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The olfactory system is the sensory system used for olfaction, or the sense of smell. 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, airborne 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 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.
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 receptors. These olfactory receptors are membrane proteins of bipolar olfactory receptor neurons in the olfactory epithelium. Rather than binding specific ligands like most receptors, olfactory receptors display affinity for a range of odor molecules. Olfactory neurons transduce receptor activation 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. The complex set of olfactory receptors on different olfactory neurons can distinguish a new odor from the background environmental odors and determine the concentration of the odor.
Axons from the olfactory sensory neurons converge in the olfactory bulb to form tangles called glomeruli (singular glomerulus). Inside the glomerulus, the axons contact the dendrites of mitral cells and several other types of cells. Mitral cells send their axons to a number of brain areas, including the anterior olfactory nucleus, 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 potentials by multiple neurons corresponds to the odor) or a combination of the two. Scientists debate whether the odor code is primarily temporal or spatial.
Clinical implications 
Damage to the olfactory system can occur by traumatic brain injury, cancer, infection, inhalation of toxic fumes, or neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease. These conditions can cause anosmia. In contrast, recent finding suggested the molecular aspects of olfactory dysfunction can be recognized as a hallmark of amyloidogenesis-related diseases and there may even be a causal link through the disruption of multivalent metal ion transport and storage (M. Mahmoudi and K. S. Suslick 2012). 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. Doctors must exclude other diseases that inhibit or eliminate 'the sense of smell' such as chronic cold of sinusitus before making the diagnois that there is permanent damage to the olfactory system.
See also 
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