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. Author manuscript; available in PMC: 2015 Aug 1.
Published in final edited form as: Trends Neurosci. 2014 Jun 17;37(8):443–454. doi: 10.1016/j.tins.2014.05.005

Figure 3. Odorant concentration coding in glomerular circuits.

Figure 3

A. Each glomerulus transforms the afferent ON signals transduced by one type of olfactory receptor into the spike activities of several dozen or more mitral cells (MC) and tufted cells (TC). The two classes (MC, TC) of olfactory bulb output neurons differ in connectivity and regulation by local circuits. Various subtypes of tufted cells are located more superficially, receive more direct ON input and less granule cell inhibition, and are more easily excited. The mitral cells are located deeper, receive mostly indirect ON input (via ET cells) and more granule cell inhibition, and have higher spike thresholds. B. Coding of odorant concentration by the number or fraction of responding ‘sister’ mitral/ tufted (i.e. connected to the same glomerulus). When odorant concentration is increased, stronger afferent ON input to the glomerulus can recruit a greater proportion of sister mitral/ tufted cells, including those with higher spike thresholds. C. Coding of odorant concentration by shift in spike latency between mitral and tufted cells. When odorant concentration is increased (blue to red traces), latency and sniff cycle phase of mitral cell spike responses decreases while that of tufted cells remains approximately invariant. D. Coding of odorant concentration by medio-lateral timing difference in activation of pairs of glomeruli corresponding to the same olfactory receptor. Inhaled odorant gains access to OSNs in medial olfactory epithelium (on septum) sooner than OSNs in lateral olfactory epithelium (in recesses of nasal turbinates). This results in a latency difference between inputs to glomeruli of mirror image receptor maps in medial and lateral halves of the olfactory bulb. E. Medio-lateral gradient in concentration at fixed time due to relative lag in odorant access. This gradient may depend on sorption properties of the odorant. F. As odorant concentration is increased, the latency of spike responses drops at different rates for mitral cells connected to medial vs. lateral glomeruli. Data sources: C: [46], average spike phase of mouse mitral cell vs. tufted cell response to various odorants; F: [49], onset latencies of mitral cell spike responses in medial vs. lateral olfactory bulb of transgenic mice with expression of olfactory receptor I7 in all OSNs activated by octanal.