MOTOR-UNIT FIRING FREQUENCY CAN BE USED FOR THE ESTIMATION OF SYNAPTIC POTENTIALS IN HUMAN MOTONEURONS

This paper describes a new method that uses the frequency of firing of motor units to estimate the stimulus-induced net post-synaptic potent ial (PSP) and the synaptic noise in the membrane of voluntarily active human motoneurones. Unlike the peri-stimulus time histogram (PSTH) wh ich is the most commonly used method for assessing stimulus-induced sy naptic potentials in human motoneurones, this new approach overcomes c ontamination of the results caused by the synchronizing effect of the stimulus on the firing pattern of the motor units. However, even after overcoming the contamination by synchronized firing, the new method d oes not directly represent the true net synaptic potential in-the moto neurone membrane. Therefore, a new term estimated net synaptic potenti al (ENSP) has been introduced. This term highlights the fact that the stimulus-induced net synaptic potential has been determined indirectly and that the size and the shape of this synaptic potential may depend on the level of activity of the recording medium (i.e., pre-stimulus firing frequency of the motor unit). This paper also puts forward a no rmalization procedure that allows the value of the ENSP and the amplit ude of the synaptic noise to be read from the ENSP graph. The normaliz ation procedure, therefore, allows comparisons of those values within and between subjects.