The interrelation of neural discharge, intra-articular pressure, and joint angle in the knee of the dog.

Single- and multi-unit recordings were obtained from the medial articular nerve (m.a.n.) of knee joints in the anaesthetized dog. The single-unit recordings were confined to low threshold (group I and II) articular mechanoreceptors. Multi-unit recordings revealed that the m.a.n. discharge was maximal in extension, submaximal in flexion, and minimal at intermediate angles, i.e. a U-shaped profile. Subatmospheric intra-articular pressures do not appear to influence the m.a.n. discharge. Intra-articular infusion of even small quantities of fluid, although not affecting the U-shaped profile, reversed the m.a.n. discharge pattern with maximum neural activity occurring in flexion and being submaximal in extension. Recordings from single units indicated that the enhanced discharge after fluid infusion was a result of increased discharge frequency and 'recruitment' of individual afferents.