Electron microscopic studies of serially sectioned cat spinal α‐motoneurons: V. motoneurons innervating fast‐twitch (type FF) units of the gastrocnemius muscle

Two intracellularly stained gastrocnemius a‐motoneurons of the FF‐type (Burke et al., ′73) were studied ultrastructurally. The architecture and synaptology of the cell bodies and proximal dendrites were analyzed from long series of consecutive sections according to, a methgd previously described (Conradi et al., ′79a). Several of the dendrites had a base diameter exceeding 10 μm. The proportion of the cell surface area covered by boutons was 40‐50% for the soma and about 70% for the proximal dendrites. In both regions, about 20‐25% of the boutons were of the S‐type and 60‐70% of the F‐type. In general, the type FF α‐motoneurons exhibited the same gross synaptological characteristics as those found earlier in type FR α‐motoneurons (Kellerth et al., ′79), although the large boutons of the C‐type were more frequent on the proximal dendrites and axon hillocks of the present cells. M‐boutons of dorsal root origin were found on several of the proximal dendrites. The observed regional differences in the spectrum of boutons on the motoneuron surface seem to follow gradients operating in the somato‐fugal direction, and they are apparently independent of the size or direction of the neuronal processes.