Changes of Achilles tendon reflex amplitudes during a fixed foreperiod of four seconds.

Abstract

ABSTRACTPreparation for a voluntary foot or finger response was studied by evoking Achilles tendon (T) reflexes. Reflexes were evoked simultaneously in both legs at 13 points between 100 and 4300 msec after the warning signal. With this paradigm, the time course of changes in reflex amplitudes related to preparatory processes could be measured simultaneously in the leg involved in the response and in the uninvolved leg. Immediately following the warning stimulus, the reflex amplitudes increased in both involved and non‐involved muscles, presumably reflecting a general non‐specific arousal. During the second half of the foreperiod, reflex amplitudes in non‐involved muscles remained elevated, perhaps reflecting a general activation of the motor system, but returned to baseline in the involved muscles. It is hypothesized that the consistently smaller reflex amplitudes in the involved leg during the entire ISI reflect a presynaptic inhibition of the la afferents to the motoneurons involved in the response. Following the response stimulus, reflex amplitudes increased in both involved and non‐involved muscles, although in the former at an earlier point and to a greater extent. It is suggested that this increase following the response stimulus is a manifestation of the transition from motor preparation to response execution.