Parallel Fluctuations of EPSP Amplitude and Rise Time with Latency at Single Ia-Fiber-Motoneuron Connections in the Cat

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1. Membrane potential was recorded intracellularly from motoneurons in the lumbosacral spinal cord in cats. Single sweeps triggered by impulses in a single Ia-fiber projecting to the impaled motoneuron were digitized at 20-µs resolution and stored on magnetic tape.

2. Analysis of many of these records confirmed that the latency of excitatory postsynaptic potentials (EPSPs) produced by activity in single Ia-fibers fluctuates from trial to trial. Distributions of EPSP latency differed at each Ia-motoneuron combination with regard to their variance, although all were unimodal.

3. A uniform finding was that the EPSPs evoked with short latencies from the Ia trigger pulse are larger in peak amplitude than those with longer latency. EPSP amplitudes decrease monotonically as latency increases throughout the range of latencies.

4. EPSP rise time also decreases steadily as latency increases. EPSP half-width tends to decrease with latency, but not as consistently as rise time.

5. Voltage fluctuations unrelated to the trigger, i.e., noise, did contribute to our measurements of variability in EPSP parameters. However, analyses designed to estimate this contribution suggest that the spike-triggered EPSPs vary significantly beyond what can be ascribed to noise.

6. None of the fluctuations in amplitude, latency, and rise time was correlated with the instantaneous rate of discharge of the Ia-fiber (3-40 pulses/s).

7. These fluctuations could not be related to variability in postsynaptic parameters, e.g., membrane potential, time constant, or locus of synaptic activation.

8. These data are most consistent with the view that the origin of fluctuations in these EPSP parameters is presynaptic. Our evidence supports the conclusion that fluctuations result from the stochastic nature of transmitter release rather than from variability in axonal conduction or bouton invasion.