An electron microscopic study of the types and proportions of neurons in the cortex of the motor and visual areas of the cat and rat.

The neuronal cell somata in the cortex of the motor and visual areas of the cat and rat have been studied with the electron microscope. It has been possible to recognize the cell types which have been identified in the corresponding functional areas of the cortex of the monkey: pyramidal, large and small non-pyramidal. The large non-pyramidal or stellate cell has abundant endoplasmic reticulum and mitochondria and receives numerous asymmetric and symmetric synapses; the small non-pyramidal or stellate has a small amount of cytoplasm and few synapses of both asymmetric and symmetric nature. The proportions of these types of neurons are of the same order of magnitude in the different functional areas and in the different species: approximately two-thirds are pyramidal, slightly less than one-third are small stellate, and less than 10 per cent are large stellate. There is the same mean number of synapses upon the pyramidal and small stellate cell in the visual and motor areas of the rat and cat, but this number is less in the monkey being 50 per cent less in the visual cortex and 25 per cent less in the motor area.