Aqualitative and quantitative electron microscopic study of the neurons in the primate motor and somatic sensory cortices

Abstract

A study has been made of the neuronal somata in the motor and somatic sensory cortices of the monkey. Pyramidal cells in the motor cortex are very similar to those described previously in sensory and parietal cortical areas. The largest pyramidal cells in area 4, the Betz cells of layer V, are up to 50 μm in transverse diameter. Although basically resembling smaller pyramidal cells, the nucleus of a Betz cell often has a complex indentation and is smaller in relation to the overall size of the cell soma than is that of a smaller pyramid and the cytoplasm of Betz cells contains discrete clumps of endoplasmic reticulum. As with other pyramidal cells, the synapses on to Betz cell somata are all of the symmetrical type. Previous descriptions of stellate cells have been of cells receiving a high density of axosomatic synapses of both the asymmetric and symmetrical type. Cells like this are found in both the motor and somatic sensory cortices and have been termed here large stellate cells. In addition to their high density of axosomatic synapses, they have abundant cytoplasm full of organelles and usually contain stacks of endoplasmic reticulum. Their dendrites similarly receive a high density of asymmetric and symmetrical synapses and contain prominent organelles and have a moderately varicose shape. Large stellate cells occur predominantly in layer IV in area 3 b but in the motor cortex they are also found commonly in the lower part of layer III and the upper part of layer V. A third class of neuron has been described in both the motor and somatic sensory cortices and cells of this type have been termed small stellate cells. These receive a low density of axosomatic synapses, but some of these are of the asymmetric type and they have sparse cytoplasm with few organelles. They have a small rounded or fusiform soma, frequently have a dark nucleus, have no apical dendrite and their axon initial segments are thin and may be directed towards the cortical surface. Most of the rounded small stellate cells occur in layer II whereas those with fusiform somata occur more in the deeper layers of the cortex. A quantitative study was made of the cells in a strip of the same width running through the full depth of the cortex in both cortical areas. The absolute numbers of cells in the strips of the motor and somatic sensory cortices were very similar as were the proportions of each type of neuron, 72 % in each area being pyramidal with 21 % being small stellate and 7 % large stellate in the motor cortex, and 23 % small stellate and 5% large stellate in area 3 b . The quantitative study also provided evidence that large and small stellate cells form two distinct populations rather than being a continuum.