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Glomerulus (olfaction)
The glomerulus (plural glomeruli) is a spherical structure located in the olfactory bulb of the brain where synapses form between the terminals of the olfactory nerve and the dendrites of mitral, periglomerular and tufted cells. Each glomerulus is surrounded by a heterogeneous population of juxtaglomerular neurons (that include periglomerular, short axon, and external tufted cells) and glial cells.〔A. J. Pinching, T. P. S. Powell. (1971) The neuropil of the glomeruli of the olfactory bulb. J. Cell Sci. 9: 347-377. PMID 4108057〕〔K. Kosaka, K. Toida, Y. Aika, T. Kosaka. (1998) How simple is the organization of the olfactory glomerulus?: the heterogeneity of so-called periglomerular cells. Neuroscience Research 30: 101-110. PMID 9579643〕〔(【引用サイトリンク】title=Wachowiak and Shipley (2006) Coding and synaptic processing of sensory information in the glomerular layer of the olfactory bulb. ''Semin Cell Dev Biol.'' 17(4):411-23 PMID 16765614 )〕
All glomeruli are located near the surface of the olfactory bulb. The olfactory bulb also includes a portion of the anterior olfactory nucleus, the cells of which contribute fibers to the olfactory tract.〔Morris, H., & Schaeffer, J. P. (1953). The Nervous system-The Brain or Encephalon. Human anatomy; a complete systematic treatise. (11th ed., pp. 1034). New York: Blakiston.〕 They are the initial sites for synaptic processing of odor information coming from the nose. A glomerulus is made up of a globular tangle of axons from the olfactory receptor neurons, and dendrites from the mitral and tufted cells, as well as, from cells that surround the glomerulus such as the external tufted cells, periglomerular cells, short axon cells, and astrocytes. In mammals, glomeruli typically range between 50-120 µm in diameter and number between 1100 and 2400 depending on the species, with roughly between 1100 and 1200 in humans.〔〔 The number of glomeruli in a human decreases with age; in humans that are over 80 they are nearly absent.〔R.L Doty. ''Neurobiology of Disease'' (2012). doi:10.1016/j.nbd.2011.10.026〕 Each glomerulus is composed of two compartments, the olfactory nerve zone and the non-olfactory nerve zone. The olfactory nerve zone is composed of preterminals and terminals of the olfactory nerve and is where the olfactory receptor cells make synapses on their targets.〔 The non-olfactory nerve zone is composed of the dendritic processes of intrinsic neurons and is where dendrodendritic interactions between intrinsic neurons occur.〔
==Anatomy==
Glomeruli are important waystations in the pathway from the nose to the olfactory cortex and have been found to be critical for odorant signal transduction. The olfactory receptor neurons (ORN), which originate in the nasal epithelium express only one type of olfactory receptor (OR). These ORNs then project their axons to the olfactory bulb. In the olfactory bulb, the ORNs synapse with termination in the glomeruli.〔S.J. Royal and B. Key (1999) Development of P2 olfactory glomeruli in P2-internal ribosome entry site-Tau-LacZ transgenic mice. J. Neurosci 19: 9856-9864 PMID 10559395〕 Each glomerulus receives input from olfactory receptor neurons expressing only one type of olfactory receptor. The glomerular activation patterns within the olfactory bulb are thought to represent the quality of the odor being detected. These activation patterns of glomeruli can change due to changes in airflow rate and odor concentration in the mucus layer of the nasal cavity.〔Y. Oka, Y. Taki, K. Touhara (2009) Nasal airflow rate affects the sensitivity and pattern of glomerular odorant responses in the mouse olfactory bulb. J. Neurosci 29: 12070-12078 PMID 19793965〕 A certain odorant can activate a glomeruli strongly whilst affecting others with less efficiency to very little at all. Linda Buck and Richard Axel were awarded a Nobel prize in 2004 for heavily influencing the working out of the genetic basis for this Olfactory coding.
The current dogma is that axons from all ORNs expressing the same receptor converge onto one or two glomeruli of a possible 1800 glomeruli in each olfactory bulb.〔 As the axons of the ORNs migrate towards their specific glomeruli they often overshoot into neighboring glomeruli. Thus, a glomerulus representing a specific OR develops slowly and involves considerable axonal reorganization in order to achieve the highly topographical projection observed in adult mice.〔(【引用サイトリンク】url=http://www.jneurosci.org/cgi/content/short/21/24/9713 )〕
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