Jour nal of Elect r omyogr aphy and Kinesiology Vol. 2, No. 4, pp 189-202 0 1993 Raven Press, Ltd., New York Properties of Synaptic Noise in Tonically Active Human Motoneurons Jason D. Warren, Timothy S. Miles, and Kemal S. Tiirker zyxwvutsrqponmlkjihgfedc Department of Physiology, The University of Adelaide, Adelaide, Australia Summary: The objective of these experiments was to determine the amount of synaptic noise on the cell membrane at various intervals after an action po- tential in a motoneuron firing at a specified frequency. Sources of noise such as variations in the level of voluntary drive were minimized by selecting only segments of the spike train in which the unit was running within prescribed frequency limits. The level of the membrane potential of the motoneuron dur- ing these intervals was determined using two test “pulses” (compound Ia excitatory postsynaptic potentials) of known amplitude. This enabled the prob- ability of the membrane potential falling within a voltage “window” of known size at known times after the preceding spike to be determined. The probability density histograms showed that the fluctuations of membrane potential about a target interspike trajectory (i.e., the membrane noise) increased with time after the preceding spike. These fluctuations in the membrane potential can be accounted for by a one-dimensional “random walk” model of membrane noise. This model explains the salient features of the interval histograms, such as positive skewness at low target frequencies. A quantitative test of the model demonstrated its applicability to the motor pools of tibialis and masseter. Key Words: Synaptic noise-Human motoneuron-Membrane potential-Model. The operation of the nervous system is marked by a degree of uncertainty. Information is carried in the sequence of variable time intervals separating functionally identical action potentials in what is effectively a pulse code (28). In the simplest situa- tion in which a neuron is running at a nominally constant frequency, there is variability in the dura- tion of interspike intervals (ISIS). Whether the in- formation-carrying parameter of this spike train is the mean ISI, the modal ISI, or some other charac- teristic, this residual variability constitutes “noise,” which must reflect noise processes acting on the neuronal membrane potential. Noise has been investigated in a variety of neu- Accepted September 29, 1992. Address correspondence and reprint requests to Dr. T. S. Miles at Department of Physiology, The University of Adelaide, GPO Box 498, Adelaide, SA 5001, Australia. 189 ronal types, and the ol-motoneuron in particular has been extensively studied. The discharge character- istics of th‘ese neurons, as the final common output of the motor system to the extrafusal muscle fibers, are crucial to the transfer of information between nerve and muscle (20). Possible sources of neuronal membrane noise fall into two broad classes: those intrinsic to the neuron, and those due to the effects of external inputs (5). Randomness in the first in- stance might arise from the presence of mobile charge carriers on either side of the membrane, due to thermal agitation (17) or from fluctuations in transmembrane mobility or “flicker” noise (9,36, 17); variations in threshold firing level (9); or the probabilistic basis of synaptic transmission (21). However, taken together, these sources are of small magnitude, and cannot account for the observed IS1 variability (5,17). The most obvious source of vari- ability due to external inputs is fluctuation in the