Effects of impulse frequency, PTP, and temperature on responses elicited in large populations of motoneurons by impulses in single Ia-fibers.

Excitatory postsynaptic potentials (EPSPs) were evoked in large populations of motoneurons by impulses in single Ia- and group II fibers and recorded from L7 and S1 ventral roots by means of a sucrose-gap technique and spike-triggered averaging, as previously described (33, 34). Factors known to influence conduction at branch points in peripheral nerves or transmission in the Ia projection to motoneurons were studied to determine their effects on these postsynaptic population potentials (PSPPs). When impulse frequency was plotted against the amplitude of these PSPPs, a sequence of low-frequency depression, higher frequency facilitation, and still higher frequency depression was regularly observed. Similar plots of PSPP amplitude versus impulse frequency in a small sample of group II spindle fibers did not reveal any low-frequency depression. Higher frequency facilitation and depression followed the same pattern as that produced by Ia-fibers. Early potentiation of the PSPPs was produced by applying 20 stimulating pulses at 500/s to branches of the medial gastrocnemius nerves containing single Ia-fibers. Single, posttetanic PSPPs were recorded at fixed intervals following each tetanus and averaged. A brief, initial depression followed by a rapidly declining potentiation of PSPPs was regularly observed. When the core temperature was reduced from 41 to 35 degrees C, there was an approximately linear increase in the amplitude of PSPPs. At each temperature a combination of cooling and posttetanic potentiation (PTP) elicited larger PSPPs than cooling alone. The effects of impulse frequency, PTP, and temperature on PSPPs may all be interpreted in terms of their known effects on action potentials or on conduction at branch points in peripheral nerves. Similar effects on transmission or transmission failure in the terminal arborizations of Ia-fibers would account for the influence of these factors on PSPPs. Alternatively, these same three factors might cause similar effects on PSPPs through influences they might exert on the transmitter release mechanism.