| Olfactory receptor neuron | |
|---|---|
 Labels in German. "Zellen" = "cell", "riech" = "smell", "Riechnerv" =olfactory nerve, "cillien" =cilia. | |
| Details | |
| System | Smell |
| Location | Olfactory epithelium in thenose |
| Shape | Bipolar sensory receptor |
| Function | Detect traces of chemicals in inhaled air (sense of smell) |
| Neurotransmitter | Glutamate[1] |
| Presynaptic connections | None |
| Postsynaptic connections | Olfactory bulb |
| Identifiers | |
| MeSH | D018034 |
| NeuroLex ID | nifext_116 |
| TH | H3.11.07.0.01003 |
| FMA | 67860 |
| Anatomical terms of neuroanatomy | |
Anolfactory receptor neuron (ORN), also called anolfactory sensory neuron (OSN), is asensory neuron within theolfactory system.[2]
Humans have between 10 and 20 million olfactory receptor neurons (ORNs).[3] Invertebrates, ORNs arebipolar neurons withdendrites facing the external surface of thecribriform plate with axons that pass through thecribriform foramina with terminal end at olfactory bulbs. The ORNs are located in theolfactory epithelium in the nasal cavity. The cell bodies of the ORNs are distributed among thestratified layers of the olfactory epithelium.[4]
Many tiny hair-like non-motilecilia protrude from the olfactory receptor cell'sdendrites. The dendrites extend to the olfactory epithelial surface and each ends in a dendritic knob from which around 20 to 35 cilia protrude. The cilia have a length of up to 100 micrometres and with the cilia from other dendrites form a meshwork in the olfactorymucus.[5] The surface of the cilia is covered witholfactory receptors, a type ofG protein-coupled receptor. Each olfactory receptor cellexpresses only one type of olfactory receptor (OR), but many separate olfactory receptor cells express ORs which bind the same set of odors. The axons of olfactory receptor cells which express the same OR converge to formglomeruli in theolfactory bulb.[6]
ORs, which are located on the membranes of the cilia have been classified as a complex type ofligand-gated metabotropic channels.[7] There are approximately 1000 different genes that code for the ORs, making them the largest gene family. Anodorant will dissolve into the mucus of the olfactory epithelium and then bind to an OR. ORs can bind to a variety of odor molecules, with varying affinities. The difference in affinities causes differences in activation patterns resulting in unique odorant profiles.[8][9] The activated OR in turn activates the intracellular G-protein, GOLF (GNAL),adenylate cyclase and production ofcyclic AMP (cAMP) opension channels in thecell membrane, resulting in an influx ofsodium andcalcium ions into the cell, and an efflux ofchloride ions. This influx of positive ions and efflux of negative ions causes the neuron to depolarize, generating anaction potential.
The olfactory receptor neuron has a fast working negative feedback response upondepolarization. When the neuron is depolarizing, theCNG ion channel is open allowingsodium andcalcium to rush into the cell. The influx of calcium begins a cascade of events within the cell. Calcium first binds to calmodulin to formCaM. CaM will then bind to the CNG channel and close it, stopping the sodium and calcium influx.[10]CaMKII will be activated by the presence of CaM, which will phosphorylateACIII and reduce cAMP production.[11] CaMKII will also activatephosphodiesterase, which will then hydrolyze cAMP.[12] The effect of this negative feedback response inhibits the neuron from further activation when another odor molecule is introduced.
A widely publicized study suggested that humans can detect more than one trillion different odors.[13] This finding has been disputed. Critics argued that the methodology used for the estimation was fundamentally flawed, showing that applying the same argument for better-understood sensory modalities, such as vision or audition, leads to wrong conclusions.[14] Other researchers have also showed that the result is extremely sensitive to the precise details of the calculation, with small variations changing the result over dozens of orders of magnitude, possibly going as low as a few thousand.[15] The authors of the original study have argued that their estimate holds as long as it is assumed that odor space is sufficiently high-dimensional.[16]
DogsCompared to humans, dogs have a larger number of olfactory receptor neurons and a larger olfactory bulb, resulting in a remarkably sensitive sense of smell, which is used by law enforcement to detect dangerous and illegal substances and biological scents, as well as by agricultural and conservation scientists to detect other living organisms, such as plant parasites, endangered animals, invasive species and even microorganisms.[17]
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