BRAIN TONSIL: ANATOMY, PARTS AND FUNCTIONS (PICTURES) - NEUROPSYCHOLOGY - 2024

The brain amygdala is a structure that receives that name because of its resemblance to that of an almond (almond in Greek is amýgdalo). It is also known as the tonsil complex or tonsillar body, and was discovered in the 19th century by the German physiologist Karl Burdach. It is a structure present in both complex vertebrate animals and humans.

The brain amygdala consists of two groups of neuronal nuclei located deep within our brain, specifically inside the temporal lobes. It is made up of groups of different neurons that are organized into nuclei, each with different roles.

Brain tonsil (small blue dot)

Despite its size, the amygdala has a complex function and participates in a wide variety of functions, although it stands out for its role in emotional processing, mainly fear. However, it also participates in memory and decision-making.

The amygdala is part of the limbic system, a set of interconnected brain structures that fulfill several basic functions related to instincts and the survival of the species such as hunger, thirst, sex, memory and the most primary emotions.

Importance

This structure is important since it exchanges numerous connections with many parts of the brain such as the thalamus, hypothalamus, hippocampus, cingulate gyrus, etc. This is because it is at a key site, mediating between more complex and higher (cortical) processing, the activity of the limbic system, and simpler functions associated with connections to the brainstem.

Researchers Heinrich Klüver and Paul Bucy found that removing the entire amygdala and temporal cortices in monkeys resulted in affective blunting, loss of fear, domestication, indiscriminate eating, hypersexuality, and hyperorality. The latter consists of the excessive tendency to explore objects with the mouth, even those that can cause damage such as a knife.

Another similar condition is that of Urbach-Wiethe. It consists of a degenerative disorder caused by a deposit of calcium located in the amygdala. It produces the curious deficit of being unable to recognize the facial emotions of others, in addition to other symptoms.

Parts of the brain amygdala: nuclei

Amygdala (yellow dot)

The amygdala is made up of several grouped and interconnected nuclei that we will list below:

Lateral core

It is the part of the amygdala that receives information from our senses: sight, smell, touch, hearing and taste; as well as pain. Other tonsillar areas also handle this type of information, but the lateral nucleus is the main area, since the information from all our senses converges and is integrated there.

On the other hand, this area has been shown to link the neutral stimulus (which has no meaning for us) with the noxious or harmful stimulus.

The most studied is the association established between a sound that is not relevant in principle (neutral) and an unpleasant stimulus, such as an electric shock. Thanks to the work of the lateral nucleus, when we hear that sound on another occasion, we will learn that an electric shock is going to arrive and we will try to flee from it.

In addition, it is interesting that there are two routes from which dangerous information comes: a very fast and imprecise one that allows us to react quickly to possible damage (which comes from the thalamus) and a slower, conscious and precise one (which comes from our cortex sensory).

That is why sometimes we get scared and give an uncontrolled start when we confuse a stimulus that is not dangerous (a rope) with a dangerous one (a snake), since we react before reaching a conscious and correct thought, guaranteeing our preservation.

Basal nucleus

This area of ​​the amygdala receives information from many other areas and is responsible for collecting clues about the context in which the danger occurs. Thus, we can be afraid of going through a street where we were robbed in the past.

In addition, it sends data to areas of the striatum that control behaviors called "instrumental" or what I did to make the danger probable or appear.

An injury to the nucleus basalis (as well as the lateral nucleus) eliminates the startle responses we have already learned.

Central core

MRI coronal view of the tonsil. Source: Amber Rieder, Jenna Traynor, Geoffrey B Hall

It is the part in charge of issuing the necessary answers. It connects with regions of the brain stem, controlling the expression of fear responses such as: immobilization and endocrine and autonomic reactions.

Have you ever heard that anxiety affects our hormones? This is because emotional situations activate complex interactions of the adrenergic and glucocorticoid system. This has something to do with it, since the endocrine system - which is connected to this part of the amygdala - is what controls our hormonal levels. Specifically, the hypothalamic-pituitary-adrenal (HHA) axis.

On the other hand, it activates systems such as adrenergic (adrenaline), serotonergic (serotonin), dopaminergic (dopamine) and cholinergic (acetylcholine). These systems activate our brain and prepare us to react to danger, producing the typical feelings of nervousness: increased heart rate, increased body temperature, tremors, sweating, etc.

It has been shown that, if a lesion occurs in the central nucleus of the amygdala, the signs of fear diminish in the face of stimuli previously known to be dangerous. Also, the individual would have a hard time learning what elements are harmful and should fear.

In addition, if the adrenaline receptors in the amygdala are blocked, memories are prevented from being properly consolidated in our memory.

Intercalated cells

They are groups of GABAergic neurons, and they have an inhibitory function. That is, they control the activity of the basal and lateral nuclei, "calming them down" when they are overly excited.

Medial nucleus

It is essential in innate emotional behaviors. It receives information from the olfactory bulb and transmits this olfactory information to the hypothalamic nuclei, which are related to reproduction and defense.

Functions of the amygdala

As you will have discovered if you have read to this point, the amygdala is essential for fixing both normal and pathological fear conditioning (anxiety disorders).

Tonsillar neurons have been found to be activated by stimuli such as vocalizations from other animals and smells; so, as we can see, it has very varied functions.

We are going to indicate here the fundamental functions of this brain structure.

Perception of emotions

A stimulation of the amygdala causes intense emotion, mainly fear or aggression. As if it is damaged or removed, submission and affective flattening occurs.

Produces behaviors of reaction to fear

This is due to its connections with the hypothalamus, which activates the autonomic nervous system, and therefore increases and improves attention to danger or vigilance, immobilization, or the flight response.

The amygdala also projects into areas that control facial muscles such as the trigeminal nerve, our face adopting the facial expression typical of fear (eyes wide open, raised eyebrows, tense lips and open mouth).

Emotional memory

The amygdala appears to be part of a general system for emotional memory. This type of memory is what allows us to remember which clues in the environment are associated with a dangerous or beneficial event.

Thus, before the appearance of these keys in the future, an automatic response of fear or approach can be generated, with the aim of promoting our survival.

The activation of the amygdala in the face of stimuli that cause us fear, causes an enhancement of our memory. That is, we better remember the things that happen to us when intense emotions arise simultaneously, so the arousal or emotional activation is what facilitates the memories to be consolidated.

In fact, there is a study that shows that words related to high emotional arousal are remembered better, and neutral ones are remembered worse.

For this reason, human and non-human animals learn very quickly to move away from a potentially dangerous stimulus, which has caused them great emotional activation (and they do not get closer again!).

An injury to the amygdala would override the learned fear responses both to the dangerous environment and to a stimulus associated with danger (a sound, for example).

Recognition of emotions

Location of the amygdala in the human brain. Source: User Washington irving

Recognize emotions in the facial expressions of other people and react to them. It seems that there is a connection between the brain area that analyzes the information from a face (inferior temporal cortex), and the amygdala, to which these data arrive.

Thus, the amygdala gives emotional meaning and allows us to properly relate to others, enhancing social relationships.

Pleasure responses

The amygdala not only focuses on fear, it also links data from the environment to both appetitive and non-appetitive elements of a stimulus.

That is why we prefer to spend more time in an environment that we associate with positive events than in one related to negative events. Thus, we reduce the time we spend in dangerous environments and make our survival more likely.

Sex differences

It is proven that the amygdala presents variations depending on whether we are talking about the female or male sex. This explains why slight differences are observed between men and women in emotional memory and sexual responses.

This is not surprising, because the amygdala has receptors for sex hormones such as androgens and estrogens. A greater or lesser amount of these substances can cause long-term changes in the size of the amygdala and its neurotransmitters.

In fact, men seem to have a larger amygdala than women. Although whether or not this affects behavior, making it different between the sexes, it is not clear.

Control of the amygdala develops

The amygdala is the symbol of intense emotional activation, impulsiveness, aggressiveness. One of the ways in which we achieve a more adaptive emotional control as we grow, is by the maturation of the existing connections between the amygdala and the prefrontal cortex.

The prefrontal cortex is a more complex and reflective system that is responsible for planning and establishing strategies. This structure takes years to fully develop, reaching its peak in adulthood.

For this reason, in adolescence we are more impulsive and aggressive than in adulthood, because we have not yet developed adequate strategies to regulate our emotions, such as reevaluating the situation.

Mental disorders and substance abuse

Coronal section of the human brain. The tonsil is purple in color. Source: Henry Vandyke Carter

The amygdala's goal is to maintain our survival, making us be more aware of our environment and thus be able to react to events adaptively.

However, there are various circumstances in which the amygdala is also the protagonist of mental disorders such as anxiety disorders, panic attacks and post-traumatic stress disorder. And it is that undergoing stress continuously alters our hormonal levels and, as the amygdala is sensitive to them, its work can be altered.

Like the abuse of certain substances, it can cause alterations in the amygdala and affect its proper functioning.

The amygdala has a high number of cannabinoid receptors, therefore it is not surprising that cannabis produces some change in your system. Studies show that the consumption of this substance and the consequent changes in the amygdala produce more depressive behaviors. A decrease in amygdala reactivity has also been found in threatening situations (lower fear response).

In a study it was shown that in adolescent girls who use marijuana it was more likely that an incorrect development of the amygdala occurred, manifesting itself with symptoms of anxiety and depression. This occurs because in adolescence there appear to be a greater number of cannabinoid receptors in the amygdala.

On the other hand, it is known that prolonged use of cocaine sensitizes the amygdala so that it is more easily activated. Although it is indicated that it may be due to a low prefrontal control towards tonsil activation.

In addition, depending on the initial size of our amygdala or its peculiarities, it can make you more vulnerable to starting or maintaining addictive behaviors. Do not forget that this structure is the one that establishes the links between behaviors or events and pleasant sensations, causing us to repeat those behaviors.

References

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