Ventral horn synaptology in the rat

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Abstract
The synaptology of the normal ventral horn of the rat was studied. Presynaptic boutons were classified as S (spherical vesicles), F (flattened vesicles), and G (predominance of 700–1200 Å granular vesicles). In addition, Cf, Cs, M, and T synaptic complexes were defined and quantitated. Synaptology was studied on α-motoneuron somata, α-motoneuron primary dendrites, peripheral dendrites and interneuron somata. In addition, organelles were quantified for the pre- and postsynaptic members of the synaptic complex. All counts were made on coded material and these data were analyzed statistically. Motoneuron somata had significantly more (P < 0.01) F (58%) than S (33%) boutons. This was also the case for the motoneuron primary dendrite (P < 0.01; F, 61%; S, 37%). The small dendrites had more (P < 0.05) S (56%) than F (44%) boutons. More Cf bulbs (P < 0.01) were found on motoneuron somata (9%) than on motoneuron primary dendrites (2%) or interneuron somata (3%). The C complex presynaptic bouton contained spherical (Cs) or flattened (Cf) synaptic vesicles which were attributed to the fixation employed. Cf bulbs were not observed on small dendrites. G bulbs were observed (< 1%) only on small dendrites. M bulbs were not observed on any postsynaptic structure. The boutons of the motoneuron primary dendrites (15% of total afferents) and peripheral dendrites (14% of total afferents) were frequently branched whereas there was significantly (P < 0.01) less branching of boutons on motoneuron and interneuron somata. Small postsynaptic subsurface cisterns were associated with boutons of both the S and F type on all structures. In addition, these cisterns were observed in motoneuron somata (4%) and interneuron somata (2%) without an accompanying bouton. C postsynaptic organelles were observed in motoneuron somata (3%) and primary dendrites (1%) with an overlying neuroglial cell process and no presynaptic bouton. The synaptology of the rat ventral horn is comparable to that in the cat and monkey. However, M (R) and P bulbs were not observed in the rat. This could be due to the sampling method which indicated that synapses with less than 1% occurrence fall at the level of statistical resolution in quantitative electron microscopy. The presence of postsynaptic specialization usually associated with presynaptic boutons with no presynaptic component may be a reflection of the dynamics of normal bouton renewal in the rat ventral horn.