The limbic system is a complex set of structures that are located on both sides of the thalamus, just below the brain.It includes the hypothalamus, hippocampus, tonsil and other nearby areas.
It seems to be primarily responsible for our emotional life. It is also considered of vital importance in the capacity for motivation, the formation and integration of memory, smell and the mechanisms to keep us safe. It is an important information transfer center through the thalamus, which feeds the limbic system with sensory information.
In the nineteenth century it was defined by Paul Broca integrating the structures between the cerebral hemisphere and the cerebral stem (that is, thelimbo, or edge of the brain). The term was used for approximately 70 years and suggests a functionally unified system. In 1937 the neuroanatomist J. Papez proposed that it was a circuit that linked the hippocampal formation, the mammillary nuclei and the cingulate cortex, through the anterior nucleus of the thalamus, and was called Papez circuit. Later, in 1949 P. MacLean extended Papez's circuit and coined the term limbic system.
- 1 Hypothalamus
- 2 Hippocampus
- 3 Tonsil
- 4 The amygdala and emotional processes
The hypothalamus is a small part of the brain located just below the thalamus on both sides of the third ventricle. It is located just inside the two sections of the optic nerve, and just above (and is intimately connected) with the pituitary gland.
The hypothalamus mainly deals with homeostasis. He is responsible for regulating appetite, thirst, response to pain, levels of pleasure, sexual satisfaction, anger and aggressive behavior, among others. It also regulates the functioning of the autonomic nervous system, which in turn regulates things such as pulse, blood pressure, breathing and arousal in response to emotional circumstances.
Receive information from various sources. Information about blood pressure and bowel distension comes from the vagus nerve (the stomach is full). From the reticular formation in the brain stem, it receives information about the skin temperature. From the optic nerve, information about light and darkness arrives. From unusual neurons that line the ventricles, you receive information about the content of the cerebrospinal fluid, including toxins that lead to vomiting. And from the other parts of the limbic system and the olfactory nerves, you receive information that helps you regulate food and sexuality. The hypothalamus also has some receptors of its own, which provide information on ionic balance and blood temperature.
The hypothalamus sends instructions to the rest of the body in two ways. The first is towards the autonomic nervous system. This allows the hypothalamus to have final control of things like blood pressure, heart rate, breathing, digestion, sweating and all sympathetic and parasympathetic functions.
The other way that the hypothalamus exercises control is through thepituitary gland. It connects neurologically and chemically to the pituitary gland, which in turn pumps hormones called releasing factors into the bloodstream. The pituitary is the so-called "master gland," and these hormones are vitally important for regulating growth and metabolism.
The hippocampal formation It is a curved and recurrent cortex lamina, located on the medial surface of the temporal lobe.
The cross sections show that the hippocampal formation is formed by three distinct zones: the dentate gyrus, the hippocampus and the subiculum. In these types of sections, the dentate gyrus (or dentate gyrus) and the hippocampus have the shape of two interlaced Cs. The subiculum is a transition zone that continues with the hippocampus at one end and the hippocampal cortex at the other.
The three components are organized as bands that go from the front to the back inside the temporal lobe and that together form a cylinder.
As he hippocampus as the toothed gyrus They have three layers of cells, which are:
- Molecular layer. The most superficial.
- Polymorphic layer. The deepest
- Intermediate layer. In the hippocampus they are pyramidal cells, while in the dentate gyrus they are granular cells.
The subicle it is the transition zone between the hippocampus and the parahypocampic gyrus (neocortex) and, consequently, progressively goes from having three layers to having six.
The most important activity related to the formation of hippocampus is learning and memory consolidation: allows short-term memory to consolidate and become long-term memory. If the hippocampus is damaged, a person cannot build new memories and instead lives in a strange world where everything he experiences simply vanishes, even when the oldest memories of the time before the damage remain intact. This unfortunate situation is portrayed quite accurately in the wonderful filmMemento.
Despite not being able to describe the facts or the situations experienced, this amnesia antegrade (from post-injury events, from the forward injury) only affects the specific facts and events and not the learning of new abilities or skills.
Hippocampal formation connections
The hippocampal formation receives its main entrance from a part of the limbic association cortex called entorhinal cortex, through axons called perforating path. This entorhinal region, adjacent to the hippocampal formation, collects information from other parts of the cortex. Thus, the hippocampal formation has access to almost all types of sensory information.
You also get information about the septal area and the hypothalamus, through a route called a fornix.
The main efferent pathway is the fornix, which projects mainly in the septal area, the anterior nucleus of the thalamus, the mammillary bodies of the hypothalamus and the nuclei of the trunk as the reticular formation.
Some fibers pass directly (not through the fornix) in the entorhinal cortex, in the tonsil and the cingulate cortex.
The tonsils or tonsil complex are two masses of almond-shaped neurons on each side of the thalamus, at the lower end of the hippocampus. When electrically stimulated in animals, they respond with aggression. And if the amygdala is removed, the animals become very meek and no longer respond to the stimuli that previously caused them anger. But there is something other than anger: when they are eliminated, the animals also become indifferent to the stimuli that would otherwise have caused fear and even sexual responses.
The numerous nuclei of the amygdala can be divided into three groups: basolateral, central and corticomedial. The basolateral and central are part of the limbic system and seem to give emotional importance to stimuli, while corticomedials are more related to olfactory functions.
It receives a large amount of sensory (visual, auditory, somatosensory) and visceral information in a very processed way. This information comes from various areas such as the hypothalamus, the septal area, the thalamus, the cortex, olfactory regions, the brain stem, etc.
Most of them leave through the pathway called terminal stria and end mainly in the septal area and the anterior part of the hypothalamus. There are more projections, secondary, in areas such as the trunk, the thalamus, the cortex, the hippocampus, etc.
In summary, the tonsil receives all kinds of sensory and visceral information, and sends projections, through the terminal stria, in the septal area and the hypothalamus.
The amygdala and emotional processes
The amygdala is a key element for our emotional experiences, since this depends on the stimuli to which we respond, the way in which the responses we manifest to these stimuli are organized, as well as the internal responses of our organs.
It allows an aversive stimulus (for example, the vision of a snake) to be interpreted as a threat (danger), to feel fear and act accordingly, fleeing or facing it.
The electrical stimulation of the amygdala, depending on the specific place where it is applied, evokes visceral reactions, defense, fear, aggressiveness, etc.
After tonsil lesions, stop producing an appropriate emotional response to the sensory experience present. For example, objects that were threatening cease to cause fear, aggression is lost, or edible objects are not distinguished from inedible ones.
The amygdala gives emotional meaning to the experience and causes the following actions to occur:
- The subjective emotional response.
- Endocrine, autonomic and behavioral responses appropriate to the situation.
In relation to the general function of the tonsil, we find that it is more specifically related to the following facts:
- Learning and memory processes with emotional component (for example, aversive learning)
- Control of motivated behaviors (hunger, thirst, sexual behavior) and control of visceral responses through their influence on the hypothalamus
- Body's response to stress
- Social and affective behavior (together with other structures such as the frontal lobes of the cortex and the limbic association cortex)
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Nolte, J. (1994) The human brain: introduction to functional anatomy. Madrid: Mosby-Doyma.Related tests
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