Layer VI in cat primary auditory cortex: Golgi study and sublaminar origins of projection neurons

The organization of layer VI in cat primary auditory cortex (AI) was studie d in mature specimens. Golgi-impregnated neurons were classified on the bas is of their dendritic and somatic form. Ipsilateral and contralateral proje ction neurons and the corticogeniculate cells of origin were labeled with r etrograde tracers and their profiles were compared with the results from Go lgi studies. Layer VI was divided into a superficial half(layer VIa) with m any pyramidal neurons and a deeper part (layer VIb) that is dominated by ho rizontal cells. Nine types of neuron were identified; four classes had subv arieties. Classical pyramidal cells and star, fusiform, tangential, and inv erted pyramidal cells occur. Nonpyramidal neurons were Martinotti, multipol ar stellate, bipolar, and horizontal cells. This variety of neurons disting uished layer VI from other AI layers. Pyramidal neuron dendrites contribute d to the vertical, modular organization in Al, although their apical proces ses did not project beyond layer IV. Their axons had vertical, intrinsic pr ocesses as well as corticofugal branches. Horizontal cell dendrites extende d laterally up to 700 mu m and could integrate thalamic input across wide e xpanses of the tonotopic domain. Connectional experiments confirmed the! su blaminar arrangement seen in Nissl material. Commissural cells were concent rated in layer VIa, whereas corticocortical neurons were more numerous in l ayer VIb. Corticothalamic cells were distributed more equally. The cytologi cal complexity and diverse connections of layer VI may relate to a possible role in cortical development. Layer VI contained most of the neuronal type s found in other layers in AT, and these cells form many of the same intrin sic and corticofugal connections that neurons in other layers will assume i n adulthood. Layer VI, thus, may play a fundamental ontogenetic role in the construction and early function of the cortex. (C) 1999 Wiley-Liss, Inc.