Facilitatory effect of jaw opening on somatosensory (SI) cortical neurones sensitive to tooth pressure in the cat.

In anaesthetized cats, an upper canine tooth was stimulated mechanically at two different levels of jaw opening, the resting position and an open position with 20 +/- 2 mm between the upper and lower canines. The evoked field potentials and neuronal discharges were recorded from the caudal part of the contralateral coronal gyrus (SI cortex). The waveforms of the evoked potentials appeared in a positive-negative sequence. There were no significant changes in them when the jaws were open. Discharge patterns elicited in the cortical neurones by mechanical stimulation of the teeth consisted of initial 'burst' discharges, inhibitory pauses and/or large after-discharges. Jaw opening did not influence any phases of these responses to suprathreshold stimulation, spontaneous activities, or the sizes of the receptive fields. However, jaw opening did affect the initial 'burst' phase of the response to threshold stimulation, i.e. that which caused the neurones to fire with a probability of 30-50% with the jaw closed. Jaw opening enhanced this response probability in half (6/12) of the units that had very small receptive fields restricted to the canine tooth, but did not influence it in the majority (21/24) of the units that had larger receptive fields including the oral mucosa and the facial skin. There was no difference in distribution in the coronal gyrus between the two groups of neurones categorized by whether or not they were influenced by jaw position.