Influence of stretch-evoked synaptic potentials on firing probability of cat spinal motoneurones.

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Abstract
Shapes of post-synaptic potentials (PSP) in cat motoneurons were compared with the time course of correlated changes in firing probability during repetitive firing. Excitatory and inhibitory post-synaptic potentials (EPSP and IPSP) were evoked by brief triangular stretches of the triceps surae-plantaris muscles. Depolarizing current was injected through the recording micro-electrode to evoke repetitive firing and the post-stimulus time histogram of motoneuron spikes was obtained. EPSP (n = 80) of different sizes (30-1040 .mu.V) and rise times (1.1-8.2 ms) were investigated in 59 motoneurons. The majority of the EPSP were recorded in triceps surae-plantaris motoneurons with high levels of synaptic noise (estimated peak-to-peak fluctuations of 1.5-3.5 mV). This noise was generated by keeping the triceps surae-plantaris muscles stretched to a near maximal degree. The remaining EPSP were recorded in motoneurons to other hind-limb muscles with a low level of synaptic noise. The height of the primary peak of the correlogram with respect to base-line firing rate increased in proportion to both amplitude and rising slope of the EPSP. Using normalization procedures or using EPSP of constant amplitude but different slopes and vice versa, the relative peak height increased with EPSP peak derivative with a slope of around 6/mV per ms and with EPSP peak amplitude with a slope of .apprx. 1/mV. The shape of the correlogram (peak and trough) seemed well described by a linear combination of the shape of the EPSP derivative and that of the EPSP itself. The relative EPSP contribution (EPSP:EPSP derivative ratio) varied with EPSP amplitude and noise level, being largest (mostly 0.25-1.0) for small EPSP (100-300 .mu.V) in high levels of synaptic noise and smaller (0-0.25) for larger EPSP and for EPSP in a low noise background. Conforming with this, a leaky integration of the correlograms gave time courses that in most cases closely resembled the shape of the EPSP. The time constant of the leaky circuit (= inverse value of EPSP:EPSP derivative ratio) varied with EPSP amplitude and noise level in the same manner as obtained by direct fitting of EPSP and EPSP derivative shape to correlogram shape. Stretch-evoked IPSP (n = 10) of amplitude (90-360 .mu.V as measured close to firing level) were investigated in pre-tibial flexor motorneurons with low levels of synaptic noise. These IPSP generated correlogram troughs closely resembling, although somewhat wider than, the shape of the IPSP derivatives. Intracellular negativities related to extracellular fields were obtained, either in isolation or clearly preceding synaptic potentials, in hamstring motoneurons following large stretches (15-35 .mu.m) of the triceps surae-plantaris muscles. These negativities could give, if large enough (100-300 .mu.V), clear peaks in the correlogram corresponding to a net depolarization of the membrane of 100-300 .mu.V.