Thin section and freeze-fracture studies on the synapses in the ventral nucleus of the cat medial geniculate body.

Ultrastructure of the synapses in the ventral nucleus of the cat medial geniculate body was examined with freeze-fracture and thin section techniques. Two types of synapses were recognized in this nucleus. One was the asymmetrical type such as the afferent nerve fiber to the principal cell dendrite or to the process of Golgi type 2 cell. The other was the symmetrical type such as the process of Golgi type 2 cell to the principal cell dendrite. These synapses made conglomerate on the principal cell dendrite called the glomerulus, which was ensheathed by thin sheets of the astroglia and easily recognized by freeze-fracturing. A random distribution of intramembrane particles (IMP) and synaptic vesicle attachment sites were found on the presynaptic P-face of both type synapses. Asymmetrical synapses had distinct aggregates of IMP on the postsynaptic E-face and the area of IMP aggregates on this face is often larger than 0.07 micrometer2. Similar aggregates of IMP occurred on the same face of symmetrical synapses. However, the area of IMP aggregates of this type synapses is frequently smaller than 0.05 micrometer2. IMP composing the aggregate on the postsynaptic E-face showed bimodal distribution in size, having peaks at 10 nm and 13 nm in diameter. The ratio of large IMP (13 nm) to the total number of IMP increased as the synaptic contact on the postsynaptic membrane widened in area. Non-synaptic contacts, having a complex of dense materials and neurofilaments at the subjunctional area and being similar to the filamentous contact in the lateral geniculate body, were frequently observed on the dendrite of the principal cell in thin sections. Clusters of IMP were scattered on the dendritic P-face near around the postsynaptic area and may represent the area being attached with dense materials of the contacts.