Synaptic patterns on different cell types in the different laminae of the cat visual cortex. An electron microscope study

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Abstract

Two types of synaptic membrane differentiation can be seen in formalin fixed cerebral cortex, corresponding closely, but not strictly analogous to Gray's type I and 2 described on OsO4 immersed, PTA stained material. They are referred to as asymmetrical and symmetrical synapses. On the whole, asymmetrical synapses are associated with the spheroidal type of vesicle population, symmetrical synapses with the flattened type of vesicle population. On pyramidal cells of the second cortical layer, most synaptic contacts are found on dendritic spines. The vast majority of these spine contacts are of the asymmetrical type with spheroidal vesicles. Rarely a symmetrical synapse with flattened vesicles is found coupled to an asymmetrical one on a common spine. The spine bearing dendritic branches receive comparatively few contacts. Those present are of one or the other of the afore-said types. The spine-free initial portion of apical and parent stem dendrites as well as the soma also receive relatively few contacts. These, however, virtually all appear to be of the symmetrical type with flattened vesicle. Stellate cells in the first and fourth layers receive both types of contacts on both their cell bodies and dendrites. Some have relatively few synapses. Others, especially in layer IV are quite remarkably covered by them.