The strength of the reflex response to sinusoidal stretch of monkey jaw closing muscles during voluntary contraction.

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Abstract
Rhesus monkeys were trained to exert steady biting forces of 3-60 N for 1-2 s. This behavior was well maintained while sinusoidal or step opening and closing movement were imposed on the jaw. The force modulation amplitude during sinusoidal stretching was divided by the amplitude of movement to obtain the magnitude of stiffness. This estimate was made at frequencies from 2-50 Hz at amplitudes of 100 and 500 .mu.m (half the peak-to-peak movement at the incisors). Peak magnitudes of stiffness were seen with frequencies of 8-15 Hz when the amplitude of movement was small; there was a great deal of variation between individual animals. This variation was most striking with mean forces of 25-35 N. The stiffness was greatest in animals that showed considerable spontaneous tremor, and the highest levels of stiffness were often recorded with frequencies near which tremor amplitude was large. A marked phase lag in the force response was often seen during small amplitude stretching at 8-30 Hz. Estimates of stiffness for larger amplitude (500 .mu.m) stretching showed less variation; the magnitude of stiffness showed maximum values below 10 Hz and a minimum at 15-30 Hz. Force always showed a phase lead on position although this lead became small in the frequency range where with smaller movement there had been phase lags. the magnitude of stiffness increased with increasing mean force. Bilateral electrolytic lesions were made in the brain stems of 3 animals; they reduced by over 95% the expected number of cells in the mesencephalic nucleus of the 5th cranial nerve on either side. These lesions interrupted the afferent pathway for the stretch reflex and so abolished excitatory electromyogram (EMG) responses to step stretches of the jaw closing muslces. Such relfex responses as persisted after the lesions were small and inhibitory. EMG silences followed both step stretch and release; the response to release was a load compensation that could not be attributed to spindle afferents. Comparison of the stretch relfex component of stiffness with the stiffness that remained after making lesions showed that at frequences below 10 Hz the relfex makes the major contribution even with 500 .mu.m stretching. For this larger movement the relative contribution of the reflex is the same at low and intermediate mean forces. The lesions strikingly increased spontaneous tremor of low frequency in the voluntary contracting jaw muscles. The frequent emphasis on the instability of the intact reflex as a cause of tremor was not supported experimentally. Increased tremor in the absence of the stretch reflex may be a more important clue to reflex function. The stretch reflex makes a significant contribution to motor performance by increasing several-fold the resistance to external stretch and perhaps by reducing tremor at low frequencies. The uncertain significance of the reflex responses to small amplitude stretching is discussed.