EFFECTS OF BACKGROUND-NOISE ON THE RESPONSE OF RAT AND CAT MOTONEURONS TO EXCITATORY CURRENT TRANSIENTS

1. We studied the responses of rat hypoglossal motoneurones to excitat ory current transients (ECTs) using a brainstem slice preparation. Ste ady, repetitive discharge at rates of 12-25 impulses s(-1) was elicite d from the motoneurones by injecting long (40 s) steps of constant cur rent. Poisson trains of the ECTs were superimposed on these steps. The effects of additional synaptic noise was simulated by adding a zero-m ean random process to the stimuli. 2. We measured the effects of the E CTs on motoneurone discharge probability by compiling peristimulus tim e histograms (PSTHs) between the times of occurrence of the ECTs and t he motoneurone spikes. The ECTs produced modulation of motoneurone dis charge similar to that produced by excitatory postsynaptic currents. 3 . The addition of noise altered the pattern of the motoneurone respons e to the current transients: both the amplitude and the area of the PS TH peaks decreased as the power of the superimposed noise was increase d. 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 effe cts 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, pentobarbito ne-anaesthetized cats. These recordings yielded results similar to tho se obtained in rat hypoglossal motoneurones in vitro.5. Our results su ggest that the presence of concurrent synaptic inputs reduces the effi cacy of any one input. The implications of this change in efficacy and the possible underlying mechanisms are discussed.