Formation of functional synapses in the adult cat red nucleus from the cerebrum following cross-innervation of forelimb flexor and extensor nerves

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Abstract
The effects of cross-innervation of peripheral flexor and extensor nerves on the time course and properties of red nucleus (RN) neurons were studied in adult cats. The time course of corticorubral unitary EPSPs was examined. In operated cats, RN neurons innervating upper spinal segments (C-cells) manifested corticorubral unitary EPSPs with shorter time-to-peak and larger amplitude than in normal cats. The mean amplitude of these EPSPs was 0.41 ± 0.11 mV; the mean time-to-peak was 1.14 ± 0.51 ms (n = 18). These values differ from normal cats (0.33 ± 0.09 mV, and 2.68 ± 0.61 ms, n = 22). RN neuron membrane properties were examined in cross-innervated cats. The main time constant was 5.2 ± 0.7 ms (n = 10), the shorter equalizing time constant, 0.69 ± 0.19 ms (n = 10), the input resistance, 2.5 ± 0.8 MΩ. These values were not significantly different from those of normal cats. The electrotonic length was 1.2 (n = 10), somewhat larger than in normal cats. The number of converging interposito-rubral synapses, estimated in eight cells from cross-innervated cats, ranged from 37 to 57. This was not significantly different from normal cats, and indicated that there is no large-scale degeneration of interpositorubral synapses after cross-innervation. These results suggest that sprouting and formation of functional synapses occur after cross-innervation of peripheral flexor and extensor nerves.