Ultrastructural features of the sensori-motor cortex of the primate

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Abstract
The normal structure of the motor cortex (area 4 of Brodmann) and area 3b of the somatic sensory cortex in the monkey [Macaca mulatta] were studied by EM. They were found remarkably similar. The neuropil of all layers consists predominantly of axon terminals and dendritic spines together with dendritic shafts and myelinated and unmyelinated axons. Axon terminals which make synapses on to dendritic spines may also contact the shaft of the parent dendrite directly. Layer I of both cortical areas contains a plexus of myelinated axons shown by light microscopy and EM to be markedly more dense in the motor cortex than in the somatic sensory cortex, although the diameters of the myelinated axons in both areas were similar. In layers II and III there are numerous neurons among which pyramidal cells are conspicuous and between which run vertical lengths of apical dendrite; these layers contain relatively few myelinated axons compared to the deeper layers of the cortex. Layer IV, in both the motor and somatic sensory cortices, contains conspicuous stellate cell somata which receive large numbers of synapses; in the motor cortex these extend both above and below the narrow layer IV which is conventionally described. There is a considerable increase in the number of myelinated axons at and below this level in the cortex and in sections of layer IV cut parallel to the pial surface, the apical dendrites which run vertically through layer IV may occur in bundles of 6-12 dendrites. Layer V contains the prominent Betz cells and similar, but smaller, large pyramidal cells are present in layer V of the somatic sensory cortex. Both layers V and VI contain predominantly pyramidal and fusiform cells and these layers in the motor cortex are considerably thicker than in the somatic sensory cortex; this difference in overall thickness between the 2 cortical areas. Occasional dendro-dendritic synapses were found in the deep layers of the motor cortex and gap junctions in the deep layers of motor and somatic sensory cortices. Subsurface cisternae in cell somata and dendrites are related to axon terminals making symmetrical synapses on to these same structures. The dense apposed membranes of the subsurface cisternae are closely apposed to the overlying plasma membrane opposite the non-synaptic part of the symmetrical axon terminal and the membranes of both the terminal and soma or dendrite may show some degree of specialization at this site. The dense regions of subsurface cisternae are specifically stained by ethanolic phosphotungstic acid, thus resembling the spine apparatus. The role of these structures and of cisternal organs is probably similar. Ethanolic phosphotungstic acid also stains cilia and nucleoli and the synaptic membrane complexes.