Studies on Neuromechanisms of the Jaw Opening Reflex elicited by Palate Pressing

Abstract

Sustained jaw-opening is elicited when pressure stimulation is applied to the hard palate of the cat. In this study, the neural mechanisms of this sustained “jaw-opening reflex” was evaluated. The results are as follows.1) EMG activities of the jaw-closing muscles were significantly suppressed during a sustained jaw-opening elicited by pressure stimulation of the hard palate, but the jaw-opening muscle did not seem to partake in this reflex.2) The anterior part of the hard palate, including the papilla incisiva, most effectively induced a “sustained jaw-opening reflex”. Suppression of jaw-closing muscle activities at this reflex was bilateral, but an ipsilateral predominance was witnessed.3) The “sustained jaw-opening reflex” was completely eliminated after a bilateral dissection of the palatal nerves, and the pressure receptors in the palatal mucosa turned out to be a primary factor on induction of this reflex.4) Amplitude of the monosynaptic reflex (MSR) of the masseter nerve, evoked by stimulating the mesencephalic trigeminal nucleus (MesV), decreased while the hard palate was pressed, though that of the antidromic spike remained unaffected. This result indicates that the inhibition of the masseter nerve was attributable to the postsynaptic inhition in the masseteric alpha motoneurons, but not to the presynaptic inhibition.5) Activities of the masseteric fusimotor neurons were also inhibited with a pressing of the palate, though the inhibition was not so complete as was the case with the alpha motoneurons.6) As a reflection of inhibition of the fusimotor neurons, muscle spindle discharges were also inhibited when the jaw-closing muscles were in a isometric contraction state.7) On the basis of the foregoing results, it has been demonstrated that the sustained jaw-opening elicited with a pressing of the palate is attributable to the co-inhibition of alpha and fusimotor neurons which innervate the jaw-closing muscles.