Mitral cell

Mitral cells are a key part of the olfactory bulb microcircuit. Mitral cells receive input from at least four cell types: olfactory sensory neurons, periglomerular neurons, external tufted cells and granule cells. The synapses made by external tufted cells and olfactory sensory neurons are excitatory, whereas those of granule cells and periglomerular neurons are inhibitory. In addition, sister mitral cells are reciprocally connected by gap junctions. The mitral to granule and mitral to periglomerular cell synapse was the first description of the rather atypical reciprocal dendrodenritic synapses (in contrast to the more common axodendritic synapse). The action of the full glomerular microcircuit is a topic that is under intense scientific investigation. Certain principles are starting to emerge. One discovery points to the idea of the microcircuit between mitral, tufted and periglomerular cells in separating the output of mitral and tufted cells in time.[6] It appears that tufted cells receive strong olfactory nerve input,[7] fire close to inhalation onset and their firing phase is relatively concentration insensitive, whereas mitral cells receive relatively weak olfactory nerve input[8] and strong periglomerular inhibition, which delays their firing relative to the tufted cells. This escape from inhibition can be sped up by increasing the stimulating odorant concentration, and thus mitral cell firing phase acts as one possible way the olfactory system encodes concentration. The role of the mitral cell lateral dendrite and granule cell circuit is currently a bit more uncertain. One possible hypothesis implicates the system in forming sparse representation which enable more effective pattern separation.[9] The action of this circuit is heavily influenced by both short term and long term plasticity and ongoing granule cell neurogenesis.[10] The circuit requires the animal to be awake if it is to have full functionality.

Mitral and tufted cells project to various targets in the brain. Most importantly, projections target the olfactory cortex, where odor information can be integrated with input from other sensory modalities and used to drive behavior. Tufted cells project mainly to the anterior olfactory nucleus, a center that also performs comparison between left and right side olfactory input. Mitral cells project to the olfactory tubercle, where chemical information is integrated with auditory signals. Mitral cells carrying pheromonal inputs project to the amygdala and hypothalamus to drive instinctive behaviors. A major integrative center is the piriform cortex, where mitral cells make non-topographic projections to pyramidal cells which integrate information across glomeruli. Projections also go to the entorhinal cortex. Anatomical connectivity of a mitral cell axon can be quite different depending on the target structure. Whereas piriform cortex is innervated mostly randomly, projections to the anterior olfactory nucleus and amygdala retain some topographic order. Finally, mitral cell axons also make intrabulbar connections to granule cells and in the mouse olfactory system they project selectively to granule cells underlying the second ipsilateral homotypic (expressing the same olfactory receptor) glomerulus.