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

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Abstract
To determine the presence and importance of receptors active at intermediate angles (mid range units), the discharge of knee joint afferents in the cat posterior articular nerve was recorded from the dorsal roots and the intact joint nerve. No receptors were isolated which exhibited intermediate adaptation rates (similar to phasic receptors). The proportion of mid range units isolated (17.8%) was substantially larger than in previous experiments. Direct recordings from the posterior articular nerves of a series of cats revealed that there were always receptors tonically active at all intermediate positions. 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 in 9 cats. Succinylcholine (SCh) in low doses (0.05-0.1 mg/kg) was not specific for spindle afferents since these doses of SCh could elevate the resting discharge of mid range units of true articular origin. Slowly adapting joint receptors can adequately signal joint angle throughout the full normal range of movement and could make an important contribution to position sense.