CODING OF PULSATILE MOTOR OUTPUT BY HUMAN MUSCLE AFFERENTS DURING SLOW FINGER MOVEMENTS

1. Impulse activities of thirty-eight muscle spindle and tendon organ afferents from the finger extensor muscles were recorded in the radial nerve of human subjects while the subjects performed voluntary flexio n and extension finger movements at a single metacarpo-phalangeal join t. 2. The afferent firing was analysed in relation to the 8-10 Hz disc ontinuities which previously have been shown to characterize these mov ements. Spike-triggered averaging and frequency domain analyses demons trated that all Ia muscle spindle afferents and a large proportion of group II spindle afferents responded in close association with local p eaks in the joint acceleration. During muscle lengthening the impulses appeared during phases of rapid muscle stretch, whereas they appeared during the phase of minimal speed during muscle shortening. 3. The Go lgi tendon organ (Ib) afferents displayed a reverse pattern of activit y in relation to the discontinuities, i.e. the impulses tended to appe ar in the phase of minimal speed during lengthening movements and clos e to maximal shortening speed during shortening movements. Hence, thei r firing often coincided with the phasic increases of the parent muscl e activity which account for the 8-10 Hz discontinuities. 4. A close a nalysis of the time relations between spindle firing and the kinematic s of the 8-10 Hz discontinuities revealed that the population spindle response was too delayed and too dispersed to support the hypothesis t hat the discontinuities are accounted for by the stretch reflex. 5. If , as suggested in a previous paper, the 8-10 Hz discontinuities are pr oduced by a pulsatile descending motor command, the coding of the peri odic but tenuous kinematic events by the population of proprioceptors may have a role in relation to an alleged pulsatile command generator.