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 trig eminal nucleus (MTN) were studied in adult cats by transmission electr on microscopy and more precisely quantified with an automated image an alysis system. Two subpopulations of MTN neurons were identified withi n the nucleus: large spherical or ovoid (pseudo)unipolar cells amounte d to about 60% of the total population that resemble typical primary s ensory neurons and small multipolar neurons (estimated 40%), some of w hich are possibly interneurons. By electron microscopy, most neurons i n the MTN proved to have a rich cytoplasm in the perikaryon with abund ant rough endoplasmic reticulum, a large number of free ribosomes and polysomes, also well-developed Golgi complex, and numerous mitochondri a and neurofilaments indicating a high rate of protein synthesis and a xonal transport in these cells. Three type of synaptic contacts were o bserved in the MTN: axodendritic, axosomatic and axospinic of both sym metric and asymmetric morphology. Most of them (almost 90%) were axode ndritic and axodendritic spine. Approximately 70% of axon terminals co ntained small round vesicles (S-type boutons) whereas the other 30% be longed to the P-type boutons filled with a pleomorphic vesicle populat ion. Axosomatic synapses were comparatively rare accounting for 10% of the total. About two-third of them were of S-type and almost 25% of t he remaining third were F-type in which flat synaptic vesicles could b e seen, and less than 10% were P- and G-types with granular vesicles.