PYRAMIDAL NEURONS IN LAYER-5 OF THE RAT VISUAL-CORTEX .1. CORRELATIONAMONG CELL MORPHOLOGY, INTRINSIC ELECTROPHYSIOLOGICAL PROPERTIES, ANDAXON TARGETS

Previous work has established two structure/function correlations for pyramidal neurons of layer 5 of the primary visual cortex of the rat. First, cells projecting to the superior colliculus have thick apical d endrites with a florid terminal arborization in layer 1, whereas those projecting to the visual cortex of the opposite hemisphere have thinn er apical dendrites that terminate below layer 1, without a terminal t uft (e.g., Hallman et al.: J Comp Neurol 272:149, '90). Second, intrac ellular recording combined with dye injection has revealed two classes of cells: the first has a thick, tufted apical dendrite and fires a d istinctive initial burst of two or more impulses, of virtually fixed, short interspike interval, in response to current injection; and the o ther, with a slender apical dendrite lacking a terminal tuft, tends to have a longer membrane time constant and higher input resistance, and does not fire characteristic bursts (e.g., Larkman and Mason: J Neuro sci 10:1407, '90). The present study combined intracellular recording in isolated slices of rat visual cortex and injection of carboxyfluore scein, to reveal soma-dendritic morphology, with prior injection of rh odamine-conjugated microspheres into the superior colliculus or contra lateral visual cortex to label neurons according to the target of thei r axons. This permitted a complete correlation of morphology, intrinsi c electrophysiological properties, and identity of the projection targ et for individual pyramidal cells. Neurons retrogradely labeled from t he opposite visual cortex were found in all layers except layer 1 whil e those labeled from the superior colliculus lay exclusively in layer 5. Within layer 5 interhemispheric cells were more concentrated in the lower half of the layer but extensively overlapped the distribution o f corticotectal cells. Every cell studied that projected to the superi or colliculus was of the bursting type and had a thick apical dendrite with a terminal tuft. Every cell in this study projecting to the oppo site visual cortex was a ''nonburster'' and had a slender apical dendr ite with fewer oblique branches that ended without a terminal tuft, us ually in the upper part of layer 2/3. Interhemispheric cells also had rounder, less conical somata and generally had fewer basal dendrites t han corticotectal neurons. Many cells with the physiological and morph ological characteristics of interhemispheric cells were not back-label ed from the opposite visual cortex, implying that pyramidal cells of t his type can have other projection targets (e.g., other cortical sites in the ipsilateral hemisphere). On the other hand, few cells of the ' 'thick/tufted,'' bursting class were found that were not back-labeled from the superior colliculus, implying that the majority of such cells project to that target.