Effects of background noise on the response of rat and cat motoneurones to excitatory current transients.

Abstract

1. We studied the responses of rat hypoglossal motoneurones to excitatory current transients (ECTs) using a brainstem slice preparation. Steady, repetitive discharge at rates of 12-25 impulses s-' was elicited from the motoneurones by injecting long (40 s) steps of constant current. Poisson trains of the ECTs were superimposed on these steps. The effects of additional synaptic noise was simulated by adding a zero-mean random process to the stimuli. 2. We measured the effects of the ECTs on motoneurone discharge probability by compiling peristimulus time histograms (PSTHs) between the times of occurrence of the ECTs and the motoneurone spikes. The ECTs produced modulation of motoneurone discharge similar to that produced by excitatory postsynaptic currents. 3. The addition of noise altered the pattern of the motoneurone response to the current transients: both the amplitude and the area of the PSTH peaks decreased as the power of the superimposed noise was increased. Noise tended to reduce the efficacy of the ECTs, particularly when the motoneurones were firing at lower frequencies. Although noise also increased the firing frequency of the motoneurones slightly, the effects of noise on ECT efficacy did not simply result from noise-induced changes in mean firing rate. 4. A modified version of the experimental protocol was performed in lumbar motoneurones of intact, pentobarbitone-anaesthetized cats. These recordings yielded results similar to those obtained in rat hypoglossal motoneurones in vitro. 5. Our results suggest that the presence of concurrent synaptic inputs reduces the efficacy of any one input. The implications of this change in efficacy and the possible underlying mechanisms are discussed.

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“…* * 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.…”

“…In Poliakov et al (1996Poliakov et al ( , 1997, PSTH responses to Poisson-distributed trains of current pulses were recorded. The pulses were injected into the soma of rat hypoglossal motoneurons during repetitive discharge.…”

“…A: PSTH "high" noise (noise power level 30 nA 2 µs). B: "Low" noise (courtesy of M. Binder; data from Poliakov et al, 1996). postsynaptic currents generated by presynaptic spike arrival. PSTHs of motoneuron discharge occurrences were compiled when the pulse trains were delivered either with or without additional current noise which simulated noisy background input.…”

“…PSTHs of motoneuron discharge occurrences were compiled when the pulse trains were delivered either with or without additional current noise which simulated noisy background input. Figure 1 shows examples of responses from a rat motoneuron taken from the work of Poliakov et al (1996). The effect of adding noise can be seen clearly: the low-noise peak is followed by a marked trough, whereas the high-noise PSTH has a reduced amplitude and a much smaller trough.…”

“…The effect of stochastic background input on the peristimulus time histogram (PSTH) response to spike input is calculated analytically. Model results are compared with the experimental data of Poliakov et al (1996). The linearized theory shows that the PSTH response to an input spike is proportional to a filtered version of the postsynaptic potential generated by the input spike.…”