The synaptic connexions to intercostal motoneurones as revealed by the average common excitation potential.

Abstract

The hypothesis was advanced that the joint occurrence of unitary EPSP [excitatory postsynaptic potential] evoked in motoneurons by branches of common stem presynaptic fibers caused on average, a transient depolarization in 1 motoneuron at the time of discharge in another motoneuron of the same pool. The hypothesis was tested in anesthetized, paralyzed cats by averaging the naturally occurring synaptic noise of thoracic inspiratory motoneurons with an averager triggered by spikes from other inspiratory motoneurons. These spikes were obtained as efferent discharges in nerve filaments supplying the proximal regions of the external intercostal muscles. A transient depolarization centered around the time of the trigger spikes was consistently observed and was designated the average common excitation (ACE) potential. The peak depolarization lay between -1.0 and +4.6 ms (mean +0.7 ms) with respect to the trigger spikes and the rise times of its most prominent component ranged from 4-16 ms (mean 8.4 ms). The amplitudes of the ACE potentials ranged from 6-104 .mu.V (mean 32 .mu.V) when the trigger spikes were derived from a filament in the same segment as the relevant motoneurons, and from 3-42 .mu.V (mean 19 .mu.V) when the filament was 2 segments rostral to the motoneuron. Cells innervating the proximal region of the intercostal space gave larger ACE potentials than those innervating more distal regions and also showed larger central respiratory drive potentials. ACE potentials were observed for either .alpha. or .gamma. spikes as triggers. The potentials were usually smaller for the .gamma. than for the .alpha. spikes, the mean ratio being about 0.6. The presence of the ACE potentials from the .gamma. spikes was taken as evidence for .alpha.-.gamma. coactivation by common presynaptic axons. A theory was developed which quantitatively accounts for the main features of both the ACE potential and the short term synchrony observed by Sears and Stagg. The theory includes the proposition that the raised probability of firing of a motoneuron due to a unitary EPSP has a time course which may be described by the sum of the EPSP time course and its time differential. Thus, via the measurements, the theory predicts at least to a 1st approximation the expected probability of firing due to EPSP of various shapes and sizes.