We aimed to study the receptor origin and postsynaptic potential profile of the medium latency reflex (MLR) response that develops in the soleus muscle when common peroneal nerve of antagonist tibialis anterior (TA) muscle is electrically stimulated. To achieve this aim, we electrically stimulated common peroneal nerve and recorded surface electromyography (SEMG) responses of soleus and TA muscles of informed volunteers. Additionally, we recorded intramuscular EMG from the soleus muscle. Stimulation of common peroneal nerve induced a direct motor response (M-response) in the TA and MLR in SEMG of the soleus. Using voluntarily-activated single motor units (SMUs) from the soleus muscle we noted that there were two distinct responses following the stimulus. The first response was a reciprocal inhibitory reflex probably originating from the antagonist muscle spindle primary (Ia) afferents. This was followed by an indirect reflex response activated by the contraction of the TA muscle during the M-response. This contraction generated a rapid acceleration in the direction of dorsiflexion hence inducing a stretch stimulus on soleus muscle. The response of soleus to this stimulus was a stretch reflex. We suggest that this stretch reflex is the main contributor to the so-called soleus MLR in the literature. This study illustrated the importance of using SMUs and also using discharge-rate based analysis for closely examining previously 'established' reflexes.
Keywords: Electrical stimulation; Human reflex; Muscle spindle; Peroneal nerve; Soleus motor units; Tibialis anterior.