Facial (VII) Nerve

THE MOTOR DIVISION

Fibers arise from the motor facial nucleus, located in the reticular formation of the lowest part of the pons. The nucleus is posterior to the superior olive, medial to the nucleus of the spinal tract of the trigeminal nerve, and anterolateral to the nucleus of the abducens nerve. The supranuclear control of facial movements occurs through the corticonuclear fibers originating in the precentral gyrus. These fibers course through the corona radiata, genu of the internal capsule, and the medial portion of the cerebral peduncle to the pons. The posterior portion of the facial nucleus controls the upper facial musculature and receives bilateral supranuclear input, while the anterior facial nucleus controls the lower facial muscles and receives predominantly contralateral input. Supranuclear lesions, such as with stroke, would therefore produce a pattern of contralateral predominantly lower facial weakness. The efferent fibers of the motor nucleus form a motor root and course around the abducens nucleus superiorly and exit the brainstem laterally in the cerebellopontine angle. The motor root travels with the nervus intermedius and CN VIII in the cerebellopontine angle and enters the internal auditory meatus of the temporal bone. Within the temporal bone, there are four portions of the facial nerve. (1) In the meatal (canal) segment, the motor division is on the superoanterior surface of CN VIII, and the nervus intermedius is in between them. (2) In the labyrinthine segment, the motor root, and nervus intermedius enter the facial canal in the petrous bone. The labyrinthine segment passes above the labyrinth and reaches the geniculate ganglion, which contains the sensory fibers of the nervus intermedius. Here the greater superficial petrosal nerve arises from the geniculate ganglion. This nerve is composed of preganglionic parasympathetic efferents that innervate the nasal, lacrimal, and palatal glands via the pterygopalatine ganglion. The greater superficial petrosal nerve also carries sensory fibers from the external auditory meatus, lateral pinna, and mastoid. (3) The horizontal (tympanic) segment contains the facial nerve as it runs horizontally backward below and medial to the horizontal semicircular canal. (4) In the mastoid (vertical) segment the facial nerve bends inferiorly. The nerve to the stapedius muscle branches off in this segment. The chorda tympani also branches off here and joins the lingual nerve. It contains the preganglionic parasympathetic fibers from the superior salivatory nucleus and innervates the submandibular and sublingual glands via the submaxillary ganglion. The chorda tympani also contains afferent taste fibers from the anterior two thirds of the tongue that then continue on to the nucleus of the solitary tract. CN VII exits the facial canal through the stylomastoid foramen and gives off the posterior auricular nerve (to the posterior auricular, transverse and oblique auricular muscles, and occipitalis) and digastric and stylohyoid branches. The facial nerve pierces the parotid gland and divides into temporofacial and cervicofacial branches, which further divide into temporofrontal, zygomatic, buccal, marginal mandibular, and cervical branches.

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