Olfactory system
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The '''olfactory system''' is the [[sensory system]] used for [[olfaction]]. Most mammals and reptiles have two distinct parts to their olfactory system: a '''main olfactory system''' and an [[accessory olfactory system]]. The main olfactory system detects volatile, airborn substances, while the accessory olfactory system senses fluid-phase stimuli. Behavioral evidence indicates that most often, the stimuli detected by the accessory olfactory system are [[pheromones]].
The olfactory system is often spoken of along with the [[gustatory system]] as the '''chemosensory senses''' because both [[transduction (physiology)|transduce]] chemical signals into [[perception]].
[[Image:1543,Vesalius'OlfactoryBulbs.jpg|150px|thumb|[[Vesalius]]' ''Fabrica'', 1543. [[Human]] [[Olfactory bulb]]s and Olfactory tracts outlined in red]]
==Function==
The mechanism of the olfactory system can be divided into a peripheral one, sensing an external [[stimulus]] and encoding it as an electric signal in [[neurons]], and a central one, where all signals are integrated and processed in the [[central nervous system]].
===Peripheral===
[[Image:Olfactory system.svg|thumb|280px|1: [[Olfactory bulb]] 2: [[Mitral cell|Mitral cells]] 3: Bone 4: Nasal [[Epithelium]] 5: [[Glomerulus (olfaction)|Glomerulus]] 6: [[Olfactory receptor|Olfactory receptor cells]]]]
[[Image:Amyg.png|thumb|150px|Amygdala location in each hemisphere of the human brain]]
In mammals, the main olfactory system detects odorants that are inhaled through the [[nose]], where they contact the main [[olfactory epithelium]], which contains various [[olfactory receptor]]s. These can distinguish a new odor from the background environmental odors and determine the concentration of the odor.
These olfactory receptors are connected to [[olfactory receptor neuron]]s in the olfactory epithelium, which transduce receptoractivation into electrical signals in [[neurons]]. The signals travel along the [[olfactory nerve]], which belongs to the [[peripheral nervous system]]. This nerve terminates in the [[olfactory bulb]], which belongs to the [[central nervous system]].
===Central===
[[Axon]]s from the olfactory sensory neurons converge in the olfactory bulb to form tangles called [[glomerulus (olfaction)|glomeruli]] (singular glomerulus). Inside the glomulerus, the axons contact the [[dendrite]]s of [[mitral cells]] and several other types of cells. Mitral cells send their axons to a number of brain areas, including the [[piriform cortex]], the medial [[amygdala]], and the [[entorhinal cortex]].
The piriform cortex is probably the area most closely associated with identifying the odor. The medial [[amygdala]] is involved in social functions such as mating and the recognition of animals of the same species. The [[entorhinal cortex]] is associated with memory, e.g. to pair odors with proper memories. The exact functions of these higher areas are a matter of scientific research and debate.
In the central nervous system, odors are represented as patterns of neural activity. These representations may be encoded by space (a pattern of activated [[neurons]] across a given olfactory region corresponds to the odor), time (a pattern of [[action potential]]s by multiple neurons corresponds to the odor) or a combination of the two. Scientists debate whether the odor code is primarially temporal or spatial.
==Clinical implications==
Damage to the olfactory system can occur by traumatic [[brain injury]], [[cancer]], inhalation of toxic fumes, or neurodegenerative diseases such as [[Parkinson's disease]] and [[Alzheimer's disease]]. These conditions can cause [[anosmia]]. Doctors can detect damage to the olfactory system by presenting the patient with odors via a scratch and sniff card or by having the patient close their eyes and try to identify commonly available odors like coffee or peppermint candy.
==History==
[[Linda B. Buck]] and [[Richard Axel]] won the [[2004]] [[Nobel Prize in Physiology or Medicine]] for their work on the olfactory system.
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