Propriospinal and supraspinal projections to the motor nuclei in the cat spinal cord

Abstract

Abstract Propriospinal and supraspinal projections to the motor nuclei of L 3 –S 1 were examined with light and electron microscopy in cats surviving complete transection of L 2 for periods of 1–12 days. Light microscopy revealed maximal Nauta‐Gygax and Fink‐Heimer degeneration in the motor nuclei of L 3 –L 5 at three days survival. Maximal degeneration in the motor nuclei of L 6 –S 1 did not appear until six days post‐operatively. In the three and four day animals, electron microscopy revealed degenerating synaptic knobs, making axo‐dendritic and axo‐somatic contacts, and many phagocytosed degenerating synaptic knobs, in the motor nuclei of L 3 –L 5 ; no synaptic degeneration was observed in the motor nuclei of more caudal segments. In the six to eight day animals, degenerating axo‐dendritic knobs and many phagocytosed degenerating knobs were found in the motor nuclei of S 1 ; no synaptic degeneration and very few phagocytosed degenerating knobs were present in the motor nuclei of L 3 –L 5 at these survival times. Because of the rapid phagocytosis of degenerating knobs in the motor nuclei of the early and late post‐operative animals, it was difficult to attribute the various degenerating knobs to a particular synpatic type. It is suggested that the synaptic degeneration seen in the motor nuclei near the transection of the earlier post‐operative animals is of mixed propriospinal and supraspinal origin, whereas the degeneration seen in the caudal motor nuclei of the later post‐operative animal represents supraspinal terminations. The contribution of dorsal root and long and short descending projections to the synaptic population about motoneurons is discussed.