Briefly

The role of the amygdala in emotions

The role of the amygdala in emotions

The amygdala is an almond-shaped structure in the brain. Its name comes from the Greek word that means "almond"As with most other brain structures, we actually have two tonsils. Each tonsil is located near the hippocampus, in the frontal portion of the temporal lobe.

Content

  • 1 Tonsil function
  • 2 Neurochemistry of the tonsil, stress and anxiety
  • 3 The amygdala and emotional responses
  • 4 Relationship between the amygdala and emotions
  • 5 The amygdala and emotional modulation of memory

Tonsil function

Our tonsils are essential for ability to feel certain emotions and perceive them in other people. This includes fear and the many changes it generates in our body. If a suspicious individual is following us at night and we feel that our heart is beating hard, our tonsils are most likely very active.

The amygdala is a structure of the forebrain (anterior primitive brain: anterior portion of the brain during the development phase of the embryo).

During the embryonic development of the neural tube, 3 dilations called Primary cephalic vesicles, which are the Prosencephalon, the Midbrain and the Rhombencephalon.
They are the portions of the brain when the development of the central nervous system begins. Subsequently the forebrain is divided into diencephalon (thalamus and hypothalamus), and telencephalon (cerebral hemispheres).

The tonsil, or tonsil complex, is constituted from a heterogeneous set of approximately thirteen nuclei located in the medial rostral pole of the temporal lobe. In turn, these can be grouped into three groups of nuclei widely connected to each other and with a specific pattern of projections in other brain regions: basolateral nuclei, corticomedial nuclei and central nucleus.

  • The Central coreAnatomically and functionally, it is closely related to various structures of the brain stem, with the hypothalamus and with different areas of processing visceral sensory information.
  • The corticomedial nuclei they receive interference from the olfactory bulb (both the main and the accessory) and send projections to the olfactory cortex and in the hypothalamus.
  • The basolateral nuclei (where the lateral, lateral basal, medial basal and accessory basal nuclei are included) have their main connections with the cerebral cortex, especially with areas of sensory association such as the inferior, superior and insular temporal gyrus. They are also closely related to the orbitomedial prefrontal cortex, the dorsomedial nucleus of the thalamus and the ventral striatum.

Lesions in the nucleus of the tonsil

The central nucleus of the tonsil has connections with the brainstem, with which various responses of the autonomic nervous system are regulated. Given this approach, Bruce Kapp and collaborators, from the University of Vermont, thought that the central nucleus could intervene in the autonomic responses produced by the conditioning of fear. The researchers trained rabbits in the conditioning of associating a sound with an electric shock in the legs. After learning, the appearance of the sound (without discharge) produced a change in the heart rate of the animals (change produced unconditionally by electric shock). Kapp and colleagues were able to observe that lesions of this nucleus affected the conditioning of the heart rhythm before the sound that had been associated with the discharge. Currently, it has been demonstrated in several laboratories that central core lesions affect all fear conditioning responses and not only those of the autonomic nervous system, such as the alteration of the rhythm of the heart.

The amygdala has two important projection pathways:

  1. The terminal stretch mark, characterized by being a bundle of fibers with connections to the lateral hypothalamus, the nucleus of the bed of the terminal stria and the nucleus accumbens.
  2. The tonsil-fugal-ventral pathway, considered as the diffuse set of fibers that send the information to different brainstem nuclei, in the dorsomedial nucleus of the thalamus, the rostral cingular gyrus and the orbitofrontal cortex.

So that, The amygdala connects the cortical areas that process all sensitive information with the effector systems of the hypothalamus and brain stem.

Neurochemistry of the tonsil, stress and anxiety

In the amygdala there are neurons capable of expressing CRF, the releasing factor of the ACTH hormone; and in turn this hormone is secreted by the adenohypophysis in response to stress.

Throughout the brain, the tonsil is considered the structure with the most receptors for benzodiazepines. Likewise, we can also find, in this nucleus, a large population of opioid peptide receptors (involved, for example, in hypoalgesia responses to an acute stressful situation that can cause pain).

Neurochemically speaking, we can relate the tonsil to the neurotransmitter systems that regulate cortical activation. In addition, in this nucleus we can find shallow and noradrenergic, dopaminergic, serotonergic and cholinergic pathways, which allow a wide cortical innervation.

Due to its neurochemical characterization, the amygdala is closely related to the processes of stress and anxiety.

Neural characterization of the fear conditioning to a sound

Joseph LeDoux and colleagues observed that Bilateral lesions of the basolateral complex of the tonsil or auditory thalamus prevented classical conditioning from the fear of a sound; conversely, this did not happen when the lesions were generated in the auditory cortex.

In the conditioning of fear, sensory information reaches the amygdala directly from the thalamus and indirectly through the cerebral cortex.

Fear and context

The conditioning of the fear of context is characterized by the fact that the conditioned stimulus is not a specific sensory stimulus such as a light or a sound, but rather is a set of stimuli.

Studies by Russ Phillips, Joseph LeDoux, Michael Fanselow and others saw that hippocampal lesions selectively eliminated fear responses caused by contextual stimuli, without affecting responses caused by sensory stimuli specific.

In the conditioning of fear of the context, the hippocampus generates an integrated representation of the stimuli that make up the context. This information on the relationships between the stimuli reaches the basal and basal accessory nuclei of the tonsil, which project towards the central nucleus (responsible for triggering fear responses).

The amygdala and emotional responses

Different experimental evidences have shown that lesions of the central nucleus of the tonsil affect all fear conditioning responses. Also, its stimulation produces increases in heart rate, respiratory rate, blood pressure, release of stress hormones, behavioral immobilization, hyperreflexia, among others.

The central nucleus intercedes as a mediator in the activation of the cortical arousal through its direct projections to the cortex (especially the rostral cingular gyrus and the orbitofrontal cortex) and through its indirect projections, through the Meynert basal nucleus.

The tonsil seems to be a structure involved in the mediation of both the emotional responses as of the conscious feeling of emotion.

Relationship between the amygdala and emotions

In some studies, researchers directly stimulated the tonsils of patients undergoing brain surgery and asked them to report their impressions. The subjective experience that these patients reported most frequently was of imminent danger and fear or anger. In other studies with a small number of patients whose only tonsil had been destroyed (as a result of a stroke, for example), they recognized the facial expressions of each emotion except fear.

In fact, the tonsil seems to modulate all our reactions to events that are very important for our survival. The events that warn us of an imminent danger are, therefore, very important stimuli for the amygdala, but so are the events that indicate the presence of food, sexual partners, rivals, children in distress, etc.

It has also been possible to verify the relationship of the amygdala with implicit memories of stimular keys that signal the emotions expressed facially.

In the Urbach-Wiethe disease occurs a bilateral degeneration of the tonsil, associated with an abnormal calcium deposition. These patients have a very impoverished emotional life, with a very depleted ability to emotionally modulate memories.

The tonsil lesions appear to impede the subjects' ability to learn the conditioning of the fear and the possibility of issuing social judgments from facial expressions.

Studies with humans have revealed the involvement of the amygdala in fear, social cognition and in the recognition of emotional facial expressions.

The amygdala and emotional modulation of memory

Situations with a lot of emotional charge are better remembered than situations that are neutral to us.

Currently there two positions on the role of the amygdala in the learning and memory processes:

  • There are authors, such as Larry Cahill and James L. McGaugh, who state that the amygdala has a modulating function of storing information that takes place in other structures.
  • Another position is the one advocated by authors such as Michael Fanselow and Joseph LeDoux, who hypothesize that in addition to this modulating function, the amygdala is a place where some type of memory can be stored, especially those of emotional content, since they have been found in The amygdala mechanisms of synaptic plasticity as a result of different learning of implicit memory tasks, such as the conditioning of fear.

The amygdala facilitates memory consolidation processes, both implicit and explicit or declarative, when the information has a considerable emotional charge.

How does the amygdala process information from the environment to implement the emotional response mechanisms?

The tonsil complex receives both information from the specific sensory nuclei of the thalamus and from the cerebral cortex.

A person walking through the forest and stands before a snake. The visual information of this stimulus reaches the lateral geniculate nucleus of the thalamus and from there it is quickly sent to the tonsil (almost without processing), to allow a rapid response from the organism. The snake's visual information is also sent from the thalamus to the visual cortex, where it is processed and sent to the amygdala.

The hippocampus sends information to the amygdala about the relationships between the stimuli that form the same context.

References

Bradford, H.F. (1988). Fundamentals of neurochemistry. Barcelona: Labor.

Carlson, N.R. (1999). Behavioral physiology. Barcelona: Ariel Psychology.

Carpenter, M.B. (1994). Neuroanatomy Fundamentals Buenos Aires: Panamerican Editorial.

Delgado, J.M .; Ferrús, A .; Mora, F .; Blonde, F.J. (eds) (1998). Neuroscience Manual. Madrid: Synthesis.

Guyton, A.C. (1994) Anatomy and physiology of the nervous system. Basic Neuroscience Madrid: Pan American Medical Editorial.

Kandel, E.R .; Shwartz, J.H. and Jessell, T.M. (eds) (1997) Neuroscience and Behavior. Madrid: Prentice Hall.

Martin, J.H. (1998) Neuroanatomy. Madrid: Prentice Hall.

Nolte, J. (1994) The human brain: introduction to functional anatomy. Madrid: Mosby-Doyma.

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