Smell

Smell is one of the basic senses in animals and humans. It works when small molecules floating in the air—called odorant molecules—reach the smell receptor in the upper part of the nose. There they are detected by receptors (sensory cells) that convert them into electrical signals sent to the brain.

Smell works closely with taste. Much of what people call "the taste of food" actually comes from smell, with odour from food reaching the nose from inside the mouth. This is why people perceive food as tasteless when the nose is blocked or congested.

The Structure of the System

The upper part of the nasal passage contains a thin sensory tissue. Smell receptors have tiny hairs embedded in a layer of mucus; odorant molecules must first dissolve in this mucus. After that, the signal travels through nerve fibres in the nose to a structure called the olfactory bulb, located at the base of the front of the brain. There the signal is processed and distributed to regions of the brain involved in memory and perception.

Unlike other senses, the smell pathway does not pass fully through the thalamus, the central sensory hub, before reaching the perception centres. This accounts for one reason why smell connects so quickly to memory and emotion.

Variation Across Animals

The strength of smell varies greatly across species. Dogs, sharks, and many other animals have a large number of smell receptors, enabling them to track scents. Humans rely more on sight than smell, but can still distinguish thousands of different odours. Insects detect smell through sensory hairs on their head antennae.

Loss and Disorders

Complete loss of smell is called anosmia; reduced smell is hyposmia. Common causes include nasal blockages and infections, chronic sinusitis, head injury, and aging. There also exist conditions in which people perceive smells that are not present (phantosmia) or experience distorted smell (parosmia). Loss of smell can affect food and safety—for example, the ability to detect gas or spoiled food.