Electron microscopic studies of serially sectioned cat spinal alpha-motoneurons. IV. Motoneurons innervating slow-twitch (type S) units of the soleus muscle.

Two intracellularly stained cat alpha-motoneurons of the soleus-S type (Burke et al., '74) 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 (Conradi et al., '79). Only few of the dendrites had a base diameter exceeding 10 micrometer. The proportion of the membrane surface of the cell body and dendrites covered by boutons was 40-45% and 50-80%, respectively. Out of this, 15-20% was constituted by S-boutons and 70-75% by F-boutons in both regions. In contrast to the situation in the gastrocnemius FR-neurons (Kellerth et al., '79) the large boutons of the C-type showed no clustering around the dendritic roots and were absent on the axon hillock. In addition, the M-boutons of dorsal root origin were more sparse on the proximal parts on the dendrites of the soleus-S neurons.