Motor cortical control of human masticatory muscles

Abstract

The trigeminally innervated masticatory muscles elevate and depress the mandible and are involved in chewing, swallowing, and speech. The motor cortex has been implicated in the initiation and subsequent control of movement of the mandible through descending corticobulbar projections to the trigeminal motor nuclei, but there are few details on the organization or operation of this pathway in humans. This chapter uses unilateral focal transcranial magnetic stimulation (TMS) to examine the nature of the projections from motor cortex to jaw-closer (masseter) and jaw-opener (anterior digastric) motoneuron pools in humans. Responses evoked by TMS in masseter and anterior digastric muscles are studied at both the whole-muscle and single motor-unit levels to provide information regarding (1) the existence of bilateral projections to the motor pools from a single hemisphere, (2) the relative strength of excitatory projections from the ipsi- and contra-lateral hemisphere, (3) the capacity for differential control of a single motor pool through the corticobulbar projection from each hemisphere, and (4) the nature of the corticobulbar projections to trigeminal motoneurons (mono- or oligosynaptic).