One of the 5 senses. In the nose, hair-like projections from smell receptor cells lie in the mucous membrane. When the receptors are stimulated by certain molecules, they transmit impulses along the olfactory nerves to the smell centres in the limbic system and frontal lobes of the brain, where smell is perceived. Possible causes of loss of the sense of smell include inflammation of the nasal membrane, as in a common cold; cigarette smoking; hypertrophic rhinitis, in which thickening of the mucous membrane obscures olfactory nerve endings; atrophic rhinitis, in which the nerves waste away; head injury that tears the nerves; or a tumour of the meninges or nasopharynx. The perception of illusory, unpleasant odours may be a feature of depression, schizophrenia, some forms of epilepsy, or alcohol withdrawal.
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