Cholinergic Synapses in Human Cerebral Cortex: An Ultrastructural Study in Serial Sections

Cholinergic axons in the human cerebral cortex were analyzed by electron microscopy. Choline acetyltransferase (ChAT) immunoreactivity was used to identify cholinergic axons in samples of anterior temporal lobe removed at surgery. A systematic survey of labeled axon varicosities, visualized in complete serial sections, showed that 67% of all varicosities formed identifiable synaptic specializations. These synapses were usually symmetric and quite small, often present in only one to two serial sections. However, an occasional synapse was asymmetric and larger, seen in five to seven serial sections. The postsynaptic processes at cholinergic synapses were often identified as spiny dendrites or spines. The existence of cholinergic axons in the human cerebral cortex has been demonstrated in numerous studies. Our findings provide the first ultrastructural evidence that these axons make synaptic contact with cortical neurons in the human brain.