Thalamocortical and other synapses involving nonspiny multipolar cells of mouse SmI cortex

Golgi-impregnated and -deimpregnated neurons having somata in layer IV of mouse posteromedial barrel subfield (PMBSF) cortex were identified with the light microscope and then extensive portions of them were examined with the electron microscope. Dendrites of nine nonspiny multipolar cells and eight of their cell bodies were reconstructed from serial thin sections to determine the numbers and types of symmetrical, asymmetrical, and thalamocortical synapses they formed. Results of this analysis show that cells of the same general morphological class may form widely different patterns of synaptic connections: some nonspiny multipolar cells had dendrites that formed a high proportion of their synapses with thalamocortical axon terminals, whereas dendrites belonging to other cells formed only very small proportions of thalamocortical synapses. A similar diversity characterized the synaptic connections of cell bodies: some formed more symmetrical than asymmetrical synapses, others the reverse. Some formed high proportions of thalamocortical synapses, others much less. Comparisons of thalamocortical synaptic input to cell bodies and dendrites showed that one cell formed about the same proportions of thalamocortical synapses with its cell body as with its dendrites. For two other cells the proportions of thalamocortical synapses formed with their somata was about double that formed with their dendrites. The remaining five cell bodies examined formed far higher proportions of thalamocortical synapses than did their dendrites. That different nonspiny multipolar cells form such contrasting synaptic patterns suggests that included within this morphological classification are cells which are likely to have very different functional roles.