MODULATION OF THE MANDIBULAR STRETCH REFLEX SENSITIVITY DURING VARIOUS PHASES OF RHYTHMIC OPEN-CLOSE MOVEMENTS IN HUMANS

The muscle spindles of the jaw elevator muscles provide positive feedb ack to the alpha motoneurons. It is generally assumed that the feedbac k is modulated during chewing so that counterproductive forces of the jaw elevator muscles can be avoided during jaw opening. Our aim was to investigate the modulation of the muscle spindle input to the alpha m otoneurons during various phases of open-close movements in man. To th at end, subjects made rhythmic open-close movements at their natural c hewing frequency. A force impulse (5 N, 10 ms), eliciting a jaw-jerk r eflex, was unexpectedly applied. The impulse was applied to the mandib le at 8 different phases during an open-close cycle, but only 1 impuls e per cycle. Jaw movement and surface EMG of the masseter and temporal muscles on both sides were recorded during 3 cycles without an impuls e and 3 succeeding cycles with an impulse. To examine whether the modu lation of the mandibular stretch reflex sensitivity depends on the foo d resistance, we applied an additional external force on the mandible, counteracting closing of the jaw each cycle. Two experimental session s were performed in random order, i.e., without force and with an addi tional force of 20 N. We observed pronounced reflexes at the onset of jaw closing, during the closing phase, and at occlusion. No or only we ak jaw-jerk reflexes were present during jaw opening. The reflex ampli tudes at occlusion were larger when an external force was present. Thi s increase in reflex amplitude may be the result of an adjusted gamma motoneuron activity, from pre-motor inhibition, or from both. The refl ex amplitudes elicited during jaw closing were not correlated with the phase of the movement.