ELECTRON-MICROSCOPIC STUDIES OF SERIALLY SECTIONED CAT SPINAL ALPHA-MOTONEURONS .4. MOTO-NEURONS INNERVATING SLOW-TWITCH (TYPE S) UNITS OF THE SOLEUS MUSCLE

Two intracellularly stained cat α‐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 sonsecutive sections (Conradi et al., '79). Only few of the dendrites had a base diameter exceeding 10 μm. 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. Copyright © 1979 The Wistar Institute Press