Corticobulbar projections and orofacial and muscle afferent inputs of neurons in primate sensorimotor cerebral cortex.

In barbiturate-anesthetized monkeys, single cortical neurons were found that could be antidromically activated by brain stem stimulation in the contralateral trigeminal motor, facial, and hypoglossal nuclei. The corticobulbar neurons were localized in the precentral gyrus and anterior bank of the central sulcus, and 20% of them could be excited by muscle afferent stimulation; none were excited by cutaneous or intraoral stimuli. However, many "nonprojection" neurons situated at the bottom of the anterior bank of the central sulcus, especially in area 3a, received a low-threshold afferent input from jaw, facial, and tongue muscles, and neurons with cutaneous or intraoral afferent inputs predominated further caudally, in areas 3b and 1. The results provided electrophysiologic evidence of a direct projection from primate face motor cortex to the brain stem and of an excitatory input from low-threshold afferent fibers of the jaw, facial, and tongue muscles to the primate sensorimotor cortex.