Bilateral cortical control of the human anterior digastric muscles

Transcranial magnetic stimulation (TCMS) was used to determine the organisa tion of cortical motor projections to the anterior digastric muscles in 12 normal human subjects. Two distinct types of potentials were evoked in ante rior digastric with a figure-of-eight coil. A short-latency (3 ms) response appeared bilaterally on the surface electromyogram (EMG), but only ipsilat erally on intramuscular recordings: this was the result of direct stimulati on of the ipsilateral trigeminal motor root. Motor evoked potentials (MEPs) were elicited in the anterior digastric muscles at variable onset latencie s of around 10 ms by stimulation of scalp areas antero-lateral to the area for the first dorsal interosseous muscle of the hand. These were evoked bil aterally in relaxed anterior digastric muscles in six of the seven subjects . In the ether subject, the responses in the relaxed muscle were exclusivel y ipsilateral. However, when the anterior digastric muscles were contracted , the responses were bilateral in all subjects. TCMS and spike-triggered av eraging revealed that the bilateral responses were not due to the branching of axone from individual digastric motoneurones to muscles on each side. B ecause the digastric motor nucleus may contain separate populations of ipsi - and contralateral projecting motoneurones, it was necessary to study sing le motor-unit responses to TCMS to demonstrate a bilateral corticobulbar pr ojection. The responses of 17 single motor units in the anterior digastric muscle to TCMS were recorded. All were activated by contralateral stimulati on. Approximately 80% were also activated by ipsilateral TCMS. although one well-characterised motor unit was inhibited by ipsilateral TCMS. When bila teral activation was present, the ipsilateral responses were more secure th an the contralateral responses, which may indicate an additional interneuro ne in the pathway to the contralateral motoneurone. The major conclusions f rom this study are that (1) the cortical representation of the anterior dig astric muscle is antero-lateral to hand muscles; (2) the cortical projectio n to the anterior digastric muscles is bilateral; (3) the corticobulbar pro jection is stronger contralaterally than ipsilaterally but may involve at l east one additional synapse; and (4) anterior digastric motoneurones do not branch to innervate the muscles bilaterally.