Cortically induced masticatory rhythm and its modification by tonic peripheral inputs in immolbilized cats.

Repetitive cortical stimulation induced rhythmical masticatory jaw movement and masseteric nerve activity in cats with the spinal cord sectioned at the C2 level. Jaw depression alone did not induce any rhythmical jaw movement, but it turned the irregular jaw movement evoked by subthreshold cortical stimulation into the regular jaw movement as evoked by suprathreshold cortical stimulation. The cortically induced rhythmical masseseteric nerve activity could be induced even in immobilized condition and after cerebellectomy, although the rhythmical nerve activity was depressed in amplitude and the rhythm became slower than before immobilization. Tonic jaw depression after immobilization facilitated the tonic activity in the masseteric nerve and also induced a remarkable increase in the amplitude of the cortically evoked rhythmical nerve activity and a slight acceleration of the rhythm, leading to the same masticatory rhythm as before immobilization. It was concluded (1) that the cortically induced masticatory rhythm in cats is basically generated centrally and (2) that the tonic inputs from the muscle spindle in the jaw-closing muscles participate in the rhythm formation by supplying tonic excitatory inputs mainly to the jaw-closer motoneuron and also to the central rhythm generator.