Modulation of stretch-evoked reflexes in single motor units in human masseter muscle by experimental pain

The interaction between muscle pain and motor function of the jaw has been examined in recent years, but the nature of the modulation of the short-lat ency stretch reflex by pain is not fully understood. In this study, the ref lex responses to stretch were measured in single low-threshold motor units that were kept discharging at a constant frequency, before, during and afte r the induction of experimental pain ill one masseter muscle by controlled infusion of hypertonic saline. The probability of evoking a reflex response in individual motor units in the painful muscle at near-monosynaptic laten cy was reduced by a mean of about 20%. However; the overall reflex response in the surface electromyogram of both the ipsi- and contralateral masseter muscles was greater during pain. This was apparently a secondary response to the pain-induced increase in pre-stimulus activity in the motoneurone po ols of both muscles, because increased motoneurone excitability may facilit ate stretch reflexes. It is concluded that the most likely explanation for the reduced reflex response of low-threshold masseter motor units during ex perimental pain is a tonic reduction in the fusimotor drive to the masseter spindles.