Velocity sensitivity of human muscle spindle afferents and slowly adapting type II cutaneous mechanoreceptors.

1. Velocity information is used in the performance of movement. This study evaluated the ability of peripheral receptors to signal velocity in human subjects. 2. The velocity sensitivity of human muscle spindle afferents from the extensor digitorum muscles and slowly adapting type II cutaneous mechanoreceptors on the dorsum of the hand was evaluated with recordings from the radial nerve during imposed flexion movements about the metacarpophalangeal joint. Twenty-degree movements at velocities ranging from 5 to 80 deg s-1 were used. 3. Three measures of dynamic response were calculated: the dynamic positional sensitivity (the relation between discharge rate and joint angle during the dynamic phase of movement), the dynamic index (the discharge rate just before ramp completion minus the rate 0.5 s later), and the incremental response (the discharge rate just before ramp completion minus the rate just before ramp onset). 4. Both muscle spindle afferents and slowly adapting type II cutaneous mechanoreceptors demonstrated significant velocity sensitivity. The magnitudes of the relations between dynamic response measures and velocity were similar in the two receptor types. 5. These findings are consistent with the view that both muscle spindle afferents and slowly adapting type II cutaneous mechanoreceptors provide reasonable velocity signals.