Two types of synaptic glomeruli and their distribution in laminae I-III of the rat spinal cord.

Systematic examination of photomontages revealed two types of synaptic glomeruli in laminae II-III. Type I glomeruli have a dark small central (C) terminal of indented contour with closely packed spherical vesicles of variable diameter and few mitochondria. Among the peripheral terminals there are dendritic spines and a few presynaptic dendritic spines (V1 terminals) and axon endings rich in discoid vesicles (V2). These glomeruli occur in groups which are particularly evident in parasagittal sections in which successive C terminals are connected by narrower portions or dark unmyelinated profiles. Type II glomeruli have an electron-lucent and large C terminal of regular contour with less packed synaptic vesicles of more uniform diameter, more mitochondria, and sometimes neurofilaments. Presynaptic dendrites are fewer and axon endings more numerous. C terminals in type II glomeruli are fusiform in longitudinal section, rarely occurring in groups. Lamina I is virtually devoid of glomeruli. Within lamina II, glomeruli are rare in the dorsalmost 20-micrometers band and abundant in the immediately ventral 20-micrometers band in which type I glomeruli are prevalent (approximately 79%). In ventral lamina II, type II glomeruli predominate (66%), being practically exclusive in lamina III where most contain neurofilaments. Considering the distribution of terminations of primary afferents, it is suggested that type I C terminals originated from unmyelinated primary afferents, type II C terminals without neurofilaments from direct myelinated fibers, the those with neurofilaments from recurrent large fibers. The distinct numbers of presynaptic dendritic and axonal endings suggest different modulatory mechanisms functioning in the two types and in ventral vs. dorsal areas of this region.