Sense of smell

Olfaction, commonly called the sense of smell, is how we perceive odors. This sense helps us find food, detect dangers, and sense pheromones, and it also contributes to how we experience taste. In humans, smelling begins when an odor molecule attaches to a receptor inside the nose, which then sends a signal along the olfactory system. A person’s sense of smell can be disrupted by many things: damage to the nose or its smell receptors, a condition called anosmia, nasal congestion, upper respiratory infections, traumatic brain injury, or neurodegenerative diseases. **History of study**

Early scientific work on smell includes Eleanor Gamble’s 1898 doctoral dissertation, which compared smell to other senses and suggested that smell has a weaker ability to distinguish between different intensities. The Roman philosopher Lucretius, writing in the 1st century BC, proposed that different odors come from atoms of different shapes and sizes that stimulate the olfactory organ. A modern version of this idea emerged when Linda B. Buck and Richard Axel cloned olfactory receptor proteins—work that earned them the 2004 Nobel Prize—and then linked specific odor molecules to specific receptor proteins. Each odor receptor recognizes only one type of molecular feature or class of odor molecules. Mammals have roughly a thousand genes for odor reception, but only some of these genes are functional. Humans have far fewer active odor receptor genes than other primates or mammals. In mammals, each olfactory receptor neuron expresses just one functional odor receptor, and these nerve cells work like a key–lock system: if an airborne chemical’s molecules fit the lock, the nerve cell fires. Several competing theories try to explain how odor coding and perception work. The shape theory holds that each receptor detects a feature of the odor molecule. The weak-shape theory, or odotope theory, suggests that different receptors detect only small pieces of molecules, and these minimal inputs combine to create a larger olfactory perception—similar to how vision builds a detailed picture from small, information-poor sensations. A newer study found a functional relationship between the molecular volume of odorants and the neural response to smell. An alternative theory, the vibration theory proposed by Luca Turin, argues that odor receptors detect the infrared-range vibrations of odor molecules through quantum tunnelling, though behavioral tests have raised doubts about this idea. No existing theory fully explains olfactory perception. **Function**

**Taste** Flavor perception combines auditory, taste, touch, and smell information. Retronasal smell—odor that reaches the nose from the mouth—is the biggest contributor to flavor. During chewing, the tongue moves food to release odorants, which enter the nasal cavity when we exhale. Food smells seem to come from the mouth because the motor cortex and olfactory epithelium activate together during chewing. Smell, taste, and trigeminal receptors (chemesthesis) all work together to create flavor. The human tongue can distinguish only five basic tastes, while the nose can identify hundreds of substances, even in tiny amounts. The smell component of flavor occurs during exhalation, unlike ordinary smell, which happens during inhalation. The olfactory system is the only human sense that bypasses the thalamus and connects directly to the forebrain. **Hearing** In rodents, smell and sound information converge in the olfactory tubercles. This neural convergence may create a perception called a “smound.” While flavor arises from interactions between smell and taste, a smound may arise from interactions between smell and sound. **Inbreeding avoidance** MHC genes (called HLA in humans) are important for the immune system; generally, offspring from parents with different MHC genes have stronger immune systems. Fish, mice, and human females can smell some aspect of the MHC genes of potential mates and prefer partners with different MHC genes. However, some research suggests that hormonal contraception can alter women’s preferences, making them more likely to choose partners with similar MHC genes. Sexual orientation also influences preferences for different body odors, and some studies indicate that the putative pheromones AND and EST may play a role. Humans can detect blood relatives by smell. Mothers can identify their biological children by body odor but not their stepchildren. Pre-adolescent children can smell their full siblings but not half-siblings or step-siblings, which may help explain incest avoidance and the Westermarck effect. Functional imaging shows that this olfactory kinship detection involves the frontal-temporal junction, the insula, and the dorsomedial prefrontal cortex, but not the primary or secondary olfactory cortices, the piriform cortex, or the orbitofrontal cortex. Because inbreeding is harmful, it tends to be avoided.