Structure of layer II in cat primary auditory cortex (AI).

The cytoarchitecture, myeloarchitecture, neuronal architecture, and intrinsic and laminar organization of layer II were studied in the primary auditory cortex (AI) of adult cats. The chief goal was to describe the different types of cells and axons to provide a framework for experimental studies of corticocortical connections or of neurons accumulating putative neurotransmitters. A further goal was to differentiate layer II from layer III. Layer II extends from 150–200 μm to about 400 μm beneath the pia and has two subparts. The superficial stratum, layer IIa, has many small, chiefly non-pyramidal neurons, primarily with round or oval perikarya, and a sparse, fine, and irregularly arranged axonal plexus. Layer IIb somata are larger and more densely packed and there is a more developed vertical and lateral axonal plexus. The border with layer III was marked by numerous large pyramidal cells with a thicker apical dendrite with more developed basal dendrite arbors than those of layer II pyramidal cells. Eight varieties of neurons were recognized in Golgi-impregnated material. These included (1) small and (2) medium-sized pyramidal cells, whose apical dendrites often ramified in layer I; (3) bipolar and bitufted cells with polarized, sparse dendrite arbors; (4) small smooth or sparsely spinous multipolar cells with radiating dendrites and small dendritic fields; (5) spinous multipolar cells, whose large dendritic fields had more extensive apical than basal arbors; (6) large sparsely spinous multipolar cells with smooth, robust apical dendrites; (7) tufted multipolar cells with highly developed apical dendrites and some dendritic appendages; and (8) extroverted multipolar cells with a broad, candelabra-shaped dendritic configuration, and with most dendrites oriented at right angles to the pia. The axons of the different cell types had the following general dispositions: those arising from the pyramidal cells could often be traced into the white matter but had many local branches as well; those of the other neurons had more or less extensive local axonal collateral systems and fewer branches which appeared to be corticofugal. However, the complete trajectory of the axons was not always impregnated in the adult material. Several types of layer II neurons, including non-pyramidal cells, project to layer III (or other cortical layers), and various kinds of neurons in layers II, IV, and V in AI send their axons to layer II. The pyramidal neurons appear similar in structure to those described in the primary somatic sensory and visual cortices. In contrast, however, their extrinsic connections in cat primary auditory cortex are strictly ipsilateral in the cat (Winguth and Winer, '85). This suggests that they do not participate in any direct way in the commissural system. It is more difficult to compare the different populations of non-pyramidal cells since they have not been described in much detail in other areas in the primary sensory cortex outside AI with regard to their extrinsic connections or affinity for various neurotransmitter candidates, compared to the pyramidal cells. However, when morphological correspondences can be identified, the sparsely spinous multipolar cell may be analogous to the chandelier cell, and the dendritic configuration of many of the non-pyramidal cells resembles that of the presumptively corresponding types of cells in the primary visual and somatic sensory cortex.