A comparison of thalamocortical and other synaptic inputs to dendrites of two non-spiny neurons in a single barrel of mouse SmI cortex.

A Golgi impregnated, non-spiny multipolar cell whose soma occurred in layer V of the region of mouse SmI cortex containing the posteromedial barrel subfield (PMBSF) (Woolsey and Van der Loos, '70) was gold-toned and deimpregnated (Fairen et al., '77). Two of its dendrites, contained within a single PMBSF barrel, were serial thin-sectioned and then reconstructed in three dimensions. Dendrites of an unimpregnated, non-spiny layer IV bitufted cell, present within the same barrel, were also reconstructed in three dimensions from the series of thin sections. This approach permitted a comparison of the distribution of synapses along dendrites of the two non-spiny neurons. Results showed dendrites of the layer IV bitufted cell formed about twice as many synapses per unit length as those of the multipolar cell. Particularly striking was the contrast between the large number of synapses made by degenerating thalamocortical axon terminals with the dendrites of the bitufted cell and the rarity with which such synapses occur on dendrites of the multipolar cell. Furthermore, the proportion of the total number of synapses made by thalamocortical axons terminals onto dendrites of the bitufted cell was six times greater than the proportion of the thalamocortical synapses onto the multipolar cell dendrites.