Glomerulus (olfaction)

Glomerulus (olfaction)
Coronal section of olfactory bulb.
Plan of olfactory neurons.
Identifiers
NeuroLex ID	Olfactory bulb main glomerular layer
Anatomical terms of neuroanatomy []

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.

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. 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. 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.

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. 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. 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.