Distribution of thalamic input to different dendrites of a spiny stellate cell in mouse sensorimotor cortex.

A Golgi impregnated, gold-toned [2] spiney stellate cell from layer IV of mouse SmI cortex was reconstructed in three dimensions from serial thin sections to assess the apatial relationships of the synapses onto its dendrites. The distribution of thalamocortical (TC) synapses with the reconstructed dendrites is presented in this report. Thalamocortical axon terminals were labeled by lesion induced degeneration which, in mouse SmI cortex, may reliably indicate the numbers of thalamocortical axon terminals. Results indicate that thalamocortical synapses, which are distributed over most regions of the dendritic tree, are arranged in a regular, periodic fashion on parts of two of the reconstructed dendrites. In these regions, the necks of spines receiving thalamocortical input attach to the dendrite shaft at intervals of about 5 micrometers. In many other regions of the dendritic tree, two spines receiving thalamocortical synapses are separated by a similar interval. Further studies are expected to determine the extent to which dendrites of spiny stellate cells and of other kinds of cortical neurons are contacted in a periodic fashion by thalamocortical axon terminals.