Figure 2.

Circuit diagram of the mammalian olfactory bulb (two glomeruli shown, with corresponding postglomerular circuitry). The axons of olfactory sensory neurons (OSNs) expressing the same odorant receptor type (denoted by the shape and color of the receptor) converge together to form glomeruli (shaded ovals) on the surface of the olfactory bulb. Multiple classes of olfactory bulb neuron also innervate each glomerulus. Glomerular interneuron classes are heterogeneous, and include olfactory nerve-driven periglomerular cells (PGo), external tufted cell-driven periglomerular cells (PGe), and multiple subtypes of external tufted cells (ET). Superficial short-axon cells (sSA) are not associated with specific glomeruli but project broadly and laterally within the deep glomerular layer, interacting with glomerular interneurons. Principal neurons include mitral cells (Mi), which interact via reciprocal connections in the external plexiform layer (EPL) with the dendrites of inhibitory granule cells (Gr), thereby receiving recurrent and lateral inhibition. Middle/deep tufted cells, another class of olfactory bulb principal neurons, are not depicted. OE, olfactory epithelium (in the nasal cavity); GL, glomerular layer; EPL, external plexiform layer; MCL, mitral cell layer; IPL, internal plexiform layer; GCL, granule cell layer. Filled triangles denote excitatory (glutamatergic) synapses; open circles denote inhibitory (GABAergic) synapses. Speckles surrounding OSN terminals connote volume-released GABA and dopamine approaching presynaptic GABA_B and dopamine D2 receptors. Figure adapted from (Cleland, 2010).