An ultrastructural study of serially sectioned Renshaw cells. I. Architecture of the cell body, axon hillock, initial axon segment and proximal dendrites.

Seventeen neurons which were postsynaptic to axon collateral boutons of intracellularly HRP-stained triceps surae alpha-motoneurons were studied ultrastructurally. All 17 neurons were situated in lamina VII, ventro-medially to the main motor nuclei. This and other facts support the assumption that the observed neurons are morphological correlates to the physiologically defined Renshaw cells. The contours of the cell bodies, as observed in the midnucleolus plane, were elongated. The axons originated either from the cell bodies or from dendrites. The number of dendrites of each neuron varied between 3 and 7. The appearance of the presumed Renshaw cells was also compared with that of a larger sample of neurons from the ventral part of lamina VII which was studied light microscopically in semithin sections. It was suggested that the Renshaw cells belong to the larger and more elongated neurons in the area.