Light and electron microscopy of dorsal spinocerebellar tract neurones in the cat: an intracellular horseradish peroxidase study.

Intracellular injections of horseradish peroxidase were made into dorsal spinocerebellar (d.s.c.t.) neurones in Clarke's column of the cat. All the d.s.c.t. neurones were excited from Group I muscle afferent fibres. The stained neurones were examined at the light and electron microscope level, and light microscope material was subjected to computer-aided reconstruction and quantitative analysis. The dendritic trees of d.s.c.t. neurons extended about 3 mm in the long axis of the spinal cord and were confined, in the transverse plane, within or very close to Clarke's column. The dendrites branched extensively and carried various irregularities: complex clusters of fine branches, spine-like protuberances and bead-like varicosities. None of the axons of the cells showed any sign that they gave off collaterals. Computer-aided reconstruction allowed the dendritic trees to be viewed from any angle and enabled measurements of their total dendritic lengths, surface area and volume. The dendritic diameters at branch points were tested against Rall's 3/2 power rule. Electron microscopical analysis confirmed and extended previous studies. Of particular significance was the observation that boutons containing flattened vesicles could be presynaptic to both a d.s.c.t. neurone and to a giant synaptic terminal that made contact with the same d.s.c.t. dendrite.