Activity of spindle afferents from jaw-closing muscles and tooth mechanoreceptors was recorded during the production of controlled isometric bites in monkeys. The stretch sensitivity of muscle afferents was determined by imposing small sinusoidal displacements of the jaws at frequencies from 1 to 40 Hz during production of a bite force of 10 N. 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 ranging from 4 to 26 N. These endings were very sensitive to small displacements of the mandible, being about as sensitive, in spikes per second per millimeter, as muscle spindle primary endings of the cat hindlimb at frequencies from 1 to 10 Hz. 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. If it is assumed that the high-sensitivity and low-sensitivity afferents recorded in the present study are functionally analogous to primary and secondary endings, respectively, as studied in cat hindlimb muscles, the behavior and stretch sensitivities of jaw muscle afferents may be taken to suggest that dynamic fusimotor fibers are activated with, but not in proportion to, activation of the α-motoneuron pool. Static fusimotor fibers of jaw-closing muscles, however, appear to be at best only weakly coactivated with α-motoneurons during production of voluntary isometric contractions.
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