The adequacy of stretch receptors in the cat knee joint for signalling joint angle throughout a full range of movement.

1. The present experiments were performed to resolve the discrepancy between the experiments of Boyd & Roberts (1953) in which receptors active at intermediate angles (mid range units) were frequently encountered, and the experiments of Burgess & Clark (1969) where mid range units were rarely encountered. 2. The discharge of knee joint afferents in the cat posterior articular nerve was recorded from both the dorsal roots and from the intact joint nerve. 3. No receptors were isolated which exhibited intermediate adaptation rates (similar to the "phasic" receptors described by Burgess & Clark, 1969). 4. The proportion of mid range units isolated in the present experiments (17.8%) was substantially larger than in the experiments of Burgess & Clark (4.8%). 5. Direct recordings from the posterior articular nerves of a series of cats revealed that there were always receptors tonically active at all intermediate positions. 6. Afferents from the popliteus muscle did not make a major contribution to the population of mid range units in the posterior articular nerve as removal of the popliteus muscle did not significantly alter the mean number of mid range units in the joint nerves of a series of nine cats. 7. Succinylcholine (SCh) in low doses (0.05-0.1 mg/kg) was found not to be specific for spindle afferents as these doses of SCh could elevate the resting discharge of mid range units of true articular origin. 8. It is concluded that slowly adapting joint receptors can adequately signal joint angle throughout the full normal range of movement and hence could make an important contribution to position sense.