Electron microscopy of the somatic sensory cortex of the cat III. The fine structure layers III to VI

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Abstract
Variations in the fine structure of layers III to VI of the somatic sensory cortex have been described. Layers III and IV may be readily distinguished from one another and from layers V and VI, but within the latter two layers there is such a slow gradient of change that no clear-cut line of junction can be drawn between them. Layer III is characterized by the presence of many large apical dendrites ascending vertically through it from pyramidal cells in all layers to reach layer I. In parallel with these are many small unmyelinated axons which contain flattened synaptic vesicles and terminate on transversely orientated dendrites in symmetrical synaptic complexes. The remainder of the neuropil is filled by large numbers of dendritic spines receiving axon terminals which contain spherical vesicles and which terminate asymmetrically. In layer IV there is a marked increase in the number of small myelinated axons ascending from below and ramifying within it. Embedded in the neuropil among these are many small non-pyramidal neurons whose somata and small, irregular dendrites are covered in axon terminals. Also present, and particularly concentrated at the junction with layer III, is a meshwork of fine unmyelinated axons which contain flattened vesicles and terminate in an en passant manner as symmetrical type synapses. Most of these axons are orientated transversely. A larger axon terminal which ends in asymmetrical complexes on small dendritic shafts and spines and which may be the terminal of thalamo-cortical axons is only found in any quantity in this layer. On descending into layers V and VI there is a progressive increase in the number of large myelinated fibres and glial cells, and a progressive diminution of neuronal elements, particularly dendritic spines. Some large non-pyramidal cells resembling the smaller ones on layer IV are present in layer VI.