Stroboscopic Cinematographic and Videorecording of Dynamic Bag1 Fibres During Rapid Stretching of Isolated Cat Muscle Spindles

When an isolated cat muscle spindle is subjected to a ramp and hold stretch the primary sensory annulo-spiral round the dynamic bag1 (Db1) fibre is extended and then creeps back part way towards its original state over several seconds since the poles of the fibre give way (Boyd, 1976). This ‘creep’ at the final length is small, or absent, when the fibre is inactive, and is greatly enhanced by activation of the fibre (Fig. 1; Boyd, Gladden & Ward, 1981). The length sensitivity of the primary sensory ending is greatly increased during the dynamic phase of stretching, but the static length sensitivity is little increased because the creep causes pronounced slow adaptation of the Ia discharge (Fig. 2).