The oral-facial region is innervated by nerve cells whose axonal origins and terminations lie in the brain stem of the central nervous system. The brain stem extends from the medulla to the midbrain and is a direct extension of the spinal cord. The central canal of the spinal cord continues forward into the medulla to form the fourth ventricle. In the floor of the fourth ventricle, the sensory areas lie dorsolateral and the motor areas, mainly ventromedial. These areas represent the nuclei of origin and termination of the various branchial or cranial nerves innervating the face and mouth. Some of these structures are just special differentiations of dorsal and ventral spinal roots. Others have very different embryonic origins. For example, a special visceral efferent group of nerves arises that supplies the striated gill musculature of lower vertebrates. These branchial muscles form the muscles of facial expression and jaw muscles. They are striated and resemble spinal segmental musculature. However, because of their primitive relationship to the viscera, they are regarded as special visceral muscles. On the sensory side, the VII, IX, and X cranial nerves mainly carry impulses concerned with general sensations from visceral surfaces and from special receptors such as taste buds. The sensory innervation of the trigeminal or V nerve is quite different. It is without a visceral sensory component and exhibits a hypertrophy of innervation of cutaneous structures. The caudal extension of the descending root of V replaces the reduced somatic sensory fibers in other branchial nerves and becomes contiguous with the sensory nuclei of the cervical spinal cord. In addition, sensory nerves arising from muscles often have their cell bodies in the central nervous system and are anatomically associated with central projections of other sensory nerves (e.g., optic and vestibular) involved in conveying information about the position of the body relative to near and distant objects in visual space.
KeywordsFourth Ventricle Cervical Spinal Cord Unmyelinated Fiber Tooth Pulp Golgi Tendon Organ
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