Sense of Smell: What is it, its process, curiosities and disorders

What is the sense of smell and how does it work? It can be said that the sense of smell is one of the five senses that humans have. Regardless of this, humans have a sense of smell much less developed than most animals. This is perhaps due to the fact that we do not depend on this sense to survive, feed ourselves, look for partners, etc. However, the sense of smell is the most sensitive of our senses. Only a small amount of matter is enough to stimulate the olfactory cells.

We don’t have the ability to guide ourselves through our sense of smell as some animals do, but we are very sensitive to odors.

Sense of smell

Sense of smell

Sense of Smell Organ: The nose

To begin to understand what is smell we must talk about the organ responsible for this sense: the nose. The nose is a protuberance located between the eyes and the mouth. It is part of our respiratory system allowing us to breathe in and out. On the other hand, in addition to breathing, the nose allows different odors to penetrate from the outside and, thanks to the olfactory receptors located in the nostrils, perceive, feel and differentiate various odors.

Sense of Smell: The process of smell

Although the process of smell is more complex than any attempt to explain it, we could explain the process of smell by the following steps:

  • Odor molecules in the form of chemical compounds that float in the air, reach the nostrils and dissolve in the humidities of the yellow pituitary.
  • Once dissolved, the compounds act chemically on olfactory receptors that detect odors.
  • Activation of these receptors generates, from one of the 12 pairs of cranial nerves, nerve impulses that are sent directly to the olfactory bulb and from there to the cerebral cortex (where “sensation” occurs).
  • In the olfactory bulb, the sensory receptors responsible for transmitting the messages of the scents, send the information to two areas:
  • The frontal lobe is responsible for recognizing the odor.
  • On the other hand, the limbic system is related to memory and emotions. When they receive the nerve impulses of the scents, they trigger strong emotions based on previous experiences because the limbic system has access to situational memories, people or places related to the perceived olfactory sensations.

Sense of Smell: Why is it adaptive?

As we mentioned before, our sense of smell is much less developed than most animals. However, it still remains very sensitive, especially to bad odors. Thanks to the structure of our nose, we are able to perceive bad odors in lower concentrations than good odors.

Why is smell sensitive to these odors? Basically, because it is an adaptive trait since the bad odor perception is linked with two basic emotions that favor our survival: disgust and fear. The perception of a bad odor almost immediately triggers an avoidance behavior towards the smell.

For example, if you ever enter your apartment and it smells like gas, you immediately evacuate and call the police or firefighters. The sense of smell triggers a fight or flight response that helps survival.

Sense of Smell: relationship with our memories

As we have seen, the sense of smell directly accesses our memory and our emotions. Surely there are many situations, places or people to which we associate certain odors and, in the same way, these odors quickly lead us to remember those same stimuli.

How many times have you been at a store or just walking around and a perfume odor hits your nose and you remember someone dear to you? 

Why is the sense of smell linked to our memories? Our sensitivity for perceiving odors, combined with our capacity to associate stimuli, makes our sense of smell a direct link to our memory. The smell of bread from a bakery might make us hungry, the smell of chlorine reminds us of the summer, the smell of coffee reminds us of breakfast or work and thus an infinity of associations between different smells and memories. These are some general associations, however, each person creates their own associations.

What is clear is that there are certain smells that can trigger very specific memories that can affect how we feel. Therefore, smells have an emotional component.

Sense of smell- Memories

 Sense of smell- Memories

Sense of Smell: Curiosities

The sense of smell also allows us to appreciate the smell of things and people. However, not everybody smells like us or pleasurable to our nose. Thus we can differentiate those bodies that smell (odoriferous) or those that don’t.

For a body to release an odor it is necessary to free small volatile particles that are the ones that penetrate our olfactory system. With that in mind, the more particles emitted the more intense the body will smell.

Dr. Alan Hirsch, a neurologist, and psychiatrist who specializes in the treatment of smell and taste loss established these curiosities:

  • “We know much more about the moon than we do about smell.”
  • Bad odors can increase aggression
  • Good odors can affect the speed of learning, hand-eye coordination to our perception of time and space itself.
  • We can train ourselves to respond positively (or negatively) to certain odors.
  • Detecting a smell can induce changes in brain waves.
  • Different odors can impact sexual arousal in men versus women.

Sense of Smell: Disorders

We can find some disorders that directly affect the sense of smell, particularly leading to the loss of this sense. Some examples are: 

  • Anosmia: total or partial loss of sense of smell (permanently or for a time).
  • Hyposmia: a reduced sense of smell.
  • Hyperosmia:  A very sensitive sense of smell.
  • Parosmia: the inability of the brain to properly identify an odor’s natural smell. Identifying neutral or pleasant odors as unpleasant. 
  • Phantosmia: It is a kind of olfactory hallucination by which people detect an odor despite it being absent. They detect odors that are not there at the moment.

The sense of smell allows to increase the capacity to taste flavors, reason why many people who lose the sense of smell, they complain that they also lose the sense of taste. Although in most cases they can differentiate flavors like sweet, salty, bitter and sour.

Sense of Smell: Disorders Causes

There are various causes for olfactory disorders. Loss of smell can happen due to conditions that make it difficult for air to reach the olfactory receptors located in the upper part of the nose, or due to an injury or loss of these receptors. Loss of smell does not have to be serious, although in some cases it may be a symptom of other disorders in the nervous system. Some common causes are the following:

  • It is quite frequent that there is a temporary loss of sense of smell due to a common cold, polyps, allergic rhinitis, respiratory tract infections, sinus infection, etc.
  • There may be a normal loss of smell due to aging.
  • Smoking can also affect our sense of smell.
  • Injury or head injury, sinus or cranial surgery.
  • Hormonal alterations.
  • Exposure to some chemicals such as solvents or insecticides can adversely affect our olfactory ability.
  • Some medications change or minimize our ability to perceive odors (antibiotics, antihistamines, amphetamines, naphazoline, estrogens, phenothiazines, continued use of nasal decongestants, reserpine, etc.)
  • Radiotherapy in areas such as the neck or head.
  • Other disorders of the nervous system such as Alzheimer’s or Parkinson’s disease.

The following video a team of neuroscientists review what we just mentioned and how our nose adapts to the smell in our environment. We hope you enjoyed the article and feel free to leave a comment below.

This article is originally in Spanish written by Mario de Vicente, translated by Alejandra Salazar.

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