Discharge characteristics and stretch sensitivity of jaw muscle afferents in the monkey during controlled isometric bites.

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Abstract
Activity of spindle afferent from jaw-closing muscles and tooth mechanoreceptors was recorded during the production of controlled isometric bites in monkeys [M. mulatta]. Some muscle afferents, herein labeled high-sensitivity endings, exhibited a large, sustained increase in firing rate during bite responses, but the firing rate was only very weakly, if at all, related to the magnitude of bite forces. These endings were very sensitive to small displacements of the mandible, being about as sensitive, in spikes/s per mm, as muscle spindle primary endings of the cat hindlimb. Another group of muscle afferents, herein labeled low-sensitivity endings, did not increase their firing rate during bite responses but did increase their rate during muscle stretches occurring during mandibular movements associated with food consumption. Sensitivity of these endings, measured with sinusoidal stretch, was much lower at all frequencies than high-sensitivity endings and was comparable to the sensitivity of secondary endings of cat hindlimb muscles. The high-sensitivity and low-sensitivity afferents recorded are probably functionally analogous to primary and secondary endings, respectively, as studied in cat hindlimb muscles. The behavior and stretch sensitivities of jaw muscle afferents may suggest that dynamic fusimotor fibers are activated with, but not in proportion to, activation of the .alpha.-motoneuron pool. Static fusimotor fibers of jaw-closing muscles are apparently at best only weakly coactivated with .alpha.-motoneurons during production of voluntary isometric contractions.