How Does Smell Get from the Nose to the Brain?

Odorous substances in the air are processed through the following steps:

• Odour sensing by receptors of the olfactory sensory neurons in the nose.
• The olfactory sensory neurons in the nose transmit the odour information (olfactory information) to the olfactory nerves through a process known as neurotransmission.
• The olfactory nerves transmit the olfactory information to the olfactory bulb, a neural structure responsible of olfaction (Smell).
• The olfactory bulb transmits the olfactory information to specific areas of the brain for processing and identification of the smell.

1. What Part of the Brain Is Most Responsible for Smell?

There are several and important parts of the brain that are responsible for smell; however, the olfactory bulb is responsible for transmitting the olfactory (smell) information from the olfactory nerves to the brain for processing.

This olfactory information is chemically identified and coded by a part of the olfactory bulb known as the glomeruli. This process is known as the odor map [1].

2. What Happens in the Brain When You Smell?

When the olfactory information is transmitted from the olfactory bulb to the brain, there are parts of the brain responsible for the processing of this information including the amygdala, the piriform cortex, and the entorhinal cortex [2].

• The Amygdala and Olfaction

The amygdala is the central hub for the management of fear within a network that involves other brain organs including the thalamus, the neocortex, the hypothalamus, the pituitary gland, the hippocampus, and the adrenal glands located on the top of the kidneys.

When the amygdala receives the olfactory information for processing it interrogates the memory siege, the hippocampus, for previous similar stimuli (e.g., olfactory stimuli).

Once the information is received from the hippocampus, the amygdala triggers the hypothalamus-pituitary-adrenal axis to initiate an adaptative reaction [3].

Depending on the nature of the smell which can be pleasant or unpleasant, associated or not with potential danger, a reaction such as robust approach (pleasant olfaction) or withdrawal if unpleasant or linked with potential danger (e.g., the smell of smoke due to fire) [4].

• The Olfactory Tubercle and Olfaction

The olfactory tubercle is part of the olfactory cortex found in the frontal lobe of the brain where is connected to the amygdala and the hippocampus.

It is involved in recognizing one’s body odor from the environment odors (sensory integration), motivated behaviors in response to odor (attractiveness or repulsion), and reward recognition [1].

• The Piriform Cortex and Olfaction

The piriform cortex is part of the brain known as the rhinencephalon located in the cerebrum. It is involved in odor perception and its differentiation from other odors and from a mixture of odors, and in odor memory [5].

• The Entorhinal Cortex and Olfaction

The Entorhinal cortex is part of the cerebral cortex where it plays a role as an interface between the hippocampus that is involved in memory and the neocortex involved in sensory perception (e.g., olfaction) spatial reasoning, and navigation, and cognition [6].

4. Can Smells Trigger Emotions?

There are common brain areas that are involved in both olfaction and emotion such as the amygdala, the hippocampus, the orbitofrontal cortex, the insula, and the cingulate cortex.

When the amygdala receives the olfactory information for processing it interrogates the memory siege, the hippocampus, for previous similar stimuli (e.g., olfactory stimuli).

Depending on the nature of the smell which can be pleasant or unpleasant, associated or not with potential danger, a reaction such as robust approach (pleasant olfaction) or withdrawal if unpleasant or linked with potential danger (e.g., the smell of smoke due to fire) [7].

The orbitofrontal cortex is involved in the reward value of taste but also in the reward value of odors which initiate an emotion [8].

The insula is a part of the cerebral cortex involved in emotion and in the processing of unpleasant odors [9].

The cingulate cortex is also part of the cerebral cortex that is involved in the formation and processing of emotions related to pleasant and unpleasant odors [10].

5. Does Smell Affect Memory?

Olfaction requires the involvement of the hippocampus, the brain part that controls memory. The amygdala processing of the olfactory information involves interrogating the hippocampus about previous experiences with the olfactory information.

6. What Is It Called When a Smell Triggers a Memory?

This effect is known as the Proustian effect based on the name of the novelist Marcel Proust.

7. Why Do Smells Make Me Angry?

The smell can trigger anger if that smell was associated with a past situation that caused anger to an individual.

The amygdala processing of the olfactory information involves interrogating the hippocampus about previous experiences with the olfactory information.

The olfactory information can also trigger emotions of anger through parts of the brain such as the amygdala, the hippocampus, the orbitofrontal cortex, the insula, and the cingulate cortex.

8. What Causes Loss of Smell?

The loss of smell can be complete and is known as anosmia or diminished and is known as hyposmia. These alterations in olfaction are due to physical or mental health disorders, viral infections, inflammation, environmental exposure, or aging [2].

Disorders or conditions such as Alzheimer’s disease, Parkinson’s disease, meningiomas, facial trauma or schizophrenia, may affect the function of the olfactory system leading to a loss of smell.

Upper respiratory viral infections and neurotropic viruses can also cause a loss of smell.

Neurotoxins found in some industrial workplaces such as solvents, metals, and particulate matter can cause a loss of smell [11].

9. What Is Aromatherapy and Its Benefits?

Aromatherapy is the use of plant essential oils through skin application or olfaction for the management of chronic pain, anxiety, some cognitive disorders, depression, insomnia, and stress-related disorders [12].

However, the clinical evidence is limited, and further research and clinical trials are required.

These are some of the essential plant oils that were used in different studies:

• Bergamot
• Chamomile-Roman
• Geranium
• Jasmine
• Juniper
• Lavender
• Lemon
• Mandarin
• Marjoram
• Melissa
• Neroli
• Patchouli
• Rose
• Rosemary
• Sage
• Spearmint
• Ylang-Ylang
• Vetiver

Conclusion

Olfaction is a complex process that involves the transmission of the olfactory information (smell) by specialized sensory neurons in the nose, the olfactory nerves, the olfactory bulb, and its processing by several regions of the brain.

Meanwhile, let’s enjoy the smell of a nice dish of food and a delicate perfume.