Afferent Discharge from Human Muscle Spindles in Non‐Contracting Muscles. Steady State Impulse Frequency as a Function of Joint Angle

Abstract Impulses in single muscle spindle afferents from the antebrachial finger flexor muscles were recorded from the median nerve of waking human subjects. The discharge frequency in the steady state was measured as the metacarpo‐phalangeal joint angles were passively changed in steps. At a comfortable resting position of the hand, less than ten percent of the spindle primaries were discharging. Within intermediate ranges of muscle lengths the discharge frequency of the single units did not exceed 20 impulse per second (ips). The response to joint movements was that expected from muscle stretch and muscle release alone and there was no indication of fusimotor adjustments as a result of joint movement. The frequency of the single units increased linearly with joint extension, the mean sensitivity of the primaries being 0.18 ips per degree which is about 5 times lower than the sensitivity of de‐efferented spindles in cat ankle extensors. The afferent discharge from the secondaries was in most respects very similar to that of the primaries. The low discharge frequency, the low percentage of active units and the low position sensitivity all indicate an insignificant fusimotor outflow to relaxed human hand muscles and a comparatively poor ability of the spindles to measure muscle length under these conditions.