The human brain’s ability to detect smells is truly remarkable. It can distinguish between thousands of different scents and can even identify particular odors that are associated with specific memories. But how does the brain do this?
Scientists have long known that the ability to smell depends on special receptors in the nose that detect particular molecules in the air. These receptors send signals to the brain that are then processed and interpreted as a specific smell. But recent research has shown that there is more to the story.
It turns out that the brain itself plays a very active role in smelling. In fact, the brain is constantly “sniffing” the air around us, even when we’re not consciously aware of it.
This sniffing is done by special neurons in the brain that are sensitive to changes in air pressure. When these neurons detect a change in air pressure, they send a signal to the olfactory bulb, which is the part of the brain responsible for processing smell.
This signal causes the olfactory bulb to send a message to the rest of the brain, telling it to pay attention to the smell. This process happens so quickly that we’re usually not aware of it.
So, the next time you catch a whiff of a delicious meal cooking or a floral fragrance wafting through the air, remember that your brain is working hard to make sure you don’t miss a thing.
Scientists have long known that the ability to smell declines with age. Now, new research has pinpointed how this happens at the level of the brain.
The research, published in the journal Nature, shows that the number of specialised brain cells that detect smells, called olfactory receptors, decreases with age.
“Our study provides the first direct evidence that deterioration of the smelling system in the brain is a cause, rather than a consequence, of ageing,” says study senior author Davide Dulcis, from Stanford University in the US.
The findings could help to explain why the sense of smell often deteriorates with age, and why this is linked to a increased risk of problems such as dementia.
“The ability to smell is crucial for many aspects of daily life, including the enjoyment of food, and plays an important role in social interactions,” says Dulcis.
“Previous studies have shown that a poor sense of smell is associated with a heightened risk of dementia, but it was not clear if this was due to the condition itself or a general decline in brain function with age.”
To investigate, the team looked at data from over 1,000 people aged 58 to 99 who had taken part in two long-running US health studies.
The participants had their sense of smell tested using a simple smell test, in which they were asked to identify different odours, such as roses, lemons, fish, onions, and soap.
The team also looked at brain scans from a subset of participants, and analysed how many olfactory receptors were present in different parts of the brain.
They found that the number of olfactory receptors declined with age, particularly in the frontal and temporal lobes – regions of the brain important for memory and cognition.
“Our findings suggest that the loss of smell may be an early sign of neurodegeneration in the brain,” says Dulcis.
The team also found that people with a poorer sense of smell were more likely to have changes in the brain that are characteristic of Alzheimer’s disease.
“The study provides further evidence for the link between loss of smell and dementia, and suggests that interventions to improve the sense of smell could help to prevent or delay the onset of the condition,” says Dulcis.
The findings also have implications for the diagnosis of dementia.
“At the moment, diagnosis of Alzheimer’s disease is based on memory tests and brain scans,” says Dulcis.
“Our findings suggest that smell tests could be used as an inexpensive and non-invasive way to screen for the condition.”
