STRETCH REFLEXES IN HUMAN MASSETER

1. The reflex response to stretch in most contracting human muscles in cludes both a short-latency, probably monosynaptic, excitatory compone nt, and a longer-latency, polysynaptic excitation. However, it has bee n claimed that stretch of the jaw-closing muscles evokes only the shor t-latency response in masseter. This question was reexamined, using co ntrolled stretches of varied rates and durations. 2. 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 e lectromyogram at monosynaptic latency excitation, but little or no lon ger-latency excitation. This response could be produced even by stimul i that were barely detectable by the subject. However, this prominent electrical response did not produce a measurable increase in biting fo rce. 3. In contrast, slower stretches evoked both a short- and a longe r-latency excitatory response in the surface electromyogram, as in mos t Limb muscles. It is shown that the absence of a long-latency excitat ory response in earlier studies can be explained by the powerful refle x disfacilitation of the motoneurones that occurred at the end of the brief stretches used. Depending on the duration of the stretch, this d isfacilitation is often sufficient to mask or abolish the long-latency reflex. 4. 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 b e playing more than a minor role in the response. 5. The stretch-induc ed increase in force became greater as the velocity of the stretch dec reased.