AbstractThe excitability properties of the sensory nerve and its finer branches in muscle spindle have been studied in isolated preparations using electrical stimulation of the nerve in conjunction with controlled extension of the spindle. Characteristic differences in recovery cycle were demonstrated between the trunk fibre and the intracapsular branches. The excitability of the afferent nerve was modified by stretch; during stretching there was a rise in excitability and a shortening of the recovery cycle in direct relation to the amount of stretch. These changes were related to the underlying receptor potential and to local currents in the nerve. In the aftermath of stretch the excitability of the nerve as tested with electrical shocks was raised This was in contrast to the depression in mechanical sensitivity as revealed by a test stretch. When an extra impulse was elicited by an electrical shock in the course of sustained stretch the succeeding spike train was shifted, its general pattern being unchanged. It appears that the interjected spike transiently wipes out the currents leading up to the generation of the succeeding spike and resets the impulse generating process. Considerations of the functional properties of the afferent fibre and its branches as found in the present study suggest that the sensory system of the muscle spindle acts in synchrony during stretch.