Limbic System: Structure, Function and Embryology

The limbic system is a group of brain areas located lateral to the thalamus, below the cerebral cortex, and above the brainstem. Paul Broca was the first to refer to this part of the brain as the grand lobe limbique, in 1878. Later, in 1949, the American physician and neuroscientist Paul D. MacLean referred to it as the limbic lobe, a more current term for the several elements that comprise this area. 

This region was ultimately shown to be associated with emotional, memory, and motivational processes that communicate with other parts of the brain. Some disorders arise clinically when components of the limbic system are compromised. 

While a complete understanding of the limbic system is still elusive, advances in neuroscience have offered a better understanding of the roles that the various components of the limbic system play, as well as some insight into their numerous connections.

Structure And Function Of Limbic System

Neuroscience advancements have repeatedly reinterpreted the structures that form the limbic system. However, the limbic system's components are distributed in a large region lateral to the thalamus, beneath the cerebral cortex but above the brainstem. 

The limbic system's current major structures can be identified by their embryological origins. The mesencephalic components consist of visual, auditory, and somatosensory inputs processed in the region. 

Diencephalic components include the hypothalamus, anterior thalamic nuclei, and the habenular commissure. Telencephalic components include cortical and subcortical structures such as the olfactory bulbs, hippocampus, parahippocampal gyrus, fornix, fornix columns, mammillary body, septum pellucidum, amygdala, cingulate gyrus, and entorhinal cortex.

While the limbic system was formerly assumed to be the sole neurological system involved in emotion regulation, it is now understood to be just one part of the brain responsible for visceral, autonomic activities. 

In general, the limbic system is involved in a wide range of cognitive functions, including spatial memory, learning, and emotional, and social processing.

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Embryology Of Limbic System

The limbic system is made up of various components that originate in the mesencephalon, diencephalon, and telencephalon, as previously indicated.

Clinical Significance Of Limbic System

The hypothalamus has several roles in maintaining homeostasis. However, its relevance in the limbic system has received little study. Researchers have uncovered connections between the hypothalamus, nucleus accumbens, ventral tegmental area, hippocampus, and amygdala. Food seeking, escape, and predator fear all need a neural connection between these components. 

This link, known as the "limbic-motor interface," provides a model for the initiation of actions by limbic forebrain regions, describing how the "emotive brain" and "cognitive brain" collaborate to initiate a response.

The olfactory bulbs contribute to the sense of smell. They route olfactory information to the amygdala, orbitofrontal cortex, and hippocampus for processing.The amygdala then evaluates the information and uses it for associative learning. For example, encoding odor cues associated with excellent or unpleasant taste.

The hippocampus is an all-cortical area that serves to integrate information, such as short-term, long-term, and spatial memories. People who have had extensive bilateral hippocampal lesions are more likely to develop anterograde amnesia, as demonstrated in the infamous case of "H.M." It has been demonstrated that individuals with schizophrenia have smaller hippocampus. The parahippocampal gyrus is a cortical region that surrounds the hippocampus and aids in scene recognition, memory encoding, and retrieval. The parahippocampal gyrus, like the hippocampus, is asymmetrical in persons with schizophrenia.

The fornix is the hippocampus' major outflow tract. Its particular function is uncertain, however lesions along the fornix have been shown to impair recall memory.

The mammillary bodies are where the fornix columns come to an end. The mammillary body has limbic connections to the amygdala, hippocampus, and anterior thalamic nuclei. 

The mamillary bodies play an important role in episodic memory. Thiamine deficiency has been shown to produce mammillary body harm, most notably through Wernicke-Korsakoff syndrome.

The amygdala, a subcortical component of the limbic system located in the medial temporal lobe, mediates emotional responses such as fear, anxiety, and rage. In addition, the amygdala is responsible for memory and decision-making.

Fear conditioning processing takes place in the lateral nuclei of the amygdala, where memories form associations with unpleasant stimuli through long-term potentiation. Damage to the amygdala impairs fear conditioning.

The cingulate gyrus is a brain area found just above the corpus callosum. Its primary afferents originate from the thalamus and the neocortex. The cingulate gyrus, like the rest of the limbic system, is involved in emotion generation, learning, and memory. It is in charge of linking behavior with motivating outcomes. 

According to one study, people with mood disorders and schizophrenia had size changes in the cingulate gyri (especially the anterior cingulate cortex).

The entorhinal cortex, located in the medial temporal lobe, is the major connector between the hippocampus and the neocortex. The EC-hippocampus system is an important part of the limbic system, responsible for declarative and spatial memories, memory formation, and consolidation. Magnetic resonance imaging demonstrated a loss in volume in the entorhinal cortex in Alzheimer's disease patients.

With developments in our understanding of the complexities of higher cognitive processes, some contend that the name limbic system is no longer relevant, serving instead as a historical basis for our contemporary understanding of neuroscience. 

Others advocated a modified limbic system model that included three distinct networks. 

The first is the hippocampal-diencephalic and parahippocampal-retrosplenial networks, which regulate memory and spatial orientation. The second is the temporo-amygdala-orbitofrontal network, which connects emotion to cognition. Finally, there's the default-mode network, which is focused on autobiographical memory and introspection.

In brief, the limbic system is made up of various components, each of which performs a unique role in the limbic system's overall function. Emotion, memory, and social processing are all essential components of human health. 

Clinical disorders characterized by bilateral lesions of particular limbic system locations are infrequent. However, in far more common disorders, such as schizophrenia, asymmetry and cortical volume loss of limbic system components are usual.

Histology

The limbic cortex, considered the oldest part of the cerebral cortex in evolutionary terms, is generally composed of allocortex, a primitive form of cortical tissue. This cortex usually well-knownshows 3 layers, surrounding the hilum of the hemisphere. Additionally, the juxtacortex, a transitional cortex among the allocortex and neocortex, features 3 to 6 layers and is located in regions much like the insula and cingulate gyrus. In evaluation, the neocortex, located in the non-limbic a part of the hemisphere, is the most specifically developed cortical tissue, providing six layers.

History

 The term "limbic" end up coined by means of Paul Pierre Broca in 1878, derived from the Latin word which means "for the border." This time period turned into applied to explain a curved rim of the cortex, which incorporates systems like the parahippocampal gyrus and cingulate gyrus, located at the junction of the diencephalon and cerebral hemisphere. James Papez later proposed a neural circuit of emotion, linking systems much like the cingulate gyrus, parahippocampal gyrus, hippocampus, mammillary body, and anterior thalamus.

Neural Circuit Of Emotion

Papez's proposed neural circuit of emotion includes interconnected areas just like the cingulate gyrus, parahippocampal gyrus, hippocampus, mammillary frame, and anterior thalamus, offering a framework for expertise emotional behavior.

Components Of Limbic System 

The limbic machine comprises severa additives, inclusive of the limbic cortex (cingulate gyrus, parahippocampal gyrus), hippocampal formation (dentate gyrus, hippocampus, subicular complex, amygdala, septal area), hypothalamus, and associated thalamic and cortical regions.

Limbic Cortex

 Situated on the anteromedial problem of the cerebral hemisphere, the limbic cortex consists of  concentric gyri surrounding the corpus callosum. The outer gyrus, proposed through Broca, consists of structures just like the isthmus of the cingulate gyrus, parahippocampal gyrus, and subcallosal area. The inner gyrus is called the infralimbic gyrus. The cingulate gyrus, a prominent shape, is "C" shaped and located dorsal to the corpus callosum.

Subdivisions Of Cingulate Gyrus

The cingulate gyrus can be subdivided into areas just like the posterior cingulate cortex, midcingulate cortex, retrosplenial cingulate cortex, pregenual anterior cingulate cortex, and subgenual anterior cingulate cortex.

Habenula

 Technically part of the epithalamus, the habenula connects with the limbic device, particularly concerning projections from the lateral habenula to serotonin neurons of the raphe nuclei and dopamine cells of the ventral tegmental location, doubtlessly impacting mood law.

Parahippocampal Gyrus

Located inside the medial temporal lobe, the parahippocampal gyrus incorporates distinct cytoarchitectural areas, with the entorhinal cortex being a essential region. It serves as a primary output pathway from the hippocampal formation, facilitating the transfer of notably processed cortical information.

Hippocampal Formation

Embryologically an extension of the medial fringe of the temporal lobe, the hippocampal formation accommodates zones with awesome cytoarchitecture, together with the dentate gyrus and hippocampus right (with CA1, CA2, CA3, and CA4 areas). It plays a crucial function in memory formation and spatial navigation.

Amygdala

A collection of nuclei involved in emotional processing, the amygdala includes systems similar to the basolateral complex, centromedial amygdaloid complex, olfactory group, and prolonged amygdala. It has bidirectional connections with various mind regions, impacting emotional responses and behaviors.

Septal Area

Situated above the anterior commissure, the septal place has reciprocal connections with systems like the hippocampus, amygdala, and hypothalamus, influencing capabilities related to emotion and praise processing.

Hypothalamus

Located within the diencephalon, the hypothalamus serves as a critical hub for regulating severa physiological methods, inclusive of feelings, autonomic functions, and endocrine responses. It accommodates terrific nuclei liable for functions like circadian rhythm law, hormone secretion, and behavioral responses.

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