A three dimensional study of layer I of the rat parietal cortex

Serial thin sections have been used to reconstruct neuronal processes in the outer portions of layer I of adult rat somatosensory cortex. The reconstructions show that two distinct forms of dendrites exist in this area: one that is smooth and varicose and another that bears spines along a shaft of relatively constant diameter. These two dendritic types, which originate from nonpyramidal and pyramidal neurons respectively, have differences in the distribution of organelles. Cortical dendrites terminate with no apparent morphological modification which would permit their identification in isolated thin sections. A pyramidal dendritic ending is blunt, about 0.5 μ in diameter, with microtubules and smooth endoplasmic reticulum in the terminal cytoplasm. Very long mitochondria which are invariably present within pyramidal dendrites, penetrate into an ending. A smooth dendrite terminates in an expansion containing microtubules and smooth endoplasmic reticulum, but no long mitochondrion. One or two asymmetrical synapses occur at the end of each terminating dendrite but do not encompass the tip. The principal axonal elements in layer I are small unmyelinated axons with en passant boutons. Serial examination reveals that various combinations of synaptic contacts are possible between axons and dendrites of both types.