The supranuclear input to CN XI and XII is not fully understood. Although input is thought to be bihemispherical in general, some muscles probably receive predominantly ipsilateral input (SCM), whereas others receive predominantly contralateral input (trapezius, genioglossus). A destructive unilateral supranuclear lesion (e.g., infarction,
hemorrhage, tumor, multiple sclerosis, brain abscess) may result in some weakness of neck and tongue musculature so that the chin deviates toward the lesion. Atrophy and fasciculations are not present in an upper motor neuron lesion. y , y An irritative unilateral supranuclear lesion (e.g., a seizure focus) may result in forced deviation of the head or tongue away from the lesion. y
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