Composite EPSPs in motoneurons of different sizes before and during PTP: implications for transmission failure and its relief in Ia projections.

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Abstract
Brief electrical shocks were applied to muscle nerves of the cat''s hindlimb, setting up maximal Ia volleys in them at 1-2/s. Composite excitatory postsynaptic potentials (EPSP) elicited by these volleys were recorded intracellularly form homonymous motoneurons. The input resistance and/or duration of afterhyperpolarization of each motoneuron was measured to obtain an estimate of its relative size. The amplitudes of the composite EPSP were correlated directly with the input resistances of the motoneurons in which they were recorded. Tetanization of the muscle nerves at 500/s caused subsequent potentiation of the composite EPSP. The percent increases in amplitude of these EPSP were correlated inversely with the input resistances. The largest increases in percent potentiation occurred in the smallest control EPSP. The absolute increases in EPSP amplitudes during posttetanic potentiation (PTP) varied widely among motoneurons and were not correlated with their input resistances. As the duration of the tetanus was doubled in steps from 2-32 s, percentage and duration of potentiation both increased. These effects were each greater in motoneurons with low input resistances. Response to duration of tetanus is also greater in large cells. Although the mean rise times increased from 0.63 .+-. 0.32 ms before potentiation to 0.75 .+-. 0.30 ms during the peak of PTP, in 10 of these 70 EPSP the rise times did not change during PTP and in 5 cases they decreased slightly. A substantial body of literature may be interpreted as evidence that transmission failure occurs normally in the projections of Ia-fibers to motoneurons. The smaller composite EPSP recorded from large motoneurons before PTP may be due, at least in part, to a higher incidence of transmission failures in the Ia projections to large cells. A greater percentage of inactive Ia synapses would, thus, be available on large motoneurons for activation during PTP, resulting in a higher percentage of potentiation in them. The concept of transmission without failure in Ia projections to motoneurons is an assumption with less experimental justification than the view that transmission failure is a common and normal occurrence.