The origin of the soleus late response evoked by magnetic stimulation of human motor cortex

Transcranial magnetic stimulation (TMS) of the human motor cortex elicits a primary motor evoked potential (MEP) in the soleus muscle at a latency of approximately 30 msec, which may be followed by a late potential with a variable latency of 80-120 msec (soleus late response, SLR). While the MEP is thought to arise from stimulation of the corticospinal tract, the origin of the SLR is uncertain. In the present study we have investigated the properties of the SLR in order to elucidate its origin. An SLR was evoked at a latency of 100-120 msec in 5 out of 10 normal subjects in the relaxed state and at a latency of approximately 100 msec in all subjects when tibialis anterior (TA) was slightly facilitated. The SLR was largest with 5-10% TA contraction, decreased in size with increasing levels of TA contraction and was negligible in all subjects when the foot was immobilised. The latency of the SLR fell by 23 msec when the foot was passively dorsiflexed 20 degrees. A similar response to the SLR, at a latency of 77 msec, was present in all subjects following electrical stimulation of the peroneal nerve. Our findings suggest that the SLR is a soleus stretch reflex resulting from dorsiflexion of the foot due to preferential activation of TA following cortical stimulation.