Static sensitivity of tendon organs to tetanic contraction of in-series motor units in feline peroneus tertius muscle.

1. The results of several studies have indicated an absence of any consistent relationship between the discharge of Ib Golgi tendon organ afferents and the steady-state tetanic tension generated by activating motor units. This question has been re-examined by recording the responses of individual tendon organs to tetanic, isometric contractions of one or more motor units from the peroneus tertius muscle of anaesthetized cats. 2. In three experiments, seventy-three individual tendon organ-motor unit interactions were recorded. The motor units were stimulated at 30-150 s-1 and for each tendon organ-motor unit pair a linear relationship was found between steady-state tetanic tension and Ib afferent discharge. The slopes of these relationships (the static sensitivities) were steepest for the weakest units. 3. When motor units were stimulated in combination, the relationship between discharge frequency and plateau tension was again linear but the static sensitivities were generally much lower than for single units and approached 1 impulse s-1 g-1. 4. Expression of these relationships in terms of the relative tensions generated revealed that the tendon organs were activated most strongly by the IIb muscle fibres, the static sensitivities being reduced by unloading effects. 5. The linear relationships observed during stimulation of single, and groups of, motor units suggest that the patterns of discharge from the tendon organs can mirror the steady-state contractile tensions within the muscle.