Synaptic termination of thalamic and callosal afferents in cingulate cortex of the rat

The distribution of degenerating thalamic and callosal afferents to cingulate cortex in the rat is analyzed. Both light microscopic silver impregnation and quantitative electron microscopic techniques demonstrate differences in the form, number, and laminar distribution of these two afferents in anterior and posterior cingulate cortices. Afferents from the mediodorsal thalamic nucleus terminate in area 24. Most terminals are in layer IIIb, fewer in layer Ia-b, and least in layers V and VI. In contrast, callosal afferents terminate mainly in layers Ib-c, II, IIIa, V, and VI. Thus, thalamic and callosal afferents terminate in a complementary pattern except in layers Ib and IIIb where they overlap. Quantitative analysis of degenerating axon terminals in area 24 indicates that there may be as many as seven times more callosal than mediodorsal thalamic terminals in this cortex. Projections of the anterior thalamic nuclei terminate in areas 29b and 29c, primarily in layer Ia, with fewer in layers Ib-IV and least in layers V and VI. Callosal afferents end mainly in layers V and VI and less densely in layers I-IV, which results in some overlap of thalamic and callosal afferents in layers Ic, IV, and V. In addition, patterns of termination of callosal afferents in posterior cingulate cortex change at borders between previously defined cytoarchitectural areas. Anterior thalamic terminals in area 29c differ from other thalamocortical afferents described previously in that they form two types of terminals. One is large (2-4 micrometer in diameter) and occurs mainly in layer Ia, whereas the second type is smaller and is present in layers Ib-V. Both types of terminals form asymmetric synapses mainly with dendritic spines.