The glomerulus (plural glomeruli) is a spherical structure located in the

olfactory bulb The olfactory bulb (Latin: ''bulbus olfactorius'') is a grey matter, neural structure of the vertebrate forebrain involved in olfaction, the sense of odor, smell. It sends olfactory information to be further processed in the amygdala, the orbitof ...

of the brain where synapses form between the terminals of the

olfactory nerve The olfactory nerve, also known as the first cranial nerve, cranial nerve I, or simply CN I, is a cranial nerve that contains sensory nerve fibers relating to the sense of smell. The afferent nerve fibers of the olfactory receptor neurons tr ...

and the

dendrites Dendrites (from Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the n ...

mitral The mitral valve (), also known as the bicuspid valve or left atrioventricular valve, is one of the four heart valves. It has two cusps or flaps and lies between the left atrium and the left ventricle of the heart. The heart valves are all one-w ...

, periglomerular and

tufted cells Tufted cells are found within the olfactory glomeruli. They receive input from the receptor cells of the olfactory epithelium found in areas of the nose able to sense smell. Both tufted cells and mitral cells are called projection neurons. Project ...

. Each glomerulus is surrounded by a heterogeneous population of juxtaglomerular neurons (that include periglomerular, short axon, and external tufted cells) and

glial cells Glia, also called glial cells (gliocytes) or neuroglia, are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system that do not produce electrical impulses. They maintain homeostasis, form myel ...

. 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

axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action po ...

s from the

olfactory receptor neuron An olfactory receptor neuron (ORN), also called an olfactory sensory neuron (OSN), is a sensory neuron within the olfactory system. Structure Humans have between 10 and 20 million olfactory receptor neurons (ORNs). In vertebrates, ORNs are b ...

s, 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.

Anatomy

Glomeruli are important waystations in the pathway from the

nose A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for respiration alongside the mouth. Behind the nose are the olfactory mucosa and the sinuses. Behind the nasal cavity, air next passes th ...

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. 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.

Function

The glomerulus is the basic unit in the odor map of the olfactory bulb. Each odor activates a different pattern of glomeruli, such that, simply by analyzing the different sets of activated glomeruli, one could, in theory, decode the identity of the odor. This odor map, however, is modified by the circuitry within the olfactory bulb so that the spiking pattern of the second-order mitral cells is usually different from that of the

olfactory sensory neurons An olfactory receptor neuron (ORN), also called an olfactory sensory neuron (OSN), is a sensory neuron within the olfactory system. Structure Humans have between 10 and 20 million olfactory receptor neurons (ORNs). In vertebrates, ORNs are ...

Other species

Glomerulus in Dogs

The

glomerulus ''Glomerulus'' () is a common term used in anatomy to describe globular structures of entwined vessels, fibers, or neurons. ''Glomerulus'' is the diminutive of the Latin ''glomus'', meaning "ball of yarn". ''Glomerulus'' may refer to: * the filter ...

process in dogs is divided in 3 parts: signal acquisition, signal transduction, and signal processing. Some dogs have as many as 100 times more ORNs than humans do, producing a correspondingly sharpened ability to detect and discriminate among millions of odors. Characteristics of these stages include: the role of olfactory plume and sniffing, the continuous renewal of Glomerulus receptors throughout the life cycle, and the relationship between the olfactory neuron and the glomerulus, and finally, the synthetic nature of glomerulus coding.

Glomerulus in Fish

One of the most distinctive features of fish olfaction is that it takes place entirely in the aquatic environment. The carrier of stimulant is water and therefore the chemicals must be soluble in water. The olfactory epithelium of fish consists of three cell types, like other vertebrates. These three cell types are the receptor cells, supporting cells and basal cells. Fish glomerulus differs from the mammalian glomerulus in terms of the number of dendrites that it receives from the mitral cells. In a mammalian olfactory system, a single dendrite from a mitral cell enters a single glomerulus. However, in fish, one or more dendrites from mitral cells enter one or more glomerulus.

References

{{DEFAULTSORT:Glomerulus (Olfaction) Olfactory system