The anterior cingulate cortex is situated around the rostral portion of the corpus callosum. This region has numerous projections into the motor cortex and, thus, advantageously sits where it may have a significant contribution in the control of actions. Basically, the anterior cingulate has been implicated in sensory, cognitive, and emotionally guided actions. A description of the morphology, cytoarchitecture, and connectivity of the anterior cingulate cortex is provided in order to help specify biological constraints on any theory of anterior cingulate function.
The anterior cingulate cortex lies bilaterally on the medial surface of the frontal lobes around the rostrum of the corpus callosum, bounded by the callosal sulcus and the cingulate sulcus (see Fig. 1). The cingulate sulcus is consistently observed and typically remains unsegmented from its rostral extent to its termination at the marginal ramus in the parietal lobe across individuals. However, the precise morphology of the anterior cingulate cortex is extremely variable. In particular, a second paracingulate sulcus is observed to run parallel to the cingulate sulcus in many subjects, more frequently so in the left hemisphere than the right.
On the basis of cytoarchitecture and differential patterns of connectivity, the anterior cingulate cortex has been divided into ventral (Brodmann's areas 25 and 33), rostral (BA 24 and 32), and caudal (BA 24'
and 32') regions. Area 24 has been further decomposed into a, b, and c subdivisions. Despite the variable morphology of cingulate cortex, there are some consistencies in the distribution of areas on the cortical surface. Thus, area 33 lies in the rostral bank of the callosal sulcus, with areas 25, 24a,b, and 24'a,b typically lying on the gyral surface and area 32 lying in the rostral part of the cingulate sulcus. Caudally, areas 24c and 24'c usually form the ventral bank of the cingulate sulcus, facing area 32' on the dorsal bank. The paracingulate gyrus, when present, comprises areas 32 rostrally and 32' caudally.
Brent Vogt and colleagues at Wake Forest University have characterized the cytoarchitectural divisions within the anterior cingulate cortex. In common with cortical motor areas, layer IV, the major input layer from sensory cortices, is absent from the anterior cingulate cortex proper (areas 33,25, and 24). Because layer IV is also called the internal granular layer the anterior cingulate cortex is described as agranular.
In contrast, cingulocortical transition area 32 is characterized by a dysgranular layer IV that is attenuated in area 32'. Area 32 can also be distinguished from area 24 by the presence of large pyramidal neurons that form a layer IIIc. In area 32', these pyramidal neurons are even larger.
Anterior cingulate cortex in general is characterized by a prominent layer V. Ventral areas 25 and 33 are poorly differentiated, although a laminar structure is apparent in a thin layer V and undifferentiated in layers II and III. Area 24 is characterized by a neuron-dense layer Va with prominent pyramidal cells and has a clear division between layers II and III. Area 24' can be distinguished from rostral area 24 by its lower neuron density and thinner layer V. The a, b, and c subdivisions of areas 24 and 24' are marked by generally increasing differentiation of cortex away from the corpus callosum. A further division in the most caudal sulcal region of the anterior cingulate cortex, area 24c'g, has been identified as containing gigantopyramidal neurons in layer Vb.
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