Characterization of postsynaptic potentials evoked by sural nerve stimulation in hindlimg motoneurons from acute and chronic spinal cats

Abstract

The purpose of this study was to characterize the changes in postsynaptic potentials recorted in ankle extensor motoneurons resulting from activation of the sural nerve spinal cord transection in the adult cat. Eight acute and nine chronic animals were spinalized at T 12. Intracellular recordings from motoneurons innervating the triceps surae were performed. Sural nerve stimulations evoked complex synaptic potentials consisting of early and late components in all motoneurons. Early excitatory and inhibitory postsynaptic potentials (PSPs), as well as long latency excitatory postsnnaptic potentials were recorded and averaged for assessment of PSP amplitude and duration. Early PSPs, both excitatory and inhibitory, were significantly larger in the motoneurons of cats spinalized 4–6 months earlier. Central latency of excitatory potentials were similar in the two samples of motoneurons, but the central latency associated with the initial inhibitory PSP was significantly shorter in the recordings from motoneurons of chronic spinal cats. In most recordings, an additional inhibitory PSP followed the initial excitatory PSP in motoneurons, and this secoddary inhibitory PSP was similar in peak amplitude and duration in both samples of motoneurons. Also, a long latency excitatory PSP was recorded in a large percentage of motoneurons from both samples. This potential was typically of greater amplitude and longer duration in the motonuurons from chronic animals, when compared to recordings from acute animals. Although changes in amplitude and duration of PSP activity could be documented, there was no marked alteration in the frequency of occurrence of each PSP pattern recorded from the two preparations. This suggests that the synaptic pathways mediating the sural nerve reflexes have not qualitatively changed in the chronic spinal animals. The changes in amplitude and durations of the PSPs in the chronic spatial cat indicate, however, the quantitative changes have occurred. The quantitative changes have probably occurred in the interneuronal networks activated by cutaneous nerve (sural) stimulation, since it was shown that only minor changes in motoneuron membrane properties could be recorded in these same chronic spinal animals.