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 con sistent relationship between the discharge of Ib Golgi tendon organ af ferents and the steady-state tetanic tension generated by activating m otor units. This question has been re-examined by recording the respon ses of individual tendon organs to tetanic, isometric contractions of one or more motor units from the peroneus tertius muscle of anaestheti zed cats. 2. In three experiments, seventy-three individual tendon org an-motor unit interactions were recorded. The motor units were stimula ted 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 af ferent discharge. The slopes of these relationships (the static sensit ivities) were steepest for the weakest units. 3. When motor units were stimulated in combination, the relationship between discharge frequen cy and plateau tension was again linear but the static sensitivities w ere generally much lower than for single units and approached 1 impuls e s(-1) g(-1). 4. Expression of these relationships in terms of the re lative tensions generated revealed that the tendon organs were activat ed most strongly by the IIb muscle fibres, the static sensitivities be ing 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 stead y-state contractile tensions within the muscle.