Amygdala Meaning- In complex vertebrates, including humans, the amygdala is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the cerebrum. The amygdalae are considered part of the limbic system because they have been shown to play a primary role in the processing of memory, decision-making, and emotional responses (including fear, anxiety, and aggression).
Karl Friedrich Burdach coined the phrase "amygdala" in 1822.
Where is the Amygdala Located?
Amygdala Location- The amygdala is one of two almond-shaped nuclei groups located deep within the temporal lobes, medial to the hypothalamus and adjacent to the hippocampus and inferior (temporal) horn of the lateral ventricle.
In general, the amygdala's major functions are strongly related to unpleasant, aversive stimuli and situations, as well as the subsequent reactions to them. For evolutionary reasons, these kinds of stimuli are extremely important, and the amygdala is one of the most important brain areas for those “primitive” behaviours.
The majority of the roles played by the amygdala are accomplished through its primary function: hypothalamic modulation via various neuronal pathways.
In humans and other animals, the regions referred to as the Amygdaloid nucleus encompass several structures of the cerebrum with distinct connectional and functional characteristics.
The basolateral complex, cortical nucleus, medial nucleus, central nucleus, and intercalated cell clusters are among these nuclei.
The basolateral complex is divided further into the lateral, basal, and accessory basal nuclei.
The amygdala, and particularly its central and medial nuclei, have been classified anatomically as part of the basal ganglia.
Basolateral (ventrolateral) Group-
The lateral, basal, and accessory basal nuclei are all part of this group. It is larger and newer in terms of phylogeny. It is functionally associated with the limbic system. It has reciprocal connections to all four sensory association areas of the cortex (the auditory, visual and somatosensory cortex).
It Projects to Three Distinct Locations:
The thalamic medial dorsal nucleus
The basal nucleus is a type of nucleus found in the brain (of Meynart)
The striatum ventral
This Nucleus Influences a Variety of Thalamic Functions, Including:
Corticomedial (dorsomedial) Group-
The cortical nuclei and the nucleus of the lateral olfactory tract are included in this group. It is smaller, phylogenetically older, and functionally belongs to the olfactory system than the basolateral group. It is the site where the olfactory fibres that emerge from the olfactory bulb terminate. This nucleus projects to the hypothalamic ventromedial nucleus and is involved in hunger and eating behaviours.
Centromedial Group -
The medial and central nuclei make up this group. It is actually a location where both the basolateral and corticomedial nuclei project. This nucleus is linked to the brainstem's visceral sensory and autonomic nuclei, which are involved in respiratory and cardiovascular functions.
Aside from these nuclei groups, there is a separate set of nuclei that cannot be easily classified into any of these three groups and are thus listed separately. Intercalated cell masses and the amygdalohippocampal area are examples of these.
The amygdaloid body is a collection of nuclei that connect to the cerebral cortex and are located in the dorsomedial pole of the temporal lobe, anterior to the inferior horn of the lateral ventricle. It is part of the olfactory tract, part of the autonomic nervous system, and part of the emotional system.
Parts of the Brain - Amygdala Pathway
Each amygdaloid nucleus receives input and sends output to multiple but distinct brain regions. These inputs and outputs are referred to as pathways, and they can be afferent or efferent depending on whether they are received or sent.
Afferent Projections -
The dorsal and medial thalamus, as well as many cortical areas such as the insula, prefrontal cortex, parahippocampal cortex, and temporal cortex, send impulses to the basolateral group of nuclei. It also receives a large number of neural fibres from the brainstem.
In any case, it is clear that all of these fibres transmit a broad range of somatosensory, visual, and optical information to the amygdala. Because it receives a large number of afferent fibres, the basolateral group of nuclei is also known as the amygdaloid zone of sensory convergence.
All of the impulses received by the basolateral group are projected to the centromedial group, where they are supposed to be arranged and then sent to various motor centres to stimulate a response to the received information. As a result, the central group is also known as the amygdaloid zone of motor divergence.
These are the Structures That Send Fibres to the Amygdala in Particular:
Efferent Projections -
The primary function of the amygdala, as previously stated, is to modulate the activity of the hypothalamus. It accomplishes this through two major pathways: the stria terminalis and the ventral amygdalofugal pathway.
The Stria Terminalis: This pathway connects the amygdala's centromedial nucleus to the hypothalamus's ventromedial nucleus. The pathway also transports fibres to the brain's septal nuclei and thalamic regions.
In addition to efferent fibres, the stria terminalis transports fibres from these three areas back to the amygdala. Stria terminalis is important in the stress responses of many different types of organisms.
Amygdalofugal Pathway: This is the amygdala's primary efferent pathway. It originates in the amygdaloid complex's centromedial and basolateral nuclei and transports fibres to various locations throughout the nervous system.
The basolateral group's axons travel medially through the innominate substance, terminating in the hypothalamus and septal nuclei. It is known that innominate substance sends cholinergic neurons to the cortex of the brain, which is important for social behaviour modulation.
Early humans faced constant danger, such as being killed by wild animals or other tribes. To improve survival chances, the startle circuit (fight-or-flight response) evolved, which is an automatic response to physical danger that allows you to react quickly without thinking.
Emotions such as fear, anxiety, aggression, and anger cause this response.
The amygdalae are most active in times of acute fear. The amygdalae are in charge of preparing the body for escape or defence whenever our senses detect a change in our surroundings that could be dangerous. This is part of the brain's startle circuit, which controls our response to being startled. The amygdala aids in the storage of memories of events and emotions so that an individual can recognise similar events in the future.
The size of the amygdala is linked to increased aggression and physical behaviour.
In humans, the amygdala also plays a role in sex drive. Its size and shape can change depending on the individual's age, hormonal activity, and gender. Males with low testosterone, or who have been castrated, for example, have smaller amygdalae and, as a result, a lower sex drive.
Destroying an animal's amygdala has disastrous consequences for the animal's natural alarm system. However, this does not make the amygdala the "fear centre," because the amygdala has connections coming in and going out to several other parts of the brain, all of which are required to manifest fear.