Analysis of rhythmical jaw movements produced by electrical stimulation of motor-sensory cortex of rabbits.

The anterolateral regions of the cerebral cortex of rabbits anesthetized with urethan were stimulated with either short trains of electrical pulses at high-frequency (3 pulses, 500 Hz) or 10-s trains of shocks at 50 Hz. The movements of the mandible and the electromyographic (EMG) activity from the muscles of mastication were recorded on magnetic tape and later analyzed by computer. Two basic types of responses are reported: twitches of the digastric muscles that followed the stimulus at short-latency (4-6.7 ms) and rhythmical masticatory movements that could only be evoked by prolonged stimulation. The cortical representations of the short-latency twitch contractions and mastication overlap. Different masticatory patterns are represented in separate areas of the cortex. The movements represented in the anteromedial half of the masticatory area are made mainly in the vertical plane, whereas stimulation of the posterolateral part produces movements in which the jaw swings to the contralateral side during closure. Increases in stimulus intensity usually increase the frequency of the movements, but other changes in the pattern depend on the site of stimulation. The rhythmical bursts of EMG activity often contain stimulus-bound short-latency responses but, since these are not always present, we conclude that they are not essential components of the masticatory pattern. We suggest that the basic patterns of mastication are elaborated by brain stem circuits that are themselves controlled by specific regions of the motor-sensory cortex.