An electron microscopic study of the laminar pattern and mode of termination of afferent fibre pathways in the somatic sensory cortex of the cat.

In a series of experiments, the extrinsic afferent pathways to the somatic sensory areas have been selectively interrupted and the distribution and mode of termination of degenerating synaptic endings studied at intervals of 2 to 6 days. Degenerating commissural fibres terminate on spines attached to dendrites of medium and small size in all cortical layers, but the endings are concentrated in the deeper parts of layers I and III and in layer IV. Cortical association fibres passing from SII to SI end on spines of small and medium-sized dendrites mainly in the intermediate layers (III, IV and V) of the cortex, but a small number are invariably seen in the molecular layer. Some of the latter are probably derived from thin myelinated axons which spread radially from a lesion and run just beneath the pia mater. Degenerating thalamo-cortical axons terminate on spines and to a lesser extent on shafts of dendrites of small diameter, mainly in layer IV but with overlap into adjacent parts of layers III and V and with an additional small but consistent number ending in the molecular layer. Thus, the molecular layer and layer IV receive the terminations of all extrinsic afferents, while the relations of these to the other laminae and their mode of termination is, in each case, slightly different. One interpretation of the results is that all three sets of extrinsic afferents terminate in the middle portion of the apical dendritic tree of pyramidal neurons, and that thalamo-cortical fibres have additional terminals on stellate neurons. The functional implications of this arrangement are discussed.