Relations between identified tendon organs and motor units in the medial gastrocnemius muscle of the cat

Summary In one medial gastrocnemius muscle of each of several cats, the response was recorded of a single tendon organ to the contraction of a single motor unit which strongly excited the receptor. The motor unit was depleted of its glycogen and the depleted muscle fibres identified in PAS-stained transverse sections. The site of maximum tendon organ sensitivity was marked and the tendon organ identified in the same sections. Five pairs of tendon organs and motor units were studied completely. Each tendon organ was found to have one or two (mean 1.6) depleted muscle fibres attached to it, included in the bundle of fibres attached to the end (mean no. 14.4) and side (mean no. 5.6) of the tendon organ. A correlation was found between tendon organ discharge rate and the tension calculated from cross-sectional area measurements of the depleted muscle fibres attached to the tendon organ, with variation between individual pairs of tendon organs and motor units. One estimate of the average sensitivity of the sample was 28 imp/s/mN. A nearly linear discharge rate vs. tension relation was found for single tendon organ and motor unit pairs when tension was graded during a series of fatiguing contractions. Under these conditions the sensitivity, measured as the slope of the relation between discharge rate and motor unit tension recorded at the common tendon, varied between 0.11 and 0.30 imp/s/mN for 6 pairs.