• 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.

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However, pain is not only physical because a sensory experience can also be due to mental health conditions such as depression, anxiety, or anger.

Therefore, a difference exists between pain and the cause of the pain. The link between the two is mediated by our nervous system through a process known as nociception.

It was estimated that 20.4% of adult individuals had chronic pain and that 7.4% of adult individuals had chronic pain which limited their activities, such as working, in the past 3 months [2].

1- What is emotional pain?

Emotional pain can be defined by the heartache we feel when losing someone or something that was important to us.

2- Is pain an emotion or feeling?

A Feeling is the outcome or expression of our interpretation of an emotion that was caused by pain (physical or psychological).

3- What is the worst mental pain?

They are several mental conditions that are characterized by unbearable psychological and emotional pain that is characterized by suicidal thoughts (suicidal ideation).

These include Emotionally unstable personality disorder (borderline personality disorder) and seasonal affective disorder.

4- What are Nociceptors?

Nociceptors are sensory neurons that respond to damage or potential damage through alerting communications to the brain and spine [3].

Nociceptors can be mechanical, thermal, mechano-thermal, polymodal, or silent.

Mechanical nociceptors respond to mechanical damage or pressure, while thermal nociceptors respond to cold or hot temperatures.

Mechano-thermal nociceptors respond to both mechanical and thermal stimuli, while silent nociceptors respond to inflammatory mediators.

5- What Are the Types of Pain?

• Acute Pain

Acute pain is caused by spasms of skeletal muscles and the activation of the sympathetic nervous system that is due to an injury or a trauma. The duration of the pain is short and disappears with the healing of the injury.

• Chronic Pain

Chronic pain is also caused by an injury or a disease, but the pain would continue even after the healing period.

• Nociceptive Pain

Nociceptive pain is caused by an injury, trauma, chemicals, or surgery that results in the activation and release of inflammatory mediators by nociceptor neurons (sensory neurons) which activate inflammation in the injured area [4].

• Neuropathic Pain

Neuropathic pain is due to direct damages or compression of the sensory nerves associated with sciatica, neuropathies such as phantom limb pain, Wallerian degeneration, segmental demyelination, or diabetic neuropathies.

5- Are There Other Classifications of Pain?

• Allodynia

Allodynia is a pain that is caused by a stimulus that is not the one that initiated the pain, such as feeling pain after a gentle shake of hands or light touch on the back.

• Analgesia

Analgesia is characterized by the inability to feel the pain caused by a stimulus (cause) that should normally have caused pain.

• Hyperpathia

Hyperpathia is a neuropathic pain characterized by an abnormal, repetitive, and elevated pain reaction to the stimulus (cause).

• Causalgia

Causalgia is neuropathic pain that combines a sensation of burning pain, hyperpathia, and allodynia.

• Dysesthesia

Dysesthesia is an unpleasant and abnormal sensation such as cold sensation, tingling, burning, numbness, tickling, or pain that is caused by nerve trauma or irritation.

• Hyperalgesia

Hyperalgesia is characterized by an abnormally increased sensitivity to pain due to the release of inflammatory hormone-like substances, known as prostaglandins, that increase the sensitivity of the nociceptors.

Hyperalgesia can be caused by fibromyalgia, diabetes, infection, trauma, postherpetic neuralgia, and complex regional pain syndrome.

• Hyperesthesia

Hyperesthesia is characterized by an increased sensitivity to stimuli associated with senses such as hearing, tasting, touch, smelling,

Hyperesthesia is caused by excessive stimulation of the nervous system such as overconsumption of caffeine.

• Hypoalgesia

Unlike analgesia characterized by the inability to feel pain, hypoalgesia is associated with a reduced response to a painful stimulus.

6- What Are the Symptoms of Pain?

The symptoms of pain are categorized corresponding to the pain intensity, duration, and pain-related disability [5]. These categories allow the classification of pain into mild, moderate, or severe using assessment methods:

• Visual Analogue Scale

This method consists in drawing a line between two points. The first point is the “no pain” and the second point is the “as bad as it could be”.

The individual in pain is requested to mark the level of pain within the two endpoints.

• Graphic Rating Scale (GRS)

This method represents a drawing of a line with levels from 1 to ten representing the intensity of pain. No pain is represented by the number 0 and “as bad as it could be” is represented by the number 10.

The individual in pain is requested to indicate the number from 0 to 10 which represents the level of pain experienced by the individual.

• Verbal Rating Scale

This method evaluates the pain experienced by the patient using adjectives to describe the level of pain.

• Pain Drawing

In this method, the patient is requested to indicate the location of the pain on a drawn human body.

• McGill Pain Questionnaire (MPQ)

This questionnaire assesses the pain-rating index, the number of words describing the pain, and the level of pain on a scale of 1-5.

8- What Is the Treatment of Pain?

Pain is mainly managed.

• Non-pharmacological Management

Non-pharmacological management involves:

– Exercise such as swimming, walking, yoga, Pilates, and exercise bike.

– Physical therapy through physiotherapists and occupational therapists who will help with stretching and pain-relieving exercises.

– Psychological management through social support, psychologists, and psychotherapists.

• Pharmacological Management

For mild and moderate pain, non-opioid analgesics such as nonsteroidal anti-inflammatory drugs (NSAIDs) and acetaminophen are used.

Although there is a high risk of dependency, opioid analgesics (e.g., morphine) are used for the treatment of severe pain.

Conclusion

Although the risk factors in the progression from acute to chronic pain are well known, identifying patients who are predisposed to this progression through assessing the status of the acute pain, the physiological and reactivity of the patients to pain, should help better manage this switch from acute to chronic pain.

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