P. Svensson et al., Modulation of stretch-evoked reflexes in single motor units in human masseter muscle by experimental pain, EXP BRAIN R, 132(1), 2000, pp. 65-71
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.