Reorganization of corticorubral synapses following cross-innervation of flexor and extensor nerves of adult cat: a quantitative electron microscopic study.

A quantitative electron microscopic study of corticorubral synapses was performed in the red nucleus (RN) of adult cats to determine the morphological correlates for the changes in time course of corticorubral excitatory post-synaptic potentials, which occur following cross-innervation of forelimb extensor and flexor nerves. Corticorubral synaptic endings were identified by anterograde degeneration after lesions of the ipsilateral sensorimotor cortex. Rubrospinal neurons innervating upper spinal segments were electrophysiologically identified and filled with horseradish peroxidase (HRP). These cells were mainly situated in the dorsomedial part of RN. Electron micrographs of the degenerating corticorubral synaptic endings were taken in the region surrounding HRP-filled neurons and the diameter of the dendrites contacted by such terminals was measured. In the cross-innervated animals many degenerating terminals were found to synapse on dendrites with large diameter and the somata of neurons in RN. This is in contrast to the previous observations in normal cats, in which very few corticorubral synapses were found to synapse on proximal dendrites and somata of RN neurons. The diameter of HRP-filled neurons in cats which were cross-innervated was slightly smaller than those observed in normal animals. These results indicate that new corticorubral synapses were formed on proximal dendrites and somata of RN neurons as a consequence of cross-innervation.