J Physiol (Lond) 476 (2): 323-331 (Apr 15 1994)
The reflex response to stretch in most contracting human muscles includes
both a short-latency, probably monosynaptic, excitatory component, and
a longer-latency, polysynaptic excitation. However, it has been claimed
that stretch of the jaw-closing muscles evokes only the short-latency response
in masseter. This question was re-examined, using controlled stretches
of varied rates and durations. Very brief, rapid stretches analogous to
the stimuli used to investigate the 'jaw-jerk' reflex in earlier studies
evoked a prominent excitatory peak in the electromyogram at monosynaptic
latency excitation, but little or no longer-latency excitation. This response
could be produced even by stimuli that were barely detectable by the subject.
However, this prominent electrical response did not produce a measurable
increase in biting force. In contrast, slower stretches evoked both a short-
and a longer-latency excitatory response in the surface electromyogram,
as in most limb muscles. It is shown that the absence of a long-latency
excitatory response in earlier studies can be explained by the powerful
reflex disfacilitation of the motoneurones that occurred at the end of
the brief stretches used. Depending on the duration of the stretch, this
disfacilitation is often sufficient to mask or abolish the long-latency
reflex. The reflex response to stretches was not markedly affected by blocking
the activation of mechanoreceptors around the teeth with local anaesthetic,
indicating that receptors around the teeth cannot be playing more than
a minor role in the response. The stretch-induced increase in force became
greater as the velocity of the stretch decreased.