Motor Division

The motor root contains myelinated special visceral efferent fibers for the muscles of facial expression (including the buccinator and the platysma), muscles of the external ear, stapedius, stylohyoid, and posterior belly of the digastric.

UPPER MOTOR NEURON: SUPRANUCLEAR CONTROL

The cortical projections (corona radiata) originate from pyramidal neurons located in the lower third of the precentral gyrus of the frontal motor cortex. The cortical representation of the face, from uppermost cortex to lower, begins with the forehead, followed by the periorbital muscles, midface, and perioral muscles. Motor cortex for the tongue is next in line. These fibers join the corticobulbar tract and pass caudally within the posterior aspect of the internal capsule near its genu. When the fibers reach the midbrain, they join the medial third of the cerebral peduncle. Continuing through the basal portion of the pons with the pyramidal tract, most fibers cross in the caudal pons to reach the contralateral facial motor nucleus. Some fibers descend lower than the nucleus, cross to the other side, and then ascend to that side's nucleus (recurrent bundle of Dejerine). Other fibers, however, never decussate and synapse in the ipsilateral facial nucleus. The cortical fibers for the upper face (i.e., the frontalis, upper portion of the orbicularis oculi, and the corrugator superciliae) project to both the ipsilateral and contralateral facial nucleus. The supranuclear fibers for the lower facial muscles, however, travel only to the contralateral facial nucleus. This anatomical fact explains how unilateral supranuclear lesions characteristically spare the function of the upper face.

Emotional control of facial motion is provided by extrapyramidal input, which travels within the reticular formation from the frontal areas, thalamus, and globus pallidus to the facial motor nucleus. If a supranuclear lesion spares this input, emotional facial expressions (e.g., surprise or pain) are generally preserved despite facial palsy. Conversely, lesions of the midbrain or thalamus can cause contralateral palsy of emotional facial expressions with intact voluntary motion. In addition, lesions of the globus pallidus or its projections to the facial nucleus can cause facial bradykinesia. Despite the resulting masklike facies, some facial motion is detectable, originating from the cortical volitional and other extrapyramidal emotional pathways. '1 , '4]

PONTINE NUCLEUS

The motor nucleus of the facial nerve lies within the reticular formation of the lower third of the pons, bordered dorsally by the trigeminal nucleus and ventromedially by the superior olivary nucleus. Within the motor nucleus, four cell groups have been described that innervate specific muscles. The dorsomedial cell group sends fibers to the auricular and occipital muscles via the posterior auricular nerve. The ventromedial cell group gives rise to the cervical branch, which innervates the platysma. Fibers for the stapedius muscle and the temporal and zygomatic branches originate from the intermediate cell group. The lateral cell group contributes to the buccal and possibly to the mandibular branches. The exact cell group innervating the stylohyoid and posterior belly of the digastric is uncertain, but some feel that the origin may be the dorsal or accessory facial nucleus. '7! ,

The special visceral efferent fibers emerge from the dorsal side of the facial motor nucleus and travel dorsomedially under the floor of the fourth ventricle. In this location, the fibers are called the funiculus teres, and the elevation of this bundle within the upper half of the floor of the ventricle creates the eminentia teres. The motor root fibers extend medially and cephalad towards the abducens

ICO-ANATOMICAL CORRELATION

OF DISORDERS OF CRANIAL NERVE Vll

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