Activity of trigeminal alpha- and gamma-motoneurons and muscle afferents during performance of a biting task

Abstract

Monkeys [Macaca mulatta] were trained to bite on a strain gauge and to maintain a force between specified limits for 1 s. The firing pattern of 36 motor units in the jaw-closing muscles, 29 trigeminal .alpha.-motoneurons, 3 .gamma.-motoneurons and 7 spindle afferents, and 40 brain stem neurons of other types recorded extracellularly in the brain stem were examined during the performance of the task and during spontaneous rhythmical movements. Conduction velocities were calculated from responses to stimulation of the mandibular nerve. For analysis the task was divided into an initial dynamic, a static and a release phase. Motor units were recruited in order of increasing spike amplitude. Increasing the rate of application of force (df/dt) lowered the threshold force at which motor units and .alpha.-motoneurons were recruited. The mean firing frequency of all but 5 .alpha.-motoneurons and motor units was higher during the static phase than during the dynamic phase of the task and rose with increases in the mean force applied. Activity of motor units and .alpha.-motoneurons ceased at the beginning of the release phase. The conduction velocity of .alpha.-motoneurons ranged from 36-90 m/s. Of the 3 .gamma.-motoneurons recorded, two responded to antidromic stimulation and had conduction velocities of 19 and 38 m/s. No responses were evoked by passive jaw movement of probing muscles. Muscle spindle afferents (7) were excited by passive jaw movements and 3 by mandibular nerve stimulation (conduction velocities 75, 25 and 20 m/s). In the biting task, .gamma.-motoneurons are active at least as early as the smallest .alpha.-motoneurons, but this does not lead to significant spindle afferent input and thus, servoassistance, before contact with the bite bar. The proportionate contribution of the servo loop input to .alpha.-motoneuron depolarization appears to be greatest during isometric contractions at low forces.