Layer V in cat primary auditory cortex (AI): Cellular architecture and identification of projection neurons

The cytoarchitectonic organization and the structure of layer V neuronal po pulations in cat primary auditory cortex (AI) were analyzed in Golgi, Nissl , immunocytochemical, and plastic-embedded preparations from mature specime ns. The major cell types were characterized as a prelude to identifying the ir connections with the thalamus, midbrain, and cerebral cortex using axopl asmic transport methods. The goal was to describe the structure and connect ions of layer V neurons more fully. Layer V has three sublayers based on th e types of neuron and their sublaminar projections. Four types of pyramidal and three kinds of nonpyramidal cells were present. Classic pyramidal cell s had a long apical dendrite, robust basal arbors, and an axon with both lo cal and corticofugal projections. Only the largest pyramidal cell apical de ndrites reached the supragranular layers, and their somata were found mainl y in layer Vb. Three types departed from the classic pattern; these were th e star, fusiform, and inverted pyramidal neurons. Nonpyramidal cells ranged from large multipolar neurons with radiating dendrites, to Martinotti cell s, with smooth dendrites and a primary trunk oriented toward the white matt er. Many nonpyramidal cells were multipolar, of which three subtypes (large , medium, and small) were identified; bipolar and other types also were see n. Their axons formed local projections within layer V, often near pyramida l neurons. Several features distinguish layer V from other layers in Al. Th e largest pyramidal neurons were in layer V. Layer V neuronal diversity ali gns it with layer VI (Prieto and Winer [1999] J. Comp. Neurol. 404:332-358) , and it is consistent with the many connectional systems in layer V, each of which has specific sublaminar and neuronal origins. The infragranular la yers are the source for several parallel descending systems. There were sig nificant differences in somatic size among these projection neurons. This f inding implies that diverse corticofugal roles in sensorimotor processing m ay require a correspondingly wide range of neuronal architecture. J. Comp. Neurol. 434:379-412, 2001. (C) 2001 Wiley-Liss, Inc.