Sense of Smell

Introduction image

Sense of smell is one of the senses that is processed by our nervous system to let us know more about our surroundings, and about the things we consume to keep the body healthy and safe. The smell is a sense of the information coming through the nose.

The sensory organ comprises specialized cells and tissues that recognize odourants, transfers a signal, which is received by nerve endings of olfactory nerve, and processed finally by the nervous system. The brain interprets the signal as smell (olfaction). Smell let us know about our surrounding and gives us instincts about what is safe or dangerous.

Content: Sense of smell

  1. Definition
  2. What is Smell?
  3. How do things smell?
  4. Smell Detection
  5. Olfaction
  6. Olfactory Receptors
  7. Relation with Memory and Emotion
  8. Smell Loss

Definition: Sense of Smell

A sense of smell is one’s ability to perceive the odour of things in the surrounding through the nose. The sense of smell is also known as an olfactory sense that unable us to detect pleasant, unpleasantness, or odourless things. Our sense of smell is a chemosensory mechanism that works by detecting chemicals present in the air via the nose (sensory organ).

Chemicals inhaled along with the air will get evaporate onto the olfactory epithelium, after which the olfactory nerve contracts and triggers an electrical response. The signal then moves along the path of the olfactory tract to the olfactory bulb (processes signal concerning memory and emotion), and finally to the neocortex region of the brain, which interprets the kind of smell we perceived.

What is Smell?

Smell or odour can define as the odour molecules (mixtures of light and small molecules) that are filled within the air, which are released from the volatile materials. An odour comes out of the substances that release volatile chemicals (in low concentrations) into the air, which can be perceived as a sense of smell.

smell

Your nose can judge different smells by the presence of smell receptors that can recognize odour molecules in the air. When your nose sniffs the air, the smell receptors get alert and send a signal to your brain. Our brain can recognize different smells at a time that is entering your nose.

Example: Different dishes contain different spices that can give off many odour molecules. Our brain can piece together all this information and let us know which kind of dish is being cooked.

How do things smell?

As we all know that few things smell, while others don’t. The reason is that the smell comes out of the substances that give off particles into the air. To perceive the smell, those particles need to pass through our sensory organ that is the nose. The particles or chemicals releases by the living and non-living materials are grouped into volatile and non-volatile chemicals.

The majority of the living things release volatile chemicals or particles that evaporate easily and give off a stronger smell. Oppositely, non-volatile objects often do smell, rarely smell, or none at all. Generally, Smell can be either pleasant or unpleasant.

kinds of smell

Smell Detection

Olfactory receptors recognize the odourants or volatile chemicals mixed with the air. A single odorant molecule shows varying affinity to attach with the number of olfactory receptors, depending on their physio-chemical properties.

  1. Once the odourants enter through the nasal cavity, the olfactory receptors sense such chemicals.
  2. Then the odorants selectively bind to the olfactory receptor, which results in structural changes in it.
  3. A structural change will turn on the olfactory-type G protein.
  4. Then, the G protein activates the adenylyl cyclase enzyme, which turns ATP into cyclic AMP (cAMP).
  5. After that, cAMP unfolds the cyclic nucleotide-gated ion channels, which allows passage for the calcium and sodium ions to get into the cell.
  6. The entry of calcium and sodium ions cause depolarization of the olfactory receptor neuron and generates an electrical impulse up to the olfactory bulb that first processes the information signal, and transfers it to the forebrain that decides the kind of smell.

Therefore, in the process of olfaction, the chemical information of the odourants converts into electrical information, which can be easily interpreted by the brain. The pathway of chemical signals to the brain is also linked with the regions (amygdala and hippocampus) of emotion and memory.

The sense of smell can change how we feel or what we think. This is the reason, where some people have a good sense of smell, while others have no sense of smell (anosmia). Smell loss can have a big impact on our psychological wellbeing.

Olfaction

Our nose acts like a chemosensory organ, which comprises numerous nerve endings that act differently on the type of chemical. While we inhale, different chemicals mixed within the air drawn into our nasal cavity. Some chemicals don’t cause the nerve endings to contact or do not bring any kind of response and are considered to be odourless or non-volatile chemicals.

In contrast, the volatile chemicals that are present in the air will cause contraction of many nerve endings in our nasal cavity, which in turn fire signals towards the brain. Our brain first interacts with the memory center, and alternatively tells you if the smell that you sense is familiar or unfamiliar.

olfaction

This mechanism is called olfaction. Therefore, the olfactory sense is a protective mechanism through which we can judge the potential illnesses from the things we consume or we use by knowing the pleasantness or unpleasantness of odour.

Olfactory Receptors

These also refers as odourant receptors (millions in the number), which are present on the dendrites of the olfactory sensory neurons. There is a characteristic feature of the olfactory receptors that these possess thin hair-like olfactory cilia. These thread-like filaments float in the nasal mucosa that detects the odorants.

Once it recognizes the presence of volatile odourants, then it gets activated and eventually conducts an electrical signal to the brain about the odour. These receptors act like G protein-coupled receptors of class-A rhodopsin family.

Unique Characteristics

  1. The olfactory receptors are the nerve cells located on the olfactory neuron that is directly connected to the brain. In contrast, other sensory nerve cells conduct information to the brain via nerve to nerve signal transmission.
  2. Olfactory receptors cells can regenerate from the basal stem cells, which is a unique character than the other nerve cells. There is ongoing research about the olfactory nerve cell regeneration to study the potential treatments of anosmia.
  3. Instead of transferring the chemical signal directly to the thalamus like other sensory organs, it transfers signals to the regions of the brain dealing with (Entorhinal cortex and hippocampus) memory and (Amygdala) emotions to the thalamus, and finally to the neocortex.

Sense of Smell in Relation with Memory and Emotion

Amygdala and hippocampus are the regions of the brain that controls the emotion and memory of a person. Therefore, your brain can remind you of  some good and bad experiences associated with particular smells, which are called “olfaction-associated memories”. The smell goes through the olfactory bulb to the limbic system of the brain that controls the mood, memory, and emotion.

Example of smell and memory: When one gets a pleasant smell of your favourite dish, eventually your brain evokes particular memories about the person who makes it for you. Thus, your brain spontaneously reminds you of long-forgotten memories or experiences. Similarly, an unpleasant smell can make you think of awful memories.

Example of smell and emotion: If you like the aroma or smell of some food, your brain will eventually release chemicals that can make you feel more starving and excited. Likewise, if you did not like the smell of the food that went bad, then your brain stops you from consuming that food.

Smell Loss

Like other senses, a sense of smell is also essential to maintain psychological wellbeing, through which we connect with the things around us. Smell loss is a medical condition called anosmia that imposes profound impact on our lifestyle. Anosmia patients may partially or completely lose their sense of smell, which often resulted through inflammation or blockage in the nose, head injury, viral infection, etc.

It is also termed as smell blindness that can affect one’s ability to form and maintain close personal relationships, and can sometimes lead to depression. As we have discussed earlier that smell is psychologically linked with memory and emotion, and its loss can affect your sentimental pathway to memories.

We have never received much attention of this medical condition until the time researchers studied the effects of impaired olfactory system. Smell loss is an early sign of symptom that occurs in the people suffering from both Parkinson’s and Alzheimer’s disease. According to the recent study, smell loss can also be an initial or later sign of symptom in patients who are suffering from corona disease.

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