypoglossal nuclei) or arise from a nucleus named for its indistinct appearance (nucleus ambiguus). Glands are innervated via a two-neuron chain of neurons that belong to the parasympathetic division of the autonomic nervous system. The cell bodies of the first neurons in the chain, called preganglionic parasympathetic neurons, lie within nuclei in the brain stem, and their axons exit the brain in the cranial nerve. These axons terminate on a second set of neurons that lie in a ganglion located close to the target organ and are called postganglionic parasympathetic neurons in reference to their axons, which exit the ganglion and innervate the gland. Three small muscles within the eye are also innervated by the autonomic nervous system. Two of these intraocular muscles (for pupillary constriction and control of the shape of the lens) are innervated by the axons of postganglionic parasympa-thetic neurons, whereas the third (for pupillary dilation) is innervated by postganglionic axons that are part of the sympathetic division of the autonomic nervous system, which arises from neurons located within thoracic and upper lumbar spinal cord segments. The parasympathetic cell groups of the brain stem have a variety of names, including the eponymic Edinger-Westphal nucleus, the superior and inferior salivatory nuclei, the dorsal motor nucleus of X, and some of the neurons in nucleus ambiguus.

The motor nuclei of the brain stem all receive a variety of inputs from other neuron cell groups in the brain. These inputs include relatively local connections with reticular formation neurons and long, descending projections from motor regions of neocortex. Since the latter connections arise from neurons located above (rostral to) the cranial nerve nuclei, they are referred to as supranuclear connections. The majority of supra-nuclear inputs to cranial nerve nuclei are bilateral.

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