Electron microscopic studies of serially sectioned cat spinal alpha-motoneurons. III. Motoneurons innervating fast-twitch (type FR) units of the gastrocnemius muscle.

Two intracellularly stained cat gastrocnemius alpha-motoneurons of the FR-type (Burke et al., '73) were studied ultrastructurally. The architecture and synaptology of the cell body and proximal parts of the dendrites were analyzed from a long series of consecutive sections, according to a method presented in a preceding paper (Conradi et al., '79a). Several of the dendrites had a base diameter exceeding 10 micrometer. The proportion of the surface covered by boutons was 40-50% for the cell body and about 80% for the proximal dendrites. In both regions, about 20% of the boutons were of the S-type and 70% of the F-type. The large C-boutons were clustered around the dendritic roots and were also present on the axon hillock. M-boutons of dorsal root origin were located on the proximal parts of the majority of the dendrites.