Neural basis for initiation of rhythmic digastric activity upon midbrain stimulation in the guinea pig

The central pattern generator for rhythmic jaw movements is readily activated by repetitive stimulation (40 Hz) of the midbrain in the region of the cerebral peduncle in anaesthetized guinea pigs. The pattern of rhythmic digastric EMG evoked by midbrain stimulation after acute decerebration closely resembled that evoked by electrical stimulation of the masticatory cortex. Since the decerebration boundary was caudal to the substantia nigra and the thalamus, impulses ascending to these centers and others further rostral are not necessary for the generation of rhythmic jaw movements. This leaves local midbrain nuclei, and pathways descending from the forebrain (fibers-of-passage) as possible substrates for activating the rhythmic jaw movements. To distinguish between these two possibilities, midbrain stimulation was delivered in guinea pigs that underwent unilateral ablation of the masticatory cortex, and survived long enough to permit degeneration of all corticobulbar fibers that pass through the midbrain. In these animals no response was detected from stimulation of the ipsilateral midbrain. However, stimulation of the contralateral midbrain did evoke bilateral rhythmic digastric activity. These results suggest that rhythmic digastric activity and jaw movements evoked by midbrain stimulation are caused by activation of fibers that descend from the masticatory cortex, travel down through the midbrain, probably in the cerebral peduncle, and decussate only just rostral to the motor nucleus of the Vth cranial nerve.