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1. The hypothesis is advanced that the joint occurrence of unitary e.p.s.p.s evoked in motoneurones by branches of common stem presynaptic fibres causes, on average, transient depolarization in one motoneurone at the time of discharge in another motoneurone of the same pool. 2. The hypothesis was tested in anaesthetized, paralysed cats by averaging the naturally occurring synpatic noise of thoracic inspiratory motoneurones with an averager triggered by spikes from other inspiratory motoneurones. These spikes were obtained as efferent discharges in nerve filaments supplying the proximal regions of the external intercostal muscles. 3. A transient depolarization centred around the time of the trigger spikes was consistently observed and was designated the average common excitation (a.c.e.) potential. 4. The peak depolarization lay between -1.0 and +4.6 msec (mean +0.7 msec) with respect to the trigger spikes and the rise times of its most prominent component ranged from 4 to 16 msec (mean 8.4 msec). 5. The amplitudes of the a.c.e. potentials ranged from 6 to 104 muV (mean 32 muV) when the trigger spikes were derived from a filament in the same segment as the relevant motoneurones, and from 3 to 42 muV (mean 19 muV) when the filament was two segments rostral to the motoneurone. 6. Cells innervating the proximal region of the intercostal space gave larger a.c.e. potentials than those innervating more distal regions and also showed larger central respiratory drive potentials. 7. A.c.e. 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 ration being about 0.6. The presence of the a.c.e. potentials from the gamma spikes was taken as evidence for alpha-gamma coactivation by common presynaptic axons. 8. A theory is developed which quantitatively accounts for the main features of both the a.c.e. potential and the short term synchrony observed by Sears & Stagg (1976). 9. The theory includes the proposition that the raised probability of firing of a motoneurone due to a unitary e.p.s.p. has a time course which may be described by the sum of the e.p.s.p. time course and its time differential. Thus, via the measurements, the theory predicts at least to a first approximation the expected probability of firing due to e.p.s.p.s of various shapes and sizes.

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