This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Activity in the thalamus triggers widespread cortical inhibition. This is not because thalamic neurons are inhibitory (they release the excitatory transmitter glutamate) but because they form powerful synapses with inhibitory interneurons in Layer IV of the cortex. We will examine the ultrastructure of these unusually strong connections in a joint light/electron microscopic study using dye labeling of both the thalamic inputs and their postsynaptic interneuron targets.