Structural features of synaptic connections between descending systems and spinal neurons in the cat

The degeneration of axon terminals of the lateral (cortico and rubrospinal) and ventral (vestibulo- and reticulospinal) descending and propriospinal (lateral and ventral funiculus) systems was investigated by electron microscopy in the cat spinal cord. We studied the mean diameters of axon terminals, their arrangement on the neuronal structures in the grey matter, as well as the variety of the destructive changes in synaptic terminals. The average size of the ‘dark’ type degenerating axon terminals of fibers in the lateral descending systems was smaller than that of the ‘light’ type degenerating axon terminals in the medial descending systems. The average size of the both ‘dark’ and ‘light’ type degenerating propriospinal terminals of the ventral funiculus differed from those of the lateral funiculus.The findings concerning the sizes of synaptic terminals of the different descending and propriospinal systems correlate to a certain extent with the electrophysiological data about the nature of their synaptic action upon the spinal neurons.