Limbic system facts for kids
The limbic system is a set of structures of the brain. These structures cover both sides of the thalamus, right under the cerebrum. It is not a separate system, but a collection of structures from the cerebrum, diencephalon, and midbrain. It supports many different functions, including emotion, behaviour, motivation, long-term memory, and olfaction.
Contents
Structure
The limbic system was originally defined by Paul D. MacLean as a series of cortical structures surrounding the boundary between the cerebral hemispheres and the brainstem. The name "limbic" comes from the Latin word for the border, limbus, and these structures were known together as the limbic lobe. Further studies began to associate these areas with emotional and motivational processes and linked them to subcortical components that were then grouped into the limbic system.
Currently, it is not considered an isolated entity responsible for the neurological regulation of emotion, but rather one of the many parts of the brain that regulate visceral autonomic processes. Therefore, the set of anatomical structures considered part of the limbic system is controversial. The following structures are, or have been considered, part of the limbic system:
- Cortical areas:
- Limbic lobe
- Orbitofrontal cortex: a region in the frontal lobe involved in the process of decision-making
- Piriform cortex: part of the olfactory system
- Entorhinal cortex: related to memory and associative components
- Fornix: a white matter structure connecting the hippocampus with other brain structures, particularly the mammillary bodies and septal nuclei
- Subcortical areas:
- Septal nuclei: a set of structures that lie in front of the lamina terminalis, considered a pleasure zone
- Hippocampus and associated structures: play a central role in the consolidation of new memories
- Amygdala: located deep within the temporal lobes and related with a number of emotional processes
- Nucleus accumbens: involved in reward, pleasure, and addiction
- Diencephalic structures:
- Hypothalamus: a center for the limbic system, connected with the frontal lobes, septal nuclei, and the brain stem reticular formation via the medial forebrain bundle, with the hippocampus via the fornix, and with the thalamus via the mammillothalamic fasciculus; regulates many autonomic processes
- Mammillary bodies: part of the hypothalamus that receives signals from the hippocampus via the fornix and projects them to the thalamus
- Anterior nuclei of thalamus: receive input from the mammillary bodies and involved in memory processing
Function
The structures and interacting areas of the limbic system are involved in motivation, emotion, learning, and memory. The limbic system is where the subcortical structures meet the cerebral cortex. The limbic system operates by influencing the endocrine system and the autonomic nervous system. It is highly interconnected with the nucleus accumbens.
The limbic system also interacts with the basal ganglia. The basal ganglia are a set of subcortical structures that direct intentional movements. The basal ganglia are located near the thalamus and hypothalamus. They receive input from the cerebral cortex, which sends outputs to the motor centers in the brain stem. A part of the basal ganglia called the striatum controls posture and movement. Recent studies indicate that if there is an inadequate supply of dopamine in the striatum, this can lead to the symptoms of Parkinson's disease.
The limbic system is also tightly connected to the prefrontal cortex. Some scientists contend that this connection is related to the pleasure obtained from solving problems. To cure severe emotional disorders, this connection was sometimes surgically severed, a procedure of psychosurgery, called a prefrontal lobotomy (this is actually a misnomer). Patients having undergone this procedure often became passive and lacked all motivation.
The limbic system is often incorrectly classified as a cerebral structure, but simply interacts heavily with the cerebral cortex. These interactions are closely linked to olfaction, emotions, drives, autonomic regulation, memory, and pathologically to encephalopathy, epilepsy, psychotic symptoms, cognitive defects. The functional relevance of the limbic system has proven to serve many different functions such as affects/emotions, memory, sensory processing, time perception, attention, consciousness, instincts, autonomic/vegetative control, and actions/motor behavior. Some of the disorders associated with the limbic system and its interacting components are epilepsy and schizophrenia.
Hippocampus
The hippocampus is involved with various processes relating to cognition and is one of the most well understood and heavily involved limbic interacting structure.
Spatial memory
The first and most widely researched area concerns memory, particularly spatial memory. Spatial memory was found to have many sub-regions in the hippocampus, such as the dentate gyrus (DG) in the dorsal hippocampus, the left hippocampus, and the parahippocampal region. The dorsal hippocampus was found to be an important component for the generation of new neurons, called adult-born granules (GC), in adolescence and adulthood. These new neurons contribute to pattern separation in spatial memory, increasing the firing in cell networks, and overall causing stronger memory formations. This is thought to integrate spatial and episodic memories with the limbic system via a feedback loop that provides emotional context of a particular sensory input.
While the dorsal hippocampus is involved in spatial memory formation, the left hippocampus is a participant in the recall of these spatial memories. Eichenbaum and his team found, when studying the hippocampal lesions in rats, that the left hippocampus is "critical for effectively combining the 'what', 'when', and 'where' qualities of each experience to compose the retrieved memory". This makes the left hippocampus a key component in the retrieval of spatial memory. However, Spreng found that the left hippocampus is a general concentrated region for binding together bits and pieces of memory composed not only by the hippocampus, but also by other areas of the brain to be recalled at a later time. Eichenbaum's research in 2007 also demonstrates that the parahippocampal area of the hippocampus is another specialized region for the retrieval of memories just like the left hippocampus.
Learning
The hippocampus, over the decades, has also been found to have a huge impact in learning. Curlik and Shors examined the effects of neurogenesis in the hippocampus and its effects on learning. This researcher and his team employed many different types of mental and physical training on their subjects, and found that the hippocampus is highly responsive to these latter tasks. Thus, they discovered an upsurge of new neurons and neural circuits in the hippocampus as a result of the training, causing an overall improvement in the learning of the task. This neurogenesis contributes to the creation of adult-born granules cells (GC), cells also described by Eichenbaum in his own research on neurogenesis and its contributions to learning. The creation of these cells exhibited "enhanced excitability" in the dentate gyrus (DG) of the dorsal hippocampus, impacting the hippocampus and its contribution to the learning process.
Amygdala
Episodic-autobiographical memory (EAM) networks
Another integrative part of the limbic system, the amygdala, which is the deepest part of the limbic system, is involved in many cognitive processes and is largely considered the most primordial and vital part of the limbic system. Like the hippocampus, processes in the amygdala seem to impact memory; however, it is not spatial memory as in the hippocampus but the semantic division of episodic-autobiographical memory (EAM) networks. Markowitsch's amygdala research shows it encodes, stores, and retrieves EAM memories. To delve deeper into these types of processes by the amygdala, Markowitsch and his team provided extensive evidence through investigations that the "amygdala's main function is to charge cues so that mnemonic events of a specific emotional significance can be successfully searched within the appropriate neural nets and re-activated." These cues for emotional events created by the amygdala encompass the EAM networks previously mentioned.
Attentional and emotional processes
Besides memory, the amygdala also seems to be an important brain region involved in attentional and emotional processes. First, to define attention in cognitive terms, attention is the ability to focus on some stimuli while ignoring others. Thus, the amygdala seems to be an important structure in this ability.
Foremost, however, this structure was historically thought to be linked to fear, allowing the individual to take action in response to that fear. However, as time has gone by, researchers such as Pessoa, generalized this concept with help from evidence of EEG recordings, and concluded that the amygdala helps an organism to define a stimulus and therefore respond accordingly. However, when the amygdala was initially thought to be linked to fear, this gave way for research in the amygdala for emotional processes. Kheirbek demonstrated research that the amygdala is involved in emotional processes, in particular the ventral hippocampus. He described the ventral hippocampus as having a role in neurogenesis and the creation of adult-born granule cells (GC). These cells not only were a crucial part of neurogenesis and the strengthening of spatial memory and learning in the hippocampus but also appear to be an essential component to the function of the amygdala. A deficit of these cells, as Pessoa (2009) predicted in his studies, would result in low emotional functioning, leading to high retention rate of mental diseases, such as anxiety disorders.
Social processing
Social processing, specifically the evaluation of faces in social processing, is an area of cognition specific to the amygdala. In a study done by Todorov, fMRI tasks were performed with participants to evaluate whether the amygdala was involved in the general evaluation of faces. After the study, Todorov concluded from his fMRI results that the amygdala did indeed play a key role in the general evaluation of faces. However, in a study performed by researchers Koscik and his team, the trait of trustworthiness was particularly examined in the evaluation of faces. Koscik and his team demonstrated that the amygdala was involved in evaluating the trustworthiness of an individual. They investigated how brain damage to the amygdala played a role in trustworthiness, and found that individuals with damaged amygdalas tended to confuse trust and betrayal, and thus placed trust in those having done them wrong. Furthermore, Rule, along with his colleagues, expanded on the idea of the amygdala in its critique of trustworthiness in others by performing a study in 2009 in which he examined the amygdala's role in evaluating general first impressions and relating them to real-world outcomes. Their study involved first impressions of CEOs. Rule demonstrated that while the amygdala did play a role in the evaluation of trustworthiness, as observed by Koscik in his own research two years later in 2011, the amygdala also played a generalized role in the overall evaluation of first impression of faces. This latter conclusion, along with Todorov's study on the amygdala's role in general evaluations of faces and Koscik's research on trustworthiness and the amygdala, further solidified evidence that the amygdala plays a role in overall social processing.
History
Etymology and history
The term limbic comes from the Latin limbus, for "border" or "edge", or, particularly in medical terminology, a border of an anatomical component. Paul Broca coined the term based on its physical location in the brain, sandwiched between two functionally different components.
The limbic system is a term that was introduced in 1949 by the American physician and neuroscientist, Paul D. MacLean. The French physician Paul Broca first called this part of the brain le grand lobe limbique in 1878. He examined the differentiation between deeply recessed cortical tissue and underlying, subcortical nuclei. However, most of its putative role in emotion was developed only in 1937 when the American physician James Papez described his anatomical model of emotion, the Papez circuit.
The first evidence that the limbic system was responsible for the cortical representation of emotions was discovered in 1939, by Heinrich Kluver and Paul Bucy. Kluver and Bucy, after much research, demonstrated that the bilateral removal of the temporal lobes in monkeys created an extreme behavioral syndrome. After performing a temporal lobectomy, the monkeys showed a decrease in aggression. The animals revealed a reduced threshold to visual stimuli, and were thus unable to recognize objects that were once familiar. MacLean expanded these ideas to include additional structures in a more dispersed "limbic system", more on the lines of the system described above. MacLean developed the intriguing theory of the "triune brain" to explain its evolution and to try to reconcile rational human behavior with its more primal and violent side. He became interested in the brain's control of emotion and behavior. After initial studies of brain activity in epileptic patients, he turned to cats, monkeys, and other models, using electrodes to stimulate different parts of the brain in conscious animals recording their responses.
He analyzed the brain's center of emotions, the limbic system, and described an area that includes structures called the hippocampus and amygdala. Developing observations made by Papez, he determined that the limbic system had evolved in early mammals to control fight-or-flight responses and react to both emotionally pleasurable and painful sensations. The concept is now broadly accepted in neuroscience. Additionally, MacLean said that the idea of the limbic system leads to a recognition that its presence "represents the history of the evolution of mammals and their distinctive family way of life."
In the 1960s, Dr. MacLean enlarged his theory to address the human brain's overall structure and divided its evolution into three parts, an idea that he termed the triune brain. In addition to identifying the limbic system, he pointed to a more primitive brain called the R-complex, related to reptiles, which controls basic functions like muscle movement and breathing. The third part, the neocortex, controls speech and reasoning and is the most recent evolutionary arrival. The concept of the limbic system has since been further expanded and developed by Walle Nauta, Lennart Heimer, and others.
Academic dispute
There is controversy over the use of the term limbic system, with scientists such as Joseph E. LeDoux and Edmund Rolls arguing that the term be considered obsolete and abandoned. Originally, the limbic system was believed to be the emotional center of the brain, with cognition being the business of the neocortex. However, cognition depends on acquisition and retention of memories, in which the hippocampus, a primary limbic interacting structure, is involved: hippocampus damage causes severe cognitive (memory) deficits. More important, the "boundaries" of the limbic system have been repeatedly redefined because of advances in neuroscience. Therefore, while it is true that limbic interacting structures are more closely related to emotion, the limbic system itself is best thought of as a component of a larger emotional processing plant. It is essentially responsible for sifting through and organizing lower order processing, and relaying sensory information to other brain areas for higher order emotional processing.
See also
In Spanish: Sistema límbico para niños
