Habituation behavior of the medium-latency reflex over the anterior tibial muscle after electrical stimulation of the sural nerve

Over human leg muscles, three motor responses (MR) can commonly be elicited, namely short-latency reflex (SLR), medium-latency reflex (MLR), and long-latency reflex (LLR). The MLR is less well understood than SLR and LLR. As the response to subsequent stimuli may be used to characterize central influences of an MR, we were interested, whether the MLR differs from SLR and LLR with respect to its habituation and facilitation behavior. MR were examined over the anterior tibial (TA) muscle at different contraction levels after electrical single or train stimuli (time intervals of 3 ms) over the ipsilateral sural nerve. Furthermore, MR were selectively averaged after each of four subsequent stimuli (1 Hz, 0.4 Hz, trains-of-3). After single stimuli, the peak latency values were 46.2 ± 2.3 ms, 88.0 ± 5.8 ms (MLR), and 131.7 ± 22.2 ms (LLR). All three MR gained similarly strong and significantly in amplitude when up to 10 kg of weight was loaded compared with no weight load. After train stimuli, the LLR but not SLR and MLR gained significantly in amplitude as compared with single stimuli. Different to SLR and LLR, the MLR showed significant habituation behavior at a stimulus repetition rate of 1 Hz but not of 0.4 Hz. Thus, inhibitory interneurons seem to be involved in the MLR pathway.