Dendritic morphology and axon collaterals of corticotectal, corticopontine, and callosal neurons in layer V of primary visual cortex of the hooded rat

Recent evidence indicates that corticotectal neurons belong to only one of the three morphological classes of pyramidal cells in layer V. The present study compares the dendritic morphology and axon collaterals of corticotectal, corticopontine, and layer V callosal neurons by using techniques based on the retrograde transport of horseradish peroxidase and fluorescent dyes as well as in vitro intracellular dye injections. Our results indicate that corticotectal and corticopontine neurons are located predominantly in the upper middle part of layer V. These neurons have medium to large somas with 5 or 6 primary basal dendrites and a single apical dendrite ascending to layer I. Approximately 60% of these cells send axon collaterals to both the superior colliculus and the pons. In contrast, callosal neurons form a heterogeneous group. In general, they have small pyramidal or ovoid cell bodies which give rise to 3 or 4 primary basal dendrites. Many cells have an apical dendrite that bifurcates and terminates in layer V or IV. We find that callosal neurons do not send an axon collateral to either the superior colliculus or the pons. We conclude that the corticotectal and corticopontine systems are similar in their intralaminar distribution, dendritic morphology, and pattern of axon collaterals, whereas the callosal system differs in these characteristics.