Quantification of synapses formed with apical dendrites of golgi-impregnated pyramidal cells: Variability in thalamocortical inputs,but consistency in the ratios of asymmetrical to symmetrical synapses

Five pyramidal cells from the posteromedial barrel subfield of mouse SmI cortex were labeled by Golgi impregnation and then gold-toned and de-impregnated (Fairén, Peters & Saldanha, 1977). Subsequently, 40 to 70 μm-long segments of their apical dendrites occurring in layer IV were graphically reconstructed from serial thin sections to determine the distribution of their synapses. Thalamocortical synapses onto these dendritic segments were identified by lesion-induced degeneration.The synaptic pattern of the pyramidal cell apical dendrites was consistent with previous reports in that most synapses occurred on spines and were asymmetrical and the smaller number of shaft synapses were primarily symmetrical. Some axospinous synapses were formed by degenerating thalamocortical axon terminals. The proportion of thalamocortical synapses onto reconstructed dendritic segments was different for different neurons. For example, thalamocortical axon terminals formed 15% of the synapses involving the spines of the reconstructed segment from a medium superficial layer V pyramidal cell and 10% of the synapses onto portions of the segment from a large layer VI pyramidal cell. In contrast, reconstructed dendritic segments of three other layer VI pyramidal cells formed no more than one thalamocortical synapse.An analysis of the distribution of synapses onto reconstructed dendritic segments revealed that the segments of 3 medium and large pyramidal cells had a ratio of about 12.5 asymmetrical synapses per symmetrical synapse, whereas the segments of 2 small pyramidal cells had ratios of only 6.5 asymmetrical synapses per symmetrical synapse. That these ratios fall into 2 distinct groups suggests that the relative number of asymmetrical and symmetrical synapses is stereotyped within populations of neurons.