An experimental electron microscopy study of afferent connections to the primate motor and somatic sensory cortices

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Abstract
An experimental electron microscope (e.m.) study has been made of the termination of the afferent connections to the primate sensori-motor cortex. Following large, stereotaxically placed thalamic lesions, degeneration in the motor and somatic sensory cortices was studied at survival periods of 4 and 5 days. Degenerating thalamocortical terminals had asymmetric membrane specializations. In the motor cortex 89.5% made synapses on to dendritic spines, 9% on to dendritic shafts and 1.5% on to cell somata; in the somatic sensory area 89% made synapses on to spines, 11 % on to dendritic shafts and one example contacted a cell soma and a spine. A considerable number of the spines receiving synapses from degenerating thalamo-cortical terminals were traced to their parent dendrites and these were of the pyramidal type whereas the dendritic shafts and cell somata contacted by degenerating thalamo-cortical terminals were mostly of the large stellate type. Most of the thalamo-cortical degeneration in both cortical areas occurred in a dense band in the upper two thirds of layer IV and the lower half of layer III but a number of degenerating terminals were found deep to this; in the motor cortex a second, less dense, band of degeneration was present in the lower part of layer V and top of layer VI. Degenerating thalamo-cortical terminals making synapses on to dendritic shafts and cell somata were scattered through the deep half of the cortex and not concentrated in the dense band of degeneration and so formed a greater proportion of the degeneration in the deep layers, particularly in the motor cortex. Sections cut parallel to the pial surface in layer IV of the motor cortex showed a statistically significant association between the degenerating thalamocortical axon terminals and the bundles of apical dendrites present at this level. Degeneration of commissural fibres was studied after removal of the contralateral sensori-motor cortex. Degenerating terminals had asymmetric membrane specializations. In the motor cortex 96% made synapses on to dendritic spines, 3% contacted dendritic shafts and one example made an axosomatic synapse; in area 3 97% made synapses on to dendritic spines and 3% contacted dendritic shafts. A number of the spines receiving synapses from degenerating commissural axon terminals were traced to their parent dendrites and these were of the pyramidal type. The cell soma and the majority of the dendritic shafts receiving synapses from commissural terminals were of the large stellate type although some of the dendritic shafts were probably those of small stellate cells. In the motor cortex degenerating commissural axon terminals were found in all cortical layers but were relatively more dense in layer I, the upper part of layer III, the upper part of layer V and the lowest part of layer V with layer V I; in the somatic sensory cortex most degenerating commissural terminals were found in the superficial half of the cortex. Following lesions of the primary somatic sensory cortex (SI) or of area 6 of the premotor cortex, degenerating terminals making asymmetric synapses were found in the motor cortex. Of the terminals of association fibres from SI, 82% made synapses on to dendritic spines and 18% on to dendritic shafts; of those fibres from area 6, 76% made synapses on to dendritic spines and 24% on to dendritic shafts. For both these association fibre connections, a proportion of the dendritic shafts contacted were clearly identifiable as those of large stellate cells. Terminals of both association connections occurred in all cortical layers with no obvious concentrations at any particular depth.