CONTRIBUTION OF JOINT AND MUSCLE AFFERENTS TO POSITION SENSE AT THE HUMAN PROXIMAL INTERPHALANGEAL JOINT

Experiments were carried out to examine the perceived position of the human index finger about the proximal interphalangeal joint. In protoc ol 1, the finger was moved from an intermediate position at velocities ranging from 200 deg/min to 2 deg/min, then held still at one of thre e positions. The subject's task was visually to align a finger silhoue tte that was coaxial with the joint to the kinaesthetically perceived position of the unseen finger. Judgements of position were found to be quite accurate, and unaffected by previous velocity. Protocol II show ed that although the direction of joint displacements of 0.01 and 0.1 deg could not be detected at any velocity, 1 deg could be detected at 200 deg/min and 10 deg at 20 and 2 deg/min. In protocol III the finger was moved at 2 deg/min and maintained at either 105 or 175 deg. It wa s found that the position of the unanaesthetized finger was quite accu rately known, but with digital nerve block, subjects clearly perceived the finger to be at the mid-position (approximately 130 150 deg). Thi s suggests that the absence of joint and cutaneous afference is interp reted by the CNS as indicating mid-position. The slight bias of the se nsed position towards the objective position shown by the results also indicates that muscle afferents can provide a crude signal related to joint position. This finding was further supported by the observation that splinting the distal interphalangeal joint into flexion resulted in flexion bias in the perceived angle of the proximal interphalangea l joint. Anaesthesia of the middle finger, thumb and distal portion of the index finger (leaving proximal joint unaffected), had little effe ct on position matching performance, suggesting that the large error i n position sense during anaesthesia of the whole finger is due to loss of afference specifically related to the proximal interphalangeal joi nt, and not due to loss of non-specific facilitatory influences from c utaneous and joint afferents. The results argue for an important propr ioceptive role for joint afferents at finger joints.