Contribution of Joint Afferents to Proprioception and Motor Control

Summary

Diseased joints are often deformed, but the underlying mechanisms are unclear. Previous research indicates that group II joint afferents influence the excitability of both a and y motoneurones of muscles acting at the joint and thus their tone. It is therefore possible that joint disease could disrupt proprioceptive feedback, thereby altering the balance of forces acting at the joint and producing deformity. Recent observations have shown that patients with rheumatoid arthritis affecting the proximal interphalangeal joint of the index finger have ∼50% decrement in position sense with a clear flexion bias in their judgements of finger position. Subjects with the hypermobility syndrome (double-jointedness) also show disturbed position sense even though this group have much less joint damage than patients with rheumatoid arthritis. These findings suggest that proprioceptive feedback from group II articular afferents is altered in joint diseases. Animal experiments performed in cats decerebrated under halothane anaesthesia have shown that the excitatory influence of group II joint afferents on both α and γ motoneurones is reduced and often abolished by conditioning stimuli from group IV afferents from the same joint. Thus, the activation of joint nociceptors, which almost invariably accompanies joint disease, could significantly alter reflex effects mediated via joint proprioceptors, once again resulting in altered muscle tone. Joint deformity could therefore arise by two complimentary mechanisms — altered proprioceptive feedback and enhanced nociceptive discharge from the affected joint.