Your senses may be underestimated! Today, let's take you to rediscover your sense of smell

Produced by: Science Popularization China

Author: Wang Jiayin (Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences)

Producer: China Science Expo

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In the human sensory system, the sense of smell seems to be often considered less important. According to a survey, more than half of young people would rather lose their sense of smell than give up their mobile phones or laptops.

However, the latest research from the Weizmann Institute of Science in Israel has made a thought-provoking discovery: people who lose their sense of smell not only lose the ability to perceive odors, but also change the way they breathe . This study, published in Nature Communications on October 22, 2024, has opened up a new perspective for us to understand the impact of anosmia on health.

Schematic diagram of the wearable device used in this study, with the left image showing the device fixed to the back of the neck and the right image showing the device connected via a nasal cannula to measure airflow in each nostril separately.

(Image source: Reference 1)

From molecules to the brain: the exquisite mechanism of the olfactory system

Let's first talk about how the olfactory system works. At the top of our nasal cavity, there is a special area of ​​about 5 square centimeters, where 5 million olfactory neurons are distributed. Their dendrites extend olfactory cilia, floating in a layer of mucus, forming the first checkpoint for perceiving odors.

The Brain and Sensory Perception

(Photo source: veer photo gallery)

When we inhale air, odor molecules dissolve in this mucus and bind to receptors on the olfactory cilia. This process is like a key fitting into a lock - each odor molecule is a unique "key" and the olfactory receptor is the corresponding "lock." Humans have about 400 different olfactory receptor genes, allowing us to distinguish thousands of different smells.

When odor molecules bind to receptors, they trigger a series of biochemical reactions inside neurons, ultimately generating electrical signals. These signals are first transmitted to the olfactory bulb of the brain for preliminary processing, and then directly projected to multiple brain regions including the piriform cortex and amygdala. This unique neural pathway explains why odors can so strongly affect our emotions and memories.

Even more surprising is that the olfactory system is extremely plastic. Olfactory neurons are the only sensory neurons in the adult body that can continue to regenerate, and they are partially renewed every month, which may help maintain the sensitivity of the system. When problems occur in this delicate system, it not only affects our ability to perceive odors, but may also affect overall health in many ways, such as by changing breathing patterns, which explains why anosmia has such a wide range of effects.

An underrated sense

People often say that losing vision is like losing the whole world, losing hearing is like losing the bridge to communicate with people, and losing the sense of smell is like losing the pleasure of tasting delicious food. This view has been popular since the 19th century, and even the famous psychologist Freud believed that the sense of smell is just an "animal behavior." However, the reality is far more complicated than people imagine.

Studies have shown that people who lose their sense of smell not only suffer from a loss of quality of life, but also face a range of unexpected health risks. They are more likely to experience depressive symptoms, feel lonely and emotionally numb, and even have a shorter life expectancy. Even more shocking is that statistics show that the risk of death for elderly people with anosmia is three times that of normal people.

The secret of breathing

Why does losing a "less important" sense have such serious consequences? With this question in mind, the research team of the Weizmann Institute of Science conducted a carefully designed experiment. They asked 21 people who were born without a sense of smell and 31 people with normal sense of smell to wear a special detection device to record their breathing patterns for 24 hours.

The results showed that while there was no difference in the overall breathing rate between the two groups, the difference was obvious at a more subtle level. People with normal sense of smell tend to have some tiny peaks of inhalation with each breath, and these extra inhalations are considered an instinctive "exploratory sniff" - just like we will unconsciously take a few more breaths when we smell a faint smell. People who have lost their sense of smell completely lack this pattern, with about 240 fewer such exploratory inhalations per hour.

Even more surprising is that this difference exists not only in the awake state, but also in sleep. The researchers found that the difference in breathing patterns alone can accurately determine whether a person has a sense of smell with 83% accuracy.

The left image shows a recording of 4 consecutive breaths from a participant with no sense of smell. The peaks identified by the software are marked with red circles. Here, the total number of breaths is 4, and the number of inspiratory peaks is also 4. The right image shows a recording of 4 consecutive breaths from a participant with normal sense of smell. The peaks identified by the software are marked with red circles. Here, the total number of breaths is also 4, but the number of inspiratory peaks reaches 9.

(Image source: Reference 1)

Impacts beyond smell

The findings have implications beyond that. The researchers note that changes in breathing patterns may affect the rhythm of brain activity. Each inhalation sets off a cascade of neural activity in the brain, starting in the olfactory bulb and spreading throughout the cerebral cortex. For people with a normal sense of smell, an extra 240 exploratory inhalations per hour means 240 extra waves of neural activity in the brain.

This difference may affect emotions, cognition, and even memory formation. Studies have shown that breathing rhythm is closely related to the recognition of fear emotions, memory consolidation and retrieval, the quality of mental imagination, and spatial processing ability. This may explain why people who lose their sense of smell are more likely to suffer from psychological problems such as depression and anxiety.

Rediscovering Breathing

This study also makes us realize the importance of breathing. In the history of medicine, people have long noticed the important influence of breathing patterns on health. For example, when treating polio, scientists found that it was not enough to just maintain a basic breathing rate. Patients also needed to take a deep breath (sigh) every few minutes, otherwise the alveoli in the lungs would collapse.

"It's not enough to just focus on the frequency of your breathing; subtle changes in your breathing patterns could have profound effects on your physical and mental health. This finding is particularly important for people who have lost their sense of smell, either naturally or later in life, because it means we may need to develop new treatments that focus not only on restoring the sense of smell but also on regulating breathing patterns," said Dr.

There are intricate connections between the various systems of the human body. The seemingly simple function of smell is actually closely connected to the respiratory system and the nervous system. The impact of losing the sense of smell is far more profound than we think. As the lead author of this study said: "When a person loses his sense of smell, he loses more than just the ability to perceive odors. This loss will bring about a series of chain reactions, starting with changes in breathing patterns, which may eventually affect overall health."

References:

1.Lior Gorodisky et. al., Humans without a sense of smell breathe differently