Motor cortical control of human masticatory muscles

The corticotrigeminal projections to masseter and anterior digastric motoneuron pools that are activated by TMS are bilateral, but not symmetrical. This conclusion is supported by whole-muscle data showing larger MEPs in the contralateral muscle with unilateral focal TMS, as well as evidence that TMS stimulation of one hemisphere may produce excitation in a masseter or digastric single motor unit while stimulation of the opposite hemisphere produced inhibition of the same motor unit. The asymmetry is particularly marked for masseter, in which the low-threshold motor units were most commonly excited with contralateral TMS and inhibited with ipsilateral TMS. Spike-triggered averaging of digastric motor unit activity revealed cross-talk in surface EMG recordings from digastric muscles, and no evidence that muscle fibres in both digastric muscles were innervated by a common motor axon. Narrow excitatory peaks in the PSTH of motor unit discharge elicited by TMS in masseter (either hemisphere) and digastric motor units (ipsilateral hemisphere) suggest a direct corticomotoneuronal projection. The contralateral projection to digastric motoneurons may include additional oligosynaptic connections, as judged by the broader peaks in the PSTH with contralateral TMS. The organisation of bilateral corticotrigeminal inputs revealed with TMS suggests that: (a) the contralateral hemisphere provides relatively more of the excitatory input delivered via the fast corticotrigeminal pathway for both masseter and digastric motoneuron pools, and (b) corticotrigeminal projections from either hemisphere are capable of contributing to the voluntary command mediating activation of masseter, and (to a lesser extent) anterior digastric muscles on one side, that is independent of the homologous muscles on the other side.