Habituation of exteroceptive suppression and of exteroceptive reflexes in man as influenced by voluntary contraction.

Habituation kinetics were found to vary considerably in different polysynaptic pathways of the brain stem in man. The exteroceptive reflex of the digastric muscle habituates markedly, even for stimuli repeated at 1-min intervals. The second component of the blink reflex habituates for intervals below about 8 sec, but this can largely be prevented by steady voluntary contraction of the muscle. On the other hand the exteroceptive suppressions of the masseter muscle only disclose slight habitaution of their late component ES2 and no habituation of the early component ES1. This does not appear to be affected by voluntary activation since the suppressive effects present the same pattern in relaxed masseter muscles tested by proprioceptive reflex activation. Exteroceptive reflexes and exteroceptive suppressions of similar latencies disclose genuine and important differences in their habituation kinetics.