Olfactory Bulb: Synaptic Organization

Graeme Lowe, Monell Chemical Senses Center Philadelphia, Pennsylvania, USA

Published online: December 2007

DOI: 10.1002/9780470015902.a0020289

Chemosensory information transduced by olfactory receptors in the periphery is mapped into a two dimensional array of glomeruli in the olfactory bulb, creating an orderly spatial code of odour identity. In the bulb, local interneurons implement multiple layers of recurrent and lateral inhibition both within and across glomeruli. These synaptic operations can rescale, filter and modulate neural activity patterns of principal neurons, dynamically modifying odour representations being relayed to higher cortical centres.

Keywords: olfactory bulb; glomeruli; dendrodendritic

Figure 1. Olfactory bulb circuits. Schematic showing some of the major synaptic connections in the mammalian olfactory bulb. Green arrows indicate glutamate release at excitatory synapses, red arrows indicate GABA release at inhibitory synapses (both activating ionotropic postsynaptic receptors). ON axons arriving from the periphery enter glomeruli (gray ovals) and release glutamate. This activates a cascade of local circuits: (1) mitral/tufted (M/T) to PG cell reciprocal feedback; (2) PG cell interglomerular lateral inhibition; (3) SA cell interglomerular lateral inhibition; (4) endothelin (ET) cell to PG cell reciprocal feedback; (5) mitral/tufted (M/T) cell intraglomerular synchronization by gap junctions and glutamate spillover; (6) M/T cell lateral inhibition through granule cells (GC); (7) M/T to granule reciprocal feedback; (8) Blanes cell (BC) inhibition of GC; (9) M/T axon collateral excitation of GC; (10) feedback excitation of granule cells from olfactory cortex; (11) neuromodulatory inputs to glomeruli and granule cells, including serotonergic (5-hydroxytryptamine (HT)), cholinergic (acetylcholine (ACh)) and noradrenergic (NA) fibres.
Figure 2. Dendrodendritic reciprocal synapse. Dendrites of mitral, tufted, granule and juxtaglomerular cells are multitasking-signalling devices, doubling as presynaptic and postsynaptic elements. Excitatory-inhibitory reciprocal synapses implement feedback and lateral inhibition between principal neurons (M/T and ET cells) and interneurons (PG and granule cells). The schematic shows operation of a mitral–granule reciprocal synapse: an action potential back-propagating into a mitral cell secondary dendrite (blue) opens voltage-gated calcium channels, and calcium entry releases glutamate (green arrow) from which activates AMPA and NMDA receptors on granule cell spines (yellow). Spine depolarization opens granule cell calcium channels, releasing GABA (red arrow) which inhibits the mitral cell by activating dendritic GABAA receptors.
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    Wachowiak M and Shipley MT (2006) Coding and synaptic processing of sensory information in the glomerular layer of the olfactory bulb. Seminars in Cell & Developmental Biology 17: 411–423.
    Wilson RI and Mainen ZF (2006) Early events in olfactory processing. Annual Review of Neuroscience 29: 163–201.

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Organisation of the Nervous System | Sensory Systems
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Article Title: Olfactory Bulb: Synaptic Organization
 
How to Cite close
Lowe, Graeme(Dec 2007) Olfactory Bulb: Synaptic Organization. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020289]