Central Innervation of Motor Cranial Nerves

Abstract

Each cranial nerve (CN) nucleus has central innervation. The unique neuroanatomical patterns of central innervation of motor CNs have important clinical and functional implications. Voluntary horizontal eye movements, mediated by CNs III and VI, are yoked due to connections between the frontal eye fields, pontine paramedian reticular formation (PPRF), medial longitudinal fasciculus (MLF), and their brainstem nuclei, resulting in different lateral eye deviation patterns depending on destructive versus stimulating processes affecting the frontal eye field, or destructive lesions of the pons or MLF. For facial movements mediated by CN VII, due to the pattern of bilateral corticobulbar projections from the motor cortex to the portions of the facial nucleus in the pons subserving movement of the upper part of the face and contralateral projections to the portion of the facial nucleus subserving movement to the lower part of the face, muscles of the forehead typically do not become weak in the setting of most central hemispheric lesions, such as stroke, while lesions of the facial nucleus or CN VII result in weakness of the entire ipsilateral face. The movement of the muscles of the palate and larynx that promote swallowing, mediated by CNs IX and X, receive input from bilateral motor cortices through corticobulbar projections to the contralateral and ipsilateral nucleus ambiguus of both nerves, making development of dysphagia from a unilateral hemispheric lesion unlikely. Similar to CNs IX and X, muscles promoting tongue movement mediated by CN XII receive input from bilateral motor cortices via corticobulbar projections to the hypoglossal nuclei in the medulla, making significant tongue weakness unlikely to develop from a unilateral hemispheric lesion. Functional implications of these motor CN patterns include difficulty voluntarily moving one eye, one side of the upper face, or one side of the larynx or palate independently.

Authors of this chapter: Carrie Katherine Grouse and Steven L. Lewis.