The synaptic organization of the caudate nucleus

Abstract

The synaptic organization of the caudate nucleus appears to be homogeneous with no specialized groupings of axon terminals and postsynaptic profiles. The nerve terminals in the caudate nucleus fall into two size groups, one about 1 fim in diameter and the other about 5 /itn diameter. The smaller size group, which comprises the majority of terminals, may be subdivided into three varieties on the basis of vesicle morphology and the type of membrane thickening. Most contain round, 45 nm diameter vesicles and are associated with asymmetrical membrane thickenings. Others contain 48 nm diameter, polymorphic vesicles, which become flat in material washed in cacodylate buffer, and are associated with symmetrical membrane thickenings. A few other terminals with symmetrical membrane thickenings contain flat 42 nm vesicles. The large terminals contain round 45 nm diameter vesicles and have asymmetrical membrane thickenings. Axon terminals with asymmetrical membrane thickenings are found most frequently in contact with dendritic spines, but also with dendritic shafts and cell somata. Terminals with symmetrical membrane thickenings contact dendritic shafts and cell somata, and occasionally dendritic spines with which a terminal with asymmetrical thickenings is also making contact. The two types of terminal with symmetrical contact regions also form synapses onto the initial segments of axons. It is probable that such contact regions are invariably associated with cisternal organs in the initial segments. Serial synapses are found occasionally.