Discharge in muscle spindle afferents related to direction of slow precision movements in man

Single-unit activity was recorded with needle electrodes in eighteen muscle spindle afferents (eleven primaries, seven secondaries) from finger extensor muscles in the radial nerve of awake human subjects. The discharge rate of the afferents was determined during precisely controlled voluntary movements. The subjects performed a standardized visual ramp-and-hold tracking task, which included very slow finger extension and flexion movements (2.5 deg/s) with an amplitude of 20 deg. Throughout the tracking task a constant torque load of small or intermediate size, i.e. less than 30% of maximum voluntary contraction force, opposed finger extension. Altogether, 131 trials were studied. For most units the discharge rate was lower during shortening compared with active position holding, and it was higher during lengthening contractions. Thus, the majority of units responded to phasic stretch during the active movements, although the size of the movement response varied considerably between units and was never large. A few units even exhibited a reversed stretch response pattern. Hence, when estimated from pooled data, movement responses of the unit sample as a whole were small, around 1 impulse/s. The over-all response pattern of an individual afferent during the tracking task was very similar between successive tests. Although the discharge rate of most units increased with the load during movements as well as during position holding, the presence as well as the magnitude of movement responses depended only little on the size of the load. However, a few afferents exhibited a stretch response pattern with small loads and a reversed stretch response pattern with larger loads. In spite of the predominant increase of afferent firing during muscle lengthening there was no systematic modulation of the discharge rate in relation to the joint angle during the active movements, either in the primary or in the secondary afferents. The present findings suggest that human muscle spindles provide information about the occurrence as well as the direction of slow isotonic movements at low velocities in a precision motor task. This is in contrast to the lack of accurate position response which has previously been demonstrated.