The relationship between cortically induced mandibular movements and lateral pterygoid and digastric muscle EMG activity in the anesthetized guinea pig.

Vertical and horizontal jaw movements along with lateral pterygoid (PT) and anterior digastric (DIG) muscle electromyogram (EMG) activity were recorded in anesthetized guinea pigs during electrical stimulation of the masticatory area of cortex. Both repetitive stimulation at 25 Hz and short pulse-train stimulation (3 pulse, 500 Hz) delivered 2/s produced jaw opening with a horizontal component contralateral to the side of cortex stimulated in addition to short latency (7-12 ms) evoked EMG bursts in the ipsilateral PT muscle and in the DIG muscles bilaterally. Evidence was also obtained which indicated that both types of cortical stimulation resulted in time-locked inhibition of the contralateral PT muscle. Unlike each short train of stimulus pulses, however, each pulse in the repetitive train did not evoke jaw movement and EMG bursts of a consistent amplitude. Rather, repetitive stimulation produced rhythmical movements of the jaw associated with recurrent periods of activity and inactivity in the EMG records. These results demonstrate the presence of a corticotrigeminal pathway between masticatory cortex and PT motoneurons in addition to a similar pathway to DIG motoneurons, and suggest that both pathways are similarly modulated by a central pattern generator to produce rhythmical jaw movements.