Ia reflexes and EPSPs in human soleus motor neurones

The reflex responses of single motor units in the soleus muscle to electrical stimulation of the tibial nerve were recorded in human volunteers. A feature of the experiments was the stimulation paradigm used. In order to control the peri-stimulus firing rate, a computer triggered the stimulus isolator only when 2 interspike intervals of specified duration occurred in succession. In addition, the timing of the stimulus in relation to the preceding action potential was controlled in a manner similar to a conditioning/testing paradigm. The general pattern of response was an initial, "H-reflex" excitation at monosynaptic latency, followed first by a silent period due to the refractoriness of the motor neurone, then by other phases of reduced activity. When the stimulus intensity was increased, the intensity of the excitation and the duration of the silent period increased in parallel. When the pre-stimulus firing rate of the motor unit was varied, the amplitude of the H-reflex response. normalized to the number of stimulus trials, was similar at 6, 8 and 10 Hz, but was greater at 4 Hz in most units tested. These findings were consistent with a simple model of the events occurring at the cell membrane in this reflex which was proposed by Ashby and Zilm (1982a), although some modification of the model was necessary to account for the different response at 4 Hz. The improved stimulation paradigm enabled a direct estimate to be made of the amplitude and shape of the rising phase of the Ia EPSP in human motor neurones.