Two smell systems in the human brain

Today’s Nature has a special supplement on chemical sensing, including a freely accessible article on smell and the flavour system that is full of surprising facts about one of the most neglected senses.

For me, one of the most surprising aspects of the article, was discovery that there are two distinct brain networks for smell.

One is the orthonasal system which deals with odours ‘sniffed in’ to the nose, and the other is the retronasal smell system (image on the right, click for larger version of both) which is involved in sensing odours when we breath out.

The retronasal system is particularly linked to the flavour system, because it is most commonly activated when we eat food.

The traditional view in the literature on eating behaviour in human culture is that the flavour of prepared foods is humanity’s greatest universal shared behaviour, experienced by individuals of all ages in the course of daily life. Flavour is also among the most complex and powerful of all human sensations. It engages almost all of the sensory modalities. It also engages the complex facial, swallowing and respiratory motor systems. Flavour is therefore an active sensation ‚Äî we use ‘active taste’ to palpate our food with our tongue as we use ‘active touch’ to palpate an object we are examining with our fingers. Some of these systems are indicated in the diagram [above]. Above these sensory and motor systems are the cognitive systems for memory, emotion, abstract thinking and language. The importance of retronasal smell images is illustrated by the massive extent to which they interact with these brain systems compared with orthonasal smell images

The article also discusses the how smells are transformed into spatial odour patterns in the brain depending on which sensors the odour activates, and notes that smell-related genes are the largest group in the genome.

All in all, it’s a really eye-opening article if, like me, you’re not familiar with the surprising and complex nature of our sense of smell.

Link to article ‘Smell images and the flavour system in the human brain’.

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