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-1 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.