Quantitative investigations of spinal motoneurons and their synaptic structures in a teleost: A morphometrical analysis with special reference to axosomatic synapses

The ultrastructural and morphometrical synaptology of the spinal motoneurons in Carassius auratus (goldfish) was analyzed based on profiles of 23 photomontages of large motoneurons and 25 putative small motoneurons with a semiautomatic digitizing system connected with a computer. In the 23 large motoneurons (mean circumference, 142.45 ± 39.76 μm) the total linear perikaryal circumference was 3,276.28 μm, of which 1,548.9 μm (46.1 ± 13.9%) was covered by terminal boutons. In contrast, a total linear perikaryal circumference of 1,375.24 μm (55.01 ± 14.34 μm) of the 25 putative small motoneurons was covered with terminal boutons only occupying a length of 287.45 μm (19.8 ± 11.9%). There were a total of 1,045 boutons on the large motoneurons and 204 on the small ones. The distribution of S‐ and F‐type boutons may reflect excitatory and inhibitory synapses. The relative number of S‐type boutons (58%) was larger than that of F‐type boutons (42%) in the large neurons and showed similar values (S‐type, 57%; F type, 43%) in the small ones. This is in contrast to mammalian spinal motoneurons in which F‐type boutons are more prevalent than the S‐type. The total numbers of the axosomatic boutons in large and small neurons were estimated, based on geometrical assumptions and found to differ substantially (840 vs. 59). This indicates large quantitative differences in the total number of synaptic inputs with the large motoneurons having a greater more axo‐somatic bouton density. No large M‐ and C‐type boutons occur on the spinal motoneurons of goldfish, suggesting a synaptic organization which is simple compared to that of terrestrial vertebrates. © 1993 Wiley‐Liss, Inc.