Single unit activity in cortical masticatory area during natural masticatory movement in cats

Abstract

Repetitive stimulation of a certain area in the cerebral cortex in the monkey, cat, rabbit and guinea pig is known to induce a rhythmical jaw movement (RJM) accompanied by a coordinated tongue movement as well as salivation. The response is so closely similar to the natural masticatory movement that such a cortical area is called the cortical masticatory area (CMA) . In the cat, the CMA corresponds with the rostral part of the orbital gyrus. The present study was carried out to examine the role of the CMA by making quantitative analyses of the correlation between the unit activity pattern of the CMA neurons and the parameters of the jaw movement during the natural mastication in the unanesthetized chronic cat.The single unit activity was recorded from 134 neurons in the rostral part of the orbital gyrus during the natural masticatory movement. Thirty-two of them showed spike firing related to some aspects of the RJM, but none of them responed with antidromic spikes to the stimulation of the ipsilateral cerebral peduncle. Thirteen neurons changed the firing rate (FR) rather tonically during the natural masticatory movement (non-phasic group) and 12 neurons increased the FR throughout the series of the masticatory strokes (increase type), while 1 neuron decreased the FR during the initial masticatory strokes (decrease type) . The change in the FR started preceding the onset of the RJM by 500-1, 000 ms. The remaining 19 neurons rhythmically modulated the FR at the same rhythm as the RJM (phasic group) and 9 neurons attained the highest FR in the jawopening (JO) phase (opening type), while the other 10 neurons attained it in the jaw-closing (JC) phase (closing type) .The instantaneous firing frequency (IFF) of the 4 opening type neurons showed a positive correlation with the velocity of the JO movement and in 3 of them it was also positively correlated with the jaw position in the JO phase. In 2 of these neurons, one neuron showed also a positive correlation with the jaw position in the JC phase, while the other neuron showed also a negative correlation with both the velocity and jaw position in the JC phase. The IFF of other 2 neurons showed a positive correlation with the jaw position in only the JC phase and another 1 neuron showed a negative correlation only with the velocity of the JC movement.The IFF of each of the 2 closing type neurons showed either a negative or a positive correlation with the jaw pesition in the JC phase. The latter neuron showed also a positive correlation with the jaw position in the JO phase. The IFF of other 2 neurons showed a negative correlation with the velocity of the JO movement.The results suggest that there are neurons in the rostral part of the orbital gyrus of the cat, which participate in the initiation and/or execution of the masticatory jaw movement.