Illusory Movements Produced by Electrical Stimulation of Low-threshold Muscle Afferents from the Hand

There is continuing controversy as to the role of muscle afferents in kinaesthesia. Moberg (1983) has recently claimed that much of the evidence previously used in favour of a kinaesthetic role for muscle afferents points to a crucial role for cutaneous afferents (remote from the joint but overlying the relevant muscle) and that there is no definite evidence for a contribution to kinaesthesia from 'musculotendinous' receptors. In the present study, trains of electrical stimuli delivered to the ulnar nerve at the wrist (through a surface probe, needle electrode or microelectrode within motor fascicles) produced illusory movements of the fingers and distortions in their position. These illusions occurred in the absence of overt movement (stimuli were 0.5-0.9 times motor threshold) and in the absence of cutaneous paraesthesiae. The velocity of the smooth illusory movements increased with the frequency of stimulation. The direction of the illusory movements was opposite to that produced by shortening of intrinsic muscles of the hand. Short-latency cerebral potentials were recorded from the contralateral scalp during the illusions. These illusions are most simply explained by the perception of discharges in low-threshold muscle afferents, probably those which innervate primary muscle spindle endings. Stimulation of the digital nerves sometimes produced a sensation of extreme position of one of the interphalangeal joints or of an apparent oscillation at the joint. The frequency of oscillation increased as the stimulus frequency increased. These results argue for a direct contribution to kinaesthesia from primary muscle spindle afferents though they do not exclude contributions from nonmuscle afferents.