Presynaptic depolarization of terminals of rubrospinal tract fibers in intermediate nucleus of cat spinal cord.

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Abstract
The occurrence of presynaptic depolarization (PD) of terminals of the rubrospinal (RS) tract fibers were investigated by studying the changes in their excitability to electrical stimuli applied in the intermediate nucleus of the cat spinal cord. Amplitudes of the population responses and firing index (Fi) of individual neurons were used as indicators of RS fiber activation. Conditioning stimulation both of cutaneous and group I muscle afferents increased the antidromic firing of RS fibers. Volleys in the cutaneous (superficial peroneus, SP) and mixed (tibialis, TIB) nerves were more effective than volleys in group I muscle afferents of biceps semitendinosus (BST), gastrocnemius-soleus (GS), deep peroneus (DP) and plantaris nerves. This pattern of excitability increase was opposite to that found for group I GS fibers activated from the same microelectrode position, since group I fibers were more effectively depolarized by conditioning volleys in group I muscle afferents of flexor nerves than by volleys in cutaneous nerves. The increase in excitability of red nucleus (RN) fibers produced by conditioning volleys in group I muscle afferents usually built up slowly with successive trials and was relatively weak. This was in contrast to the very effective and rapidly developing increase in firing of group I GS fibers. The increase in excitability of RS fibers evoked by conditioning volleys in cutaneous nerves appeared with 5- to 9-ms segmental latencies, reached a maximum within 20-35 ms, and lasted up to 150-200 ms. At shorter time intervals (1.5-5 ms) there was usually a reduction of firing index, which matched closely the time course of the field potentials produced by the conditioning stimuli at the same site. The latencies of the PD would be overestimated. The early phase of reduction of excitability of RS fibers appeared to be due to the current flows initiated by the activation of cells in the dorsal horn. The relevance of the presynaptic depolarization of RS fibers for the modulation of transmission between these fibers and their target cells could not be defined.