PHASIC AND TONIC STRETCH REFLEXES IN MUSCLES WITH FEW MUSCLE-SPINDLES- HUMAN JAW-OPENER MUSCLES

We investigated phasic and tonic stretch reflexes in human jaw-opener muscles, which have few, if any, muscle spindles. law-unloading reflex es were recorded for both opener and closer muscles. Surface electromy ographic (EMG) activity was obtained from left and right digastric and superficial masseter muscles, and jaw orientation and torques were re corded. Unloading of jaw-opener muscles elicited a short-latency decre ase in EMG activity (averaging 20 ms) followed by a short-duration sil ent period in these muscles and sometimes a short burst of activity in their antagonists. Similar behavior in response to unloading was obse rved for spindle-rich jaw-closer muscles, although the latency of the silent period was statistically shorter than that observed for jaw-ope ner muscles (averaging 13 ms). Control studies suggest that the jaw-op ener refer was not due to inputs from either cutaneous or periodontal mechanoreceptors. In the unloading response of the jaw openers, the to nic level of EMG activity observed after transition to the new jaw ori entation was monotonically related to the residual torque and orientat ion. This is consistent with the idea that the tonic stretch reflex mi ght mediate the change in muscle activation. In addition, the values o f the static net joint torque and jaw orientation after the dynamic ph ase of unloading were related by a monotonic function resembling the i nvariant characteristic recorded in human limb joints. The torque-angl e characteristics associated with different initial jaw orientations w ere similar in shape but spatially shifted, consistent with the idea t hat voluntary changes in jaw orientation might be associated with a ch ange in a single parameter, which might be identified as the threshold of the tonic stretch reflex. It is suggested that functionally signif icant phasic and tonic stretch reflexes might not be mediated exclusiv ely by muscle spindle afferents. Thus, the hypothesis that central mod ifications in the threshold of the tonic stretch reflex underlie the c ontrol of movement may be applied to the jaw system.