Facial Nerve

Cranial nerve VII contains four ofthe five components present in the vagal and glossopharyngeal nerves, namely, GSA, SVA, GVE, and SVE. The seventh nerve can be divided into two parts—the intermediate nerve (of Wrisberg), which comprises the GSA, SVA, and GVE components, and the facial branchial motor nerve, which is the SVE component. The two sensory components of VII both have cell bodies located in the geniculate ganglion. As is also the case for cranial nerves IX and X, the GSA component of VII is minor, innervating a small area of skin behind the ear and the external auditory meatus. The GSA fibers terminate in the spinal nucleus of V, for which the ascending sensory pathway is discussed later. Facial SVA fibers innervate taste buds that lie on the anterior two-thirds of the tongue via the chorda tympani nerve. These neurons project to the gustatory nucleus, which, as discussed previously, projects to VPMpc in the dorsal thalamus for relay to gustatory cortical areas.

Special visceral efferent fibers arise in the branchial motor nucleus of VII, which lies ventrolateral to the abducens (VI) nucleus in the pons. The axons of motor VII initially follow a dorsomedial course within the pons and then turn laterally and curve over the dorsal surface of the abducens nucleus, forming the internal genu of the nerve. This course is due to ventrolateral migration of the neuron cell bodies during embryolo-gical development. The facial motor fibers then traverse the pons in a ventrolateral direction to exit on its lateral surface near the junction of the pons with the medulla. The facial branchial motor nerve innervates muscles embryologically derived from the muscles of the branchial arches, including the platysma (a superficial muscle of the skin of the neck), buccinator (which forms the cheek), stapedius (which inserts on the stapes bone within the middle ear), auricular and occipital muscles, and the muscles of facial expression. An additional component supplies efferent innervation to the inner ear. The branchial motor nucleus of VII receives afferent input from structures that include the spinal nucleus of V for corneal and other trigemi-nofacial reflexes, auditory input from the superior olivary complex for facial reflexes to loud noise (including closing the eyes and the stapedius reflex to damp sound transmission through the middle ear ossicles), and corticobulbar input, which is bilateral to the facial motor neurons for the forehead and upper face but predominantly contralateral for the lower face and mouth region.

The GVE component of the facial nerve arises from neurons in the superior salivatory nucleus. Some preganglionic parasympathetic fibers project via the greater petrosal nerve to the pterygopalatine ganglion for innervation of the lacrimal gland. Other pregan-glionic fibers project via the chorda tympani nerve to the submandibular ganglion for innervation of the sublingual and submandibular salivary glands. Damage to cranial nerve VII results in deficits according to the location of the lesion and the components involved. Sensory loss of taste to the anterior two-thirds of the tongue occurs with involvement of the SVA component, and loss of production of tears from the lacrimal gland and saliva from the sublingual and submandibular salivary glands occurs with involvement of the GVE component. Damage to the nucleus or nerve for the SVE motor VII component results in paralysis of all the muscles of the ipsilateral face, whereas a supranuclear lesion affecting corticobulbar input results in paralysis of only the lower part of the face on the contralateral side. Motor VII damage also results in sounds being abnormally loud due to loss of the stapedius reflex.

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