User:Amb20m/Olfactory nerve

Function
The afferent nerve fibers of the olfactory receptor neurons transmit nerve impulses about odors to the central nervous system, where they are perceived as smell (olfaction).

The olfactory nerve is special visceral afferent (SVA).

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Function
The olfaction system works to ensure that people can successfully identify an extensive range of odorants and distinguish odors from one another. Odorants interact with the olfactory receptor neurons (ORNs) at the periphery and transmit olfactory information to the central nervous system via axons at the basal surface. These axons aggregate, forming the olfactory nerve. Therefore, the olfactory nerve works to transduce sensory stimuli in the form of odorants and encode them into electrical signals, which are relayed to higher-order centers through synaptic transmission.

Odor Transduction
Odorants bind to specific odorant receptor proteins contained to the outer surface of olfactory cilia within the olfactory epithelium. Odorant binding to the cilia of an ORN evokes an electrical response, kickstarting odor transduction. An individual ORN contains several microvilli, olfactory cilia, which protrude from a knoblike structure at the apical surface involved in dendritic processes. The olfactory cilia lack the cytoskeletal features of motile cilia and are, therefore, more similar to microvilli like that found in the lungs or gut. Olfactory cilia are actin-rich protrusions supported by scaffolding proteins which help to localize odorant receptors and provide an increased cellular surface for odorant binding.

Homologous to G-protein-coupled receptors (GPCRs), olfactory receptor molecules consist of seven trans-membrane, hydrophobic domains and a cytoplasmic domain with a carboxyl terminal region that interacts with G-proteins and odorants. Once an odorant is bound to an odor receptor protein, the alpha subunit of an olfactory-specific heterotrimeric G-protein, Golf, dissociates and activates olfactory-specific adenylate cyclase, adenylyl cyclase III (ACIII). Activation of ACIII leads to an increase in cyclic AMP (cAMP), which depolarizes the neuron due to an influx of Na+ and Ca2+ by opening cyclic nucleotide-gated ion channels. The neuron is further depolarized by a Ca2+-activated Cl- current travelling from the cilia, where the depolarization first occurred, to the axon hillock of the ORN. At the axon hillock, voltage-gated Na+ channels open and generate an action potential that is transmitted to the olfactory bulb. After transmission, the ORN membrane is repolarized by calcium/calmodulin kinase II-mediated mechanisms that work to extrude Ca2+ and transport Na+ via an Na+/Ca2+ exchanger, diminish cAMP levels by activating phosphodiesterases, and restore heterotrimeric Golf.

ORN axons are responsible for relaying odorant information to CNS through action potentials. The ORN axons leave the olfactory epithelium and travel ipsilaterally to the olfactory bulb where the ORN axons coalesce into multiple clusters, called glomeruli, which together form the olfactory nerve. The ORN axons of each glomerulus synapse with apical dendrites of mitral cells, the primary projection neurons of the olfactory bulb, which create and send action potentials further into the CNS.

Regeneration of Olfactory Nerves
ORNs directly interact with odorants inhaled into the olfactory epithelium which can also subject the ORNs to damage through continuous exposure to harmful substances such as airborne pollutants, microorganisms, and allergens. Therefore, ORNs maintain a normal cycle of degeneration and regeneration. The olfactory epithelium consists of three main cell types: supporting cells, mature ORNs, and basal cells. Regeneration of ORNs requires the division of basal cells, neural stem cell s, to produce new receptor neurons. This regeneration process makes ORNs unique when compared to other neurons.

ORN Specificity
In the nasal passages, inhaled odorant molecules interact with receptor proteins on localized neuronal cilia of ORNs. These dendritic extensions, cilia, express one type of protein receptor, although individual odorants can interact with multiple different receptor proteins. As new ORNs mature, they have decreased expression levels of multiple olfactory receptor genes, contrasting with mature ORNs firm rule of one neuron—one expressed olfactory receptor gene. Moreover, different odors activate specific ORNs in a molecular and spatial manner due to receptor specificity. Some ORNs contain receptor proteins with high affinity for some odorants, with distinct odor selectivity to a specific chemical structure, while other receptor proteins are less selective.