A morphological study of the axons and recurrent axon collaterals of cat sciatic alpha-motoneurons after intracellular staining with horseradish peroxidase.

Utilizing the centrifugal neuronal transport of intracellularly injected horseradish peroxidase (HRP), we have performed a light microscopic (LM) investigation of the intramedullary parts of the axons and axon collaterals of sciatic alpha-motoneurons in the adult cat. The intramedullary parts of the alpha-motor axons had comparatively short internodes (down to 75 microns) and were thinner than reported in earlier studies on the ventral root. Positive correlations were obtained when relating nodal diameters (2.8-7.8 micron) or the mean diameters of the motor axons in the white matter (4.4-9.0 micron) to the diameters of the initial axonal segments (2.3-4.9 micron). Eighty percent of the motor axons gave off one to five collaterals. There was no correlation between the numbers of collaterals and the lengths of the parent motor axons in the gray matter. The branching patterns of the axon collaterals showed considerable variation and the number of end branches from a single collateral ranged between 1 and 39. The rostro-caudal distribution of the collateral end branches was arranged symmetrically within a narrow space (+/- 300 micron) around the origins of the first order collaterals. Outbulgings of the motor axon collaterals, interpreted as synaptic terminals, were found along (59%) or at the ends (41%) of the collateral branches, and were located 200-700 micron away from the origin of the first order collateral. No characteristic LM feature of the outbulgings was distinguished.