Ultrastructure and quantitative synaptology of the sacral parasympathetic nucleus

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Abstract
This study examines the anatomical substrate for the spinal micturition reflex. Light microscopy of pyridine silver-stained sections revealed that the sacral parasympathetic nucleus (SPN) exists as a broken column or chain of cell clusters located along the intermediolateral portion of the dorsal horn in sacral segments S2–S4. Quantitative analysis of neuropil components in electron micrographs provides data for each type of bouton identified in this nucleus. On the somata of these neurons, boutons containing clear spherical vesicles (S type) comprise 70% of the bouton population. Terminals containing three or more dense core vesicles (GS boutons) account for 26% and boutons containing flattened vesicles (F boutons) comprise 4% of the population. F boutons are more common on large dendrites where they comprise 10% of the total bouton population. The actual population density of each bouton type is most evident when the number of boutons is expressed as boutons per 100 μm of membrane length (btn/100 μm). S type boutons are the most frequently encountered type. The population density of S boutons is the same on soma and dendrites at 6.66 btn/100 μm. F boutons are more numerous on large (> 2 μm) dendrites (1.28 btn/100 μm) than on small dendrites (0.63 btn/100 μm) or on somata (0.36 btn/ 100 μm). GS boutons occur more frequently on small dendrites (3.66 btn/100 μm) than on somata (2.29 btn/100 μm), large dendrites (2.88 btn/100 μm) or medium dendrites (2.27 btn/ 100 μm). These data suggest that the dense core vesicle-containing boutons are applied primarily to small (<1 μm) dendrites and that F boutons are associated mostly with large or proximal dendrites. These results provide a quantitative profile of the synaptic input to the sacral autonomic (parasympathetic) neurons which innervate the urinary bladder and demonstrate specific population differences on various postsynaptic structures in this nucleus.