Mechanical arrangement and transducing properties of Golgi tendon organs.

TLDR: The present data emphasize that tendon organs can participate in the moment to moment reflex control of normal muscle activity and give a measure of the extent to which each tendon organ was more sensitive to active than passive force.

Abstract: 1. The mechanical arrangement and transducing properties of Golgi tendon organs in soleus and anterior tibial muscles of anesthetized cats have been studied by noting responses of their Ib afferents to muscle stretch (passive force) and contraction (active force) of small portions of the muscle including functionally isolated motor units. 2. Tendon organs were shown to be arranged both in-series and in-parallel with adjacent muscle fibers. There were gradations in these relations, the tightest arrangements involving the response to contraction of a single motor unit, brisk discharge from an in-series receptor and pause in the stretch-activated firing of an in-parallel receptor. Other arrangements included those in which groups of muscle fibers neither directly in-series nor in-parallel with a receptor were still found capable of influencing its firing pattern. If in-series muscle fibers maintained their contraction while in-parallel fibers were also contracting, the receptor usually responded slightly less actively than it did to the in-series force alone. 3. Tendon organs were found to have a very low threshold to in-series force developed by muscle contraction. Responses were observed to as little as 0.5 gm of twitch tension. Minimum active force thresholds were similar for the two muscles studied, but thresholds to dynamic stretch were lower for anterior tibial receptors. Division of the dynamic stretch threshold by the minimum active force threshold gave a measure of the extent to which each tendon organ was more sensitive to active than passive force. These values (generally less than 50) did not negate the physiological significance of responses to passive stretch. 4. The present data, together with those of Houk and his co-workers (1967, 1971) emphasize that tendon organs can participate in the moment to moment reflex control of normal muscle activity.