ABSTRACT

The axons of the ORNs run in the olfactory nerve to make excitatory synapses on the dendrites of mitral cells or tufted cells (M/T) and short axon inhibitory periglomerular cells in the olfactory bulb. M/T cells send their axons into the olfactory tract. Synapses between ORNs and M/T and periglomerular cells are found in olfactory glomeruli, spherical zones some 150 μm across. The olfactory bulb contains about 2000 glomeruli, each receiving the terminals of 25000 ORNs that respond to the same odors. Hence, glomeruli are odor-specific functional units (Figure 1). Low concentrations of a given odor molecule activate cells in the single glomerulus which gets input from the ORNs bearing odorant receptors with the highest affinity for the molecule. At higher concentrations, cells in other glomeruli are activated as their ORN odorant receptors’ low-affinity binding sites for the molecule are occupied. Each glomerulus has dendrites from about 75 M/T cells. The M/T cells integrate weak inputs from a large number of ORNs within a glomerulus to generate a strong signal.