The non‐pyramidal cells in layer III of cat primary auditory cortex (AI)

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Abstract
The form and location of non-pyramidal neurons in layer III of the primary auditory cortex (AI) of adult cats is described in Golgi, Nissl, and other material. The cells were compared to the profiles of retrogradely labeled, commissurally interconnected cells. A principal finding is that certain non-pyramidal and pyramidal cells project interhemispherically to AI; a second conclusion is that the retrogradely labeled commissural cells form small clusters or narrow strips separated by unlabeled patches even after massive injections in the opposite AI. The non-pyramidal cells of origin have not yet been conclusively identified, but they must include one (or more) of the following six types of cells observed in Golgi-impregnated material: (1) tufted or bitufted cells with a radially elongated dendritic arbor; (2) sparsely spinous stellate neurons with thin, smooth dendrites and vertically disposed axonal branches; (3) small stellate cells with varicose dendrites, a restricted dendritic field, and a profusely branched local axon; (4) bipolar neurons with long, thin dendrites; (5) medium-sized multipolar cells with radiating, sparsely branched dendrites; and (6) small stellate neurons with smooth dendrites and a tiny dendritic field. These non-pyramidal cells are found throughout layer III but are more numerous in the upper part, layer IIIa, where they mingle with the small pyramidal neurons. As a rule the axonal branches of non-pyramidal cells are more numerous than those arising from layer III pyramidal neurons, and although they have many axonal collaterals, most project locally and vertically in narrow radial strips. In contrast, pyramidal cell axons have ascending and descending components which invade large, lateral territories in many cortical layers. Layer III non-pyramidal neurons are similar to those in layer IV in certain respects, although their dendritic fields are more spherical and less tufted than those of layer IV cells, and their axons have more local, limited targets. These axons appear to contribute but little to the conspicuous, lateral fiber striae in layer III. The primary intrinsic targets of non-pyramidal cell axons appear to be the apical dendrites of medium-sized and large layer III pyramidal cells, and recurrent branches to the parent cell; their fine, distal branches fortify the vertical plexus in layer III, and certain axons may descend into layer IV. Since layer III in AI receives both commissural and thalamic input, it is possible that these parallel, afferent channels are to some degree segregated, and to some degree convergent, onto particular types of cells. Whether any single cell receives both kinds of input is unknown, but the evidence suggests that not all non-pyramidal cells project only locally, nor may all project interhemispherically or from AI to AI. This implies that there may be connectionally and perhaps functionally distinct subsets among layer III non-pyramidal neurons which remain to be identified by experimental methods.

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