Fine structure and synaptic organization of the mesencephalic trigeminal nucleus of the cat: a quantitative electron microscopic study.

The ultrastructure and synaptic organization of the mesencephalic trigeminal nucleus (MTN) were studied in adult cats by transmission electron microscopy and more precisely quantified with an automated image analysis system. Two subpopulations of MTN neurons were identified within the nucleus: large spherical or ovoid (pseudo)unipolar cells amounted to about 60% of the total population that resemble typical primary sensory neurons and small multipolar neurons (estimated 40%), some of which are possibly interneurons. By electron microscopy, most neurons in the MTN proved to have a rich cytoplasm in the perikaryon with abundant rough endoplasmic reticulum, a large number of free ribosomes and polysomes, also well-developed Golgi complex, and numerous mitochondria and neurofilaments indicating a high rate of protein synthesis and axonal transport in these cells. Three types of synaptic contacts were observed in the MTN: axodendritic, axosomatic and axospinic of both symmetric and asymmetric morphology. Most of them (almost 90%) were axodendritic and axodendritic spine. Approximately 70% of axon terminals contained small round vesicles (S-type boutons) whereas the other 30% belonged to the P-type boutons filled with a pleomorphic vesicle population. Axosomatic synapses were comparatively rare accounting for 10% of the total. About two-third of them were of S-type and almost 25% of the remaining third were F-type in which flat synaptic vesicles could be seen, and less than 10% were P- and G-types with granular vesicles.