• The limbic Lobe
• Hippocampal Formation
• The Amygdala
• The Septal Area
• The Hypothalamus

1. What Is the Function of the Limbic Lobe?

The limbic lobe is a vital region of the brain involved in emotion, learning, memory, and motivation [2]. It is a C-shaped brain region that includes:

• Cingulate gyrus
• Parahippocampal gyrus
• Subcallosal Gyrus

1.1. What Is Cingulate Gyrus and Its Function?

The cingulate gyrus is a brain structure located toward the middle (medial aspect) of the cerebral hemisphere and is involved in the formation and processing of emotions, memory, and learning.

1.2. What Is Parahippocampal Gyrus and Its Function?

The parahippocampal gyrus is a brain structure located toward the middle of the temporal lobe and is involved in-memory processing, storage, and recall.

1.3. What Is Subcallosal Gyrus and Its Function?

The subcallosal gyrus is a thin layer (lamina) located towards the middle of the hemisphere involved in the regulation of stress responses by suppressing the amygdala’s responsiveness to fear.

2. What Is Hippocampal Formation?

The hippocampal formation is located towards the middle of the temporal lobe of the brain and is made of 4 structures [3]:

• The Dentate Gyrus
• The Hippocampus Proper
• The Subiculum Complex
• The Entorhinal Cortex

2.1. What Is Dentate Gyrus?

The dentate gyrus is a structure located toward the middle of the temporal lobe that receives fibers from the entorhinal cortex and may be involved in episodic memory (Type of long-term memory).

2.2. What is Hippocampus?

The hippocampus proper is the structure of the brain where episodic memories are formed and stored.

In addition to its role in long-term memory, the hippocampus is also involved in learning, flexible and goal-oriented behavior, navigation, and spatial orientation.

2.3. What Is Subiculum Complex and Its Function?

The subiculum complex is a structure located toward the middle of the temporal lobe involved in working memory and drug addiction.

2.4. What Is Entorhinal Cortex and Its Function?

The entorhinal cortex is a structure located toward the middle of the temporal lobe which connects the hippocampus and the neocortex and is involved in navigation, perception of time, and memory formation, consolidation, and optimization during sleep.

3. What Is the Amygdala and What It Controls?

The Amygdala is an almond-shaped structure located toward the middle of the temporal lobe involved in emotional response (fear, anxiety, and anger), decision making, and memory (emotional and fear associated memories) [4].

4. What Is Septal Area and Its function?

The septal area is a structure located in the inferior and posterior part of the middle of the frontal lobe involved in emotional, motivational, and spatial behavior through its nuclei connections with the hypothalamus, amygdala, olfactory bulb, cingulate gyrus, thalamus, habenula, and midbrain.

5. What Is the Hypothalamus and What It Controls?

The hypothalamus is a structure located in the diencephalon below the thalamus and is involved in functional linking between the nervous system and the endocrine system [5].

It is divided into three regions, the anterior region (supraoptic), the middle region (tuberal), and the posterior region (mammillary), and each region is subdivided into areas containing specialized nuclei.

The anterior region has 3 areas known as preoptic, medial, and lateral, involved in thermoregulation, circadian rhythms (suprachiasmatic nucleus), and endocrine secretion of hypothalamic hormones (e.g., corticotropin-releasing hormone).

The meddle region (tuberal) has 2 areas, medial and lateral, involved in the control of blood pressure, heart rate, gastrointestinal stimulation, hunger, secretion of the growth hormone-releasing hormone (GHRH), and neuroendocrine control.

The posterior region has also 2 areas, medial and lateral, involved in memory, learning, arousal, sleep, shivering, pupillary dilatation, and energy balance.

6. What Happens If the Limbic System Is Damaged?

Damage in the structures of the limbic system can result in the following disorders [1]:

• Epilepsy
• Dementia
• Limbic Encephalitis
• Schizophrenia
• Anxiety Disorders
• Affective Disorders
• ADHD
• Korsakoff’s Psychosis
• Klüver-Bucy Syndrome
• Autism

6.1. Epilepsy

Hippocampal sclerosis (tissue stiffening) can result in temporal lobe epilepsy which can be further complicated when other structures, including the amygdala and the parahippocampal gyrus, are also involved (Mesial Temporal Sclerosis).

6.2. Dementia

Alzheimer’s’ disease is associated with senile plaques and neurofibrillary tangles affecting the hippocampus and the dentate gyrus.

6.3. Limbic Encephalitis

Limbic encephalitis is an inflammation that affects parts of the limbic structures due to an autoimmune disorder (non-paraneoplastic limbic encephalitis) or a tumor (Paraneoplastic limbic encephalitis).

6.4. Schizophrenia

Schizophrenia is a complex and severe mental health disorder that is classified as a type of psychosis. It is characterized by significant alterations in thoughts, perception, mood, and behaviors.

The causes are not well known; however, it was suggested that the causes may be associated with a cognitive decline due to alterations in brain development, particularly of the cerebral cortex.

The reduced volumes of the limbic system structures including the hippocampus and the entorhinal cortex in schizophrenia patients may be one of the causes of this disorder.

6.5. Anxiety Disorders

Failure of the hippocampus and the cingulate gyrus in modulating the activity of the amygdala can result in anxiety disorders.

6.6. Affective Disorders

Variation in the volumes of the frontal lobes, amygdala, hippocampus, and basal ganglia, have been associated with affective disorders. 

6.7. ADHD

Loss of connections between the orbitofrontal cortex and the amygdala may explain the disinhibition behavior observed in ADHD individuals.

6.8. Korsakoff’s Psychosis

Korsakoff’s psychosis is caused by damage to mammillary bodies, the thalamus (e.g., tumors, ischemia, or stroke), and hypothalamus resulting in amnesia, the deficit in explicit memory, and confabulation.

6.9. Klüver-Bucy Syndrome

This disease is associated with symptoms including hypersexuality, hyperphagia, placidity, visual agnosia, and hyperorality. It is due to bilateral lesions affecting the amygdala and the inferior prefrontal cortex.

6.10. Autism

Alterations in limbic structures, including the amygdala and the cingulate gyrus, have been associated with autism and Asperger’s syndrome.

Conclusion

Although its involvement in the control of emotions, long-term memory, behavior, and olfaction is known, the complexity of the limbic system makes its clinical study difficult.

However, advances in behavioral studies, functional magnetic resonance imaging (MRI), deep brain stimulation, and perfusion studies, have allowed a better understanding of this system, and future functional imaging techniques will certainly uncover more hidden functions of this fascinating part of the brain.

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• 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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