Circuitry of the olfactory bulb glomerulus. Primary olfactory sensory neurons (OSNs) that express a given odorant receptor and hence exhibit a common molecular receptive range converge upon discrete glomeruli in the olfactory bulb input layer. OSN terminal arborizations form excitatory synapses with the primary dendrites of mitral (Mi), periglomerular (PG) and external tufted (ET) cells; the latter in turn activate short-axon (SA) and local PG cells. PG cells in turn synapse reciprocally with local mitral cell primary dendrites; the mitral cells excite the PG dendrites while the latter inhibit mitral cell primary dendrites via graded inhibition. SA cells project axons (sa) broadly into other glomeruli , within which they excite ET and PG cells; in the present model, the resulting lateral excitatory network of ET and SA cells (the ET/SA network) provides normalizing feedback inhibition to mitral cells via their excitation of PG cells. Mitral cells are the primary output neuron of the olfactory bulb, projecting via axon collaterals to several cortical and subcortical target regions . Filled triangles denote excitatory synapses; open circles denote inhibitory synapses. Lower case labels denote incoming processes originating in other glomeruli. Adapted from reference .