Three-dimensional aspects and synaptic relationships of a Golgi-impregnated spiny stellate cell reconstructed from serial thin sections.

A Golgi-impregnated spiny stellate cell was selected from layer IV of SmI cortex in a mouse whose ipsilateral ventrobasal complex had been lesioned. The neuron was gold-toned, thin sectioned and then reconstructed in three dimensions using wooden sheets of appropriate thickness. These procedures enabled the numbers and distribution of thalamocortical and other synapses onto the reconstructed neuron to be determined. Results show the cell body to be roughly spherical and to receive 49 symmetrical synapses and four synapses which are intermediate between the asymmetrical and symmetrical type. A single, clearly asymmetrical axosomatic synapse is made by a degenerating, thalamocortical axon terminal. Five primary dendrites and their branches were reconstructed and, interestingly, these processes are distinctly elliptical in cross-section. The reconstructed dendrites receive 68 symmetrical synapses onto their shafts and 373 synapses onto spines of which 359 are asymmetrical and 14 symmetrical. Forty-eight, or about 13%, of the asymmetrical axospinous synapses are made by degenerating, thalamocortical axon terminals. An intriguing finding is that in many regions of the dendritic tree, two or more spines involved in thalamocortical synapses are attached to the dendritic shaft at intervals of 5 +/- 0.5 micron.