Olfactory Sensory Neurons
An olfactory sensory neuron (OSN) is a component within the olfactory system used to detect airborne chemicals that are inhaled, which gives rise to the sense of olfaction or smell. Olfactory sensory neurons are transduction cells that total about six million in humans (Moran et al., 1982). Transduction cells convert chemical signals into electrical signals that travel to the brain, so smells can be perceived. OSNs are classified as bipolar neurons due to having two processes that emerge from the cell body. These neurons are arranged with dendrites positioned in the inferior space of the nasal cavity and an axon that projects through the cribriform plate to the olfactory nerve, and subsequently the olfactory bulb. OSNs are located in the olfactory epithelium in the nose, where its cell bodies are distributed among all three of its stratified layers.
The olfactory epithelium has a thin layer of mucus covering its surface. There are many cilia that project into this mucus layer from the olfactory sensory neuron’s dendrites. The surface of these hair-like cilia is blanketed with olfactory receptors. These olfactory receptors are a type of G protein—coupled receptor, which means the receptors are inherently metabotropic. This means the receptor is indirectly activated when ions enter an ion channel, which is done by the secondary messenger, G protein molecules.
In this activation process, an odorant molecule will dissolve into the mucous membrane of the olfactory epithelium and subsequently bind to an olfactory receptor. This ligand-receptor binding is variable. Each receptor can bind to a variety of odorants with differing affinities. The varying strength of these intermolecular interactions gives rise to variability in activating neurons and results in the detection of unique smells (Bieri et al., 2004). Olfactory receptors on different OSNs can detect new odors from background environmental odors. Activated olfactory receptors then activate intracellular G protein, guanine nucleotide-binding protein (GNAL), adenylate cyclase, and the production of cyclic adenosine monophosphate (cAMP). Molecules of cAMP cause ion channels within the cell membrane to open, which ultimately results in depolarization of the neuron and the generation of an action potential, due to an influx of sodium and calcium and an efflux of chloride ions.
The olfactory sensory neuron is equipped with a rapid negative feedback mechanism upon depolarization. During the depolarization event, when cAMP binds to the cyclic nucleotide-gated (CNG) ion channels, sodium and calcium diffuse into the cell. When calcium diffuses into the cell, a series of events occur. Initially, calcium binds to calmodulin to form CaM. Then, CaM molecules will do a couple of things. For one, they will find CNG channels and close them, halting the influx of sodium and calcium. CaM will also activate calmodulin-dependent protein kinase II (CaMKII). CaMKII will reduce cAMP levels by both deactivating adenyl cyclase and by activating phosphodiesterase, which hydrolyzes cAMP (Wei et al., 1998). This negative feedback response inhibits the OSNs from being activated again when other odor molecules are introduced. The feedback system allows vertebrates to adapt to a stimulus.
Each OSN singly expresses only one type of olfactory receptor, which is a phenomenon that has been called the “one neuron, one receptor” rule. Olfactory receptors are the largest gene family. It is estimated that there are 1,000 different genes that code for olfactory receptors. That being said, there are many separate OSNs that express olfactory receptors, which bind to the same set of odors. The axons of these OSNs that express the same olfactory receptors come together to form glomeruli in the olfactory bulb. These axons touch the dendrites of mitral cells inside the glomerulus. Mitral cells innervate the following brain areas: the medial amygdala, anterior olfactory nucleus, entorhinal cortex, olfactory tubercle, and piriform cortex.
The medial amygdala is associated with social functions like mating and recognizing others. The entorhinal cortex is involved with memory such as pairing odors with memories. The piriform cortex is an area involved with identifying odors. Individual odors are characterized by patterns of activated neurons in an olfactory region. These functions of the central nervous system are still being researched and are debatable (Scott et al., 1980).
Drake E. Sisneros
See also: Anosmia; Dysosmia; G Proteins; Odor Threshold; Olfactory Bulb; Olfactory Mucosa; Olfactory Nerve; Olfactory System; Phantosmia; Pregnancy and Sense of Smell
Bieri, Stephan, Katherine Monastyrskaia, & Boris Schilling. (2004). Olfactory receptor neuron profiling using sandalwood odorants. Chemical Senses, 29(6), 483—487.
Moran, David T., J. Carter Rowley III, Bruce W. Jafek, & Mark A. Lovell. (1982). The fine structure of the olfactory mucosa in man. Journal of Neurocytology, 11(5), 721—746.
Scott, John W., Russell L. McBride, & Stephen P. Schneider. (1980). The organization of projections from the olfactory bulb to the piriform cortex and olfactory tubercle in the rat. Journal of Comparative Neurology, 194(3), 519—534.
Wei, Jia, et al. (1998). Phosphorylation and inhibition of olfactory adenyl cyclase by CaM kinase II in neurons: A mechanism for attenuation of olfactory signals. Neuron, 21(3), 495—504.