Morphology and laminar distribution of nonpyramidal neurons in the auditory cortex of the rabbit.

A study of the morphology and laminar distribution of nonpyramidal neurons in Golgi-Nissl preparations of electrophysiologically verified auditory cortex was carried out in the adult rabbit. Nonpyramidal neurons were located primarily within laminae I-IV and were only infrequently seen in lamina V and VI. In lamina I, four nonpyramidal cell types were observed: (1) small, spine-free horizontal neurons, (2) small, sparsely spined multipolar neurons with radiate dendrites, (3) large, multipolar neurons with fusiform somata and vertically aligned, sparsely spined dendrites, and (4) small, spine-free neurogliform neurons. The horizontal and small multipolar neurons had tangentially running axons confined to lamina I. The large, fusiform cells had descending axons which arborized in lamina II and occasionally reached lamina III. In lamina II and the upper part of lamina III, seven nonpyramidal cell types were observed: (1) spine-free bipolar neurons with vertically aligned dendrites and axonal arbors; (2) large, (3) medium, and (4) small, spine-free and sparsely spined multipolar neurons, all with locally ramifying axons; (5) pear-shaped cells with highly oriented dendrites which branched toward the pial surface and vertically arborizing axons; (6) multipolar cells with tangentially and vertically oriented dendrites and ascending axons which entered lamina I, and (7) tufted cells with local axons. Three types of nonpyramidal cells were observed in lamina IV and the lower part of lamina III: (1) large, multipolar cells with radiate, spine-free dendrites and stout axons which arborized locally, (2) spiny multipolar cells with vertically aligned dendrites and ascending axons which arborized in lamina II and III via long horizontal collaterals, and (3) spine-free bipolar cells with vertical dendrites and axons which arborized in a narrow vertical column adjacent to the dendrites. Nonpyramidal neurons in lamina V and VI were primarily multipolar cells with sparsely spined and spine-free dendrites. A comparison of these data with those of other species indicates that the neuronal organization of the rabbit auditory cortex is similar to that of the sensory cortex of the rodent but is strikingly different from that of carnivores and primates.