Cerebral Cortex 1992; 2:197-216
© Oxford University Press 1992
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Characteristics of Intracellularly Injected Infragranular Pyramidal Neurons in Cat Primary Auditory Cortex
1Neural Systems Laboratory, Frontier Research Program, RIKEN Wako, Saitama 351-01, Japan, 2Department of Cell Biology and Neuroanatomy, University of Minnesota Minneapolis, Minnesota 55455, 3Department of Anatomy and Neurobiology, University of California Irvine, California 92717
Pyramidal neurons in layers V and VI of primary auditory cortex (AI) were intracellularly injected with biocytin after functional characterization according to a position relative to an anteroposterior sequence of best-frequency responses. A sample of 19 completely filled neurons was analysed, and a preliminary classification was made on the basis of dendritic morophology and axon collateral distribution.
Layer V cells could be divided into two types. Cells in the upper part of layer V and projecting toward the diencephalon had a large cell body and an apical dendrite with extensive branches in layer 1. These cells had few recurrent axon collaterals, and no terminal axonal bushes were formed in the vicinity of the dendritic field. Long horizontal collaterals with many boutons, however, extended in various directions parallel to the cortical surface. By contrast, cells in the lower part of layer V and sending an axon into the putamen, or without an abvious subcortical axon, had a medium soma and an apical dendrite with few branches in layer I. These cells had a dense bush of recurrent collaterals extending into layers II and III and surrounding the dendritic field, but few or no horizontal collaterals.
Layer VI injected neurons were more heterogeneous. All had a thin ascending dendrite with oblique branches both ending in layer III. Axon collateral distributions varied from cell to cell. Relatively small cells with an apical dendrite that branched frequently in layers III and IV had a dense network of recurrent collaterals in the dendritic field, but virtually no horizontal collaterals. This type projected toward the diencephalon. Cells with relatively long horizontal collaterals and a weak recurrent system confined to layers V and VI had a unique arborization pattern of basal dendrites. This type may have projected to the claustrum or other cortical areas. One cell with dendritie branches restricted to layer VI had horizontal collaterals predominantly in layer VI. This cell projected into the corpus callosum.
The apparent close correlation between extrinsic projections of infragranular neurons and their dendritic morphology and intracortical collateral distributions suggests that differentially projecting cells may engage different elements of intracortical circuitry in AI.
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