Identified neurons in mouse smi cortex which are postsynaptic to thalamocortical axon terminals: A combined golgi‐electron microscopic and degeneration study

Abstract

A recently developed Golgi‐electron microscopic technique (Fairén et al., '77) has been used to identify neurons in mouse SmI cortex which that thalamic axon terminals synapsed with dendrites of several distinct neuronal types whose cell bodies occurred either in layer III, IV or V. The procedures employed were as follows: electrolytic lesions were used to destroy the cortical projections of the nucleus ventralis posterior pars lateralis thalami (homologous to medial portion of ventrobasal complex) and the nucleus posterior thalami. These two thalamic nuclei project to the posteromedial barrel subfield (PMBSF) in mouse somatosensory cortex. Blocks of cortex containing the PMBSF were processed by the Golgi method, sectioned at 150 μm and examined with the light microscope. Neurons of interest were then selected for Golgi deimpregnation (Fairén et al., '77). Portions of these cells were serial thin sectioned and subsequently examined with the electron microscope to determine if they synapsed with degenerating axon terminals. Degenerating thalamocortical axon terminals synapsed at asymmetrical synaptic junctions with dendritic spines of pyramidal cells whose somata occurred in layers III and V, and with dendritic spines of spiny stellate cells with somata in layer IV. Only normal, symmetrical synapses occurred on the cell bodies of these neurons, and no degenerating axon terminals synapsed with the shafts of their dendrites. Two, unimpregnated, non‐spiny stellate cells whose somata occurred in layers IV and IV/V were partially reconstructed from serial thin sections. Many degenerating thalamocortical axon terminals synapsed with the dendrites and somata of these non‐spiny cells. In fact, for similar lengths of dendrite, the non‐spiny stellate cells made the most synapses with thalamocortical axon terminals, followed in order of decreasing frequency by the layer IV spiny stellate cells, the layer III pyramids and finally the layer V pyramids which received only few synapses from thalamocortical axon terminals. Direct evidence has been provided that at least six different types of neurons whose somata occur in several layers of the neocortex receive synapses from thalamocortically projecting neurons. It has been proposed that the cortex is organized such that thalamic input is processed sequentially by neurons of increasingly complex response properties beginning with cells in layer IV and proceeding to cells in other layers (Hubel and Wiesel, '62, '68). The results of this study now suggest that any such hierarchical processing of thalamic input must occur in conjunction with the parallel, and perhaps simultaneous, processing of thalamic input by neurons at several hierarchical levels. The evidence for, and some implications of, a newly proposed model of cortical organization which combines hierarchical and parallel processing are discussed.