Relation between shapes of post‐synaptic potentials and changes in firing probability of cat motoneurones

Abstract

SUMMARY1. The shapes of post-synaptic potentials (p.s.p.s) in cat motoneurones were compared with the time course of changes in firing probability during repetitive firing. Excitatory and inhibitory post-synaptic potentials (e.p.s.p.s and i.p.s.p.s) were evoked by electrical stimulation of peripheral nerve filaments. With the motoneurone quiescent, the shape of each p.s.p. was obtained by compiling post-stimulus averages of the membrane potential. Depolarizing current was then injected to evoke repetitive firing, and the post-stimulus time histogram of motoneurone spikes was obtained; this histogram reveals the primary features (peak and/or trough) of the cross-correlogram between stimulus and spike trains. The time course of the correlogram features produced by each p.s.p. was compared with the p.s.p. shape and its temporal derivative.2. E.p.s.p.s of different sizes (015--31 mV, mean 0-75 mV) and shapes were investigated. The primary correlogram peak began, on the average, 0-48 msec after onset of the e.p.s.p., and reached a maximum 0-29 msec before the summit of the e.p.s.p; in many cases the correlogram peak was followed by a trough, in which firing rate fell below base-line rate. The height of the correlogram peak with respect to base-line firing rate increased in proportion to both the amplitude of the e.p.s.p.s and the magnitude of their rising slope (in these data, amplitude and rising slope also covaried).3. The mean half-width of the correlogram peaks (065+0-28 msec (S.D.)) agreed better with the average half-width of the e.p.s.p. derivatives (0-55 + 033 msec) than

Untitled

“…Increasing the noise level makes the response to an input pulse less sharply dependent on the mean drive level. Our results may help to understand a broad set of experimental measurements (Fetz and Gustafsson, 1983;Kirkwood and Sears, 1978;Poliakov et al, 1996Poliakov et al, , 1997.…”

“…To our knowledge, the only direct physiological measurements of the effect of noise on the PSTH response of neurons to current pulses have been in motoneurons stimulated with a suprathreshold current (Poliakov et al, 1996(Poliakov et al, , 1997Fetz and Gustafsson, 1983). Our theory allows two general predictions to be made.…”

“…If the stimulus is a presynaptic spike, the measurement is called a cross-correlogram. It has been used extensively (Fetz and Gustafsson, 1983;Kirkwood and Sears, 1978;Knox, 1974;Moore et al, 1970;Poliakov et al, 1997;Poliakov et al, 1996) to document the input-output transform of motoneurons. The characteristics of the background noise can be as important as the PSP shape itself (Kirkwood and Sears, 1979), which has implications for the interpretation of the form of the PSTH or cross-correlogram; the amount of noisy background input must be taken into account when making comparisons (Kirkwood and Sears, 1991).…”

“…They found results consistent with this model in intercostal muscle afferents and ventral root afferents during respiration, but were limited to using excitatory PSPs (EPSPs) recorded in separate experiments. Fetz and Gustafsson (1983) were able to record the PSPs simultaneously with the action potential response of cat motoneurons. They found that while the linear combination can describe the appearance of the PSTH under some conditions, in which case the PSP derivative dominates the response, synaptic noise affects the relative contributions of the two components and that the inhibitory PSP (IPSP) response is not well described.…”

“…* * Current address: Ecole d'lngénieurs de l'Etat de Vaud (EIVD), Yverdon-les-Bains, Switzerland. tally measured passive time constant is used because the recovery processes during refractoriness following an action potential are not modeled at all. However, experimental studies of the effect of noise on the PSTH are generally conducted on tonically firing neurons (Fetz and Gustafsson, 1983;Poliakov et al, 1996Poliakov et al, , 1997. The goal of this paper is to show that the effect of noise on the PSTH demonstrated for the integrate-and-fire neuron in the companion paper can also be obtained using a model with a more realistic membrane trajectory, but without incorporating further biological details such as specific ionic conductances or adaptation.…”