Sensorimotor connections in the lumbar spinal cord of the young rat: a morphological study

A morphological investigation of sensorimotor connections was performed on the isolated lumbar spinal cord of 8-15-day-old rats using horseradish peroxidase labelling techniques. Horseradish peroxidase was applied to the filaments of dorsal and ventral roots and injected intracellularly into motoneurons. The labelled afferent fibres and their contacts on motoneurons were examined under a light microscope. Numerous afferent collaterals entered the lateral motor nuclei. In the medial motor nuclei a few afferent collaterals were found. Some fibres were visible passing through the ventral commissure. The number of boutons per afferent collateral in the motor nuclei was 40-60. A single terminal branch contained one to five boutons (average 1.5). Predominating axodendritic and apparent axosomatic contacts were found between afferent fibres and motoneurons belonging to the lateral motor nuclei. The contacting boutons were both terminaux and en passant. As a rule, the sensorimotor connection involved dorsally and rostrocaudally directed dendrites of the first to sixth orders.