The limbic system is a collection of structures involved in processing emotion and memory, including the hippocampus, the amygdala, and the hypothalamus.
The limbic system is located within the cerebrum of the brain, immediately below the temporal lobes, and buried under the cerebral cortex (the cortex is the outermost part of the brain).
The limbic system was originally called the rhinencephalon (meaning ‘smell brain’) because it was thought to be primarily involved with the sense of smell.
Psychologists now recognize that the limbic system serves a lot more functions than previously believed. These structures are known to be involved in the processing and regulating of emotions, the formation and storage of memories, sexual arousal, and learning.
The limbic system is thought to be an important element in the body’s response to stress, being highly connected to the endocrine and autonomic nervous systems.
There are two widely accepted structures of the limbic system: the hippocampus and the amygdala. There are differing opinions on which other structures are included in the system and which only interact closely with it.
The nerve cells (neurons) within the limbic system are structured differently from those in the cerebral cortex. In the cerebral cortex, the cells are primarily neocortical, meaning they are formed into six layers.
Within the limbic system, the cells are either arranged in fewer layers or are more jumbled. As there is less complexity of the cells within the limbic system, this has led people to believe that this system is evolutionarily older than the cerebral cortex itself.
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Substructures of the Limbic System
There are two hippocampi, located in each hemisphere of the brain. They are seahorse-shaped and are structures mainly associated with being the memory centers of our brains.
Episodic memories are formed in the hippocampus and then filed away into long-term storage throughout other parts of the cerebral cortex.
The hippocampus always plays a role in spatial navigation and has also been associated with learning and emotions (Tyng, Amin, Saad, & Malik, 2017).
The hippocampus is also known as a site where neurogenesis occurs – this means that new nerve cells are made here from adult stem cells.
Due to the hippocampus’s involvement in memory, damage to this area can lead to severe memory impairments.
Damage can also be detrimental to spatial memory, for instance, remembering directions to locations that should be familiar to the individual.
The amygdala is an almond-shaped structure located right next to the hippocampus. The main function of the amygdala is in emotional responses, including feelings of happiness, fear, anger, and anxiety.
This area is also key for the formation of new memories. The amygdala interacts with the hippocampus by attaching emotional content to memories.
It has a role in how memorable memories can be – memories with strong emotional components tend to stick rather than those with little emotional content.
‘Fear learning’ is also an element of the amygdala.
Fearful memories can be formed after only a few repetitions, which can result in avoidance of certain fearful stimuli. Therefore, the amygdala is linked with the fight-or-flight response, as stimulating activity can influence the body’s automatic fear response.
Damage to the amygdala can result in more aggression, irritability, loss of control of emotions, and deficits in recognizing emotions, especially recognizing fear.
There are also links between amygdala differences in those with Autism, depression, posttraumatic stress disorder, and bipolar disorder.
The following structures are not structures of the limbic system but form complex networks and interact closely with the limbic system, aiding in many functions.
The cingulate gyrus is part of the cingulate cortex of the brain and is thought to be an integral part of the limbic system.
This area is believed to help regulate emotions, behavior, and pain, as well as being responsible for controlling autonomic motor function.
This area is thought to involve fear and the prediction and avoidance of negative stimuli, through monitoring the body’s response to unpleasant experiences.
Damage to the cingulate gyrus can result in emotions being inappropriate, having a lack of fear, impaired sense of pain, and learning impairments.
This region has also shown differences in structure in those with Autism, depression, obsessive-compulsive disorder, posttraumatic stress disorder, and bipolar disorder due to its role in emotional processing (Yucel et al., 2003).
The hypothalamus’ most basic function is homeostasis (maintaining a steady internal state).
This region controls most autonomic functions, such as hunger, thirst, body temperature, blood pressure, heart rate, and sexual activity. The hypothalamus also serves as an interface between the nervous system and the endocrine system and in the regulation of sexual motivation and behavior.
The hypothalamus also has a role in controlling the body’s response to stress. To control these many functions, the hypothalamus integrates information from other parts of the brain and is responsive to a variety of stimuli, such as light, odor, stress, and arousal.
If the hypothalamus is impaired, this can lead to aggressive behavior, feeling over-stressed, hypothermia, hyperthermia, fatigue, weight gain/loss, and an active/underactive sex drive.
Differences in the hypothalamus have also been associated with conditions such as depression, bipolar disorder, and schizophrenia.
The basal ganglia are a group of structures situated at the base of the forebrain and the top of the midbrain.
Its main functions are to regulate voluntary movements, including eye movements, and help with balance as well as posture.
There is a limbic region of the basal ganglia, which has multiple components (nucleus accumbens, ventral tegmental area, and ventral pallidum).
These areas are involved in cognitive and emotional behaviors and with having a role in rewards and reinforcements. Because of this, it can be linked with addictive behaviors and the formation of habits.
Damage to the basal ganglia can result in tremors, involuntary muscle movements, abnormal posture, and links to movement disorders (Parkinson’s and Huntington’s disease).
In relation to the limbic system, the basal ganglia may also contribute to depression (Stathis et al., 2007).
Damage to the limbic system is dependent on which region is affected. Amygdala damage could affect a person’s fear processing (especially in being unable to recognize fearful situations), which could result in more risk-taking behaviors and putting themselves in dangerous situations.
Damage to the hippocampus could lead to deficits in being able to learn anything new, as well as affecting memory.
Hypothalamus damage can affect the production of certain hormones, including those which can affect mood and emotion.
Below is a non-exhaustive list of symptoms associated with limbic system damage:
- Uncontrolled emotions – more aggression, anxiety, and agitation.
- Olfactory impairments
- Memory impairments
- Abnormal sexual behavior – high/low sex drive
- Abnormal biological rhythms
Below are some cognitive disorders which have been shown to be connected to the limbic system:
- Olfactory impairments
- Obsessive compulsive disorder
- Posttraumatic stress disorder
- Bipolar disorder
- Autism spectrum conditions
- Alzheimer’s disease
- Movement disorders – Huntington’s and Parkinson’s disease
A potential treatment for limbic impairments is deep brain stimulation (DBS). Successful treatment of some cognitive disorders, such as anxiety and posttraumatic stress disorder, has come from DBS of the amygdala.
DBS has also been useful in targeting the nucleus accumbens (part of the basal ganglia) in drug addiction (Bari, Niu, Langevin, & Fried, 2014).
Similarly, the use of antidepressant medications has shown links with restoring the underlying physiological differences in the limbic system in major depressive disorder (Maletic et al., 2007).
- Okun et al. (2004) examined the emotional changes following deep brain stimulation of the dentate gyrus (an area which is interactive with the hippocampus). The researchers found that stimulation of this area caused participants to smile spontaneously, as well as report feelings of euphoria.
- Milner, Corkin & Teuber (1968) found that people with damage to the hippocampus lose the ability to learn anything new. For these people, ‘yesterday’ to them is always the time before their brain damage occurs.
- Sergerie, Chochol & Armony (2008) investigated the amygdala’s response to positive and negative emotional visual stimuli. The results showed that the amygdala responded more strongly to faces which showed emotion, rather than other visual emotional stimuli.
- Grèzes, Berthoz & Passingham (2006) found that there was increased amygdala activity when participants watched themselves being deceived, compared to a control group.
- This could be due to the personal involvement by participants when they make emotional judgments involving themselves. The participants may have seen deceit as a personal threat, therefore leading to increase amygdala activity as a result.
- Morris et al. (1996) found that some patients who had damage to their amygdala were unable to recognize fear in a person’s face or voice.
- Sahin et al. (2015) investigated patients who have multiple sclerosis (MS) using magnetic resonance imaging (MRI). It was found that a high number of patients had damage in limbic areas, which may explain why people with MS commonly experienced memory deficits and emotional dysfunction.
- DelBello, Adler & Strakowski (2006) investigated the neurophysiology of childhood and adolescent bipolar disorder. They found structural and neurochemical abnormalities in the limbic areas of the amygdala and hippocampus in young people with this disorder.
- White et al. (2008) investigated the limbic structures and networks of children and adolescents with schizophrenia. They found that the cingulate cortex, hippocampus, and amygdala had observable differences in structure. They also found that differences in white matter volumes tended to be more focal to the limbic system.
- Maletic et al. (2007) conducted a literature review regarding the neurobiology of major depressive disorder. They concluded that the literature showed that the limbic areas showed to be associated with emotion regulation.
- Lepage et al. (2018) investigated the limbic structures of former NFL football players, of which, a condition called chronic traumatic encephalopathy (CTE) tends to be diagnosed (this is a neurodegenerative disease associated with repeated head impacts).
- Compared to a control group, the former NFL players had reduced volumes of the amygdala, hippocampus, and cingulate gyrus. Therefore, this may be a potential indicator of neurodegeneration in those at risk of developing CTE.
- Milner (2003) found that the inferior (lower) temporal cortex is responsible for visual perception, therefore, lesions in this area can lead to an inability to recognize faces (also known as prosopagnosia).
- There may be a link between the temporal lobes and dyslexia. As dyslexia affects the reading ability and gives meaning to language, people with dyslexia may have reduced activity in their left temporal lobes.
- Valdois et al. (2019) found that impaired attentional systems within the temporal lobes can be associated with developmental dyslexia.
- Lowndes & Savage (2007) proposed that the earliest neuropsychological identification of Alzheimer’s disease may lie in the medial temporal lobe.
- Due to the superior temporal sulcus (STS) role in interpreting social and speech input, Redcay (2008) suggested that impairments in STS function may underlie many of the social and language difficulties in those with Autism Spectrum Disorders.
- Kasai et al. (2003) used neuroimaging techniques on patients with Schizophrenia and found structural abnormalities in their superior temporal gyrus.
- There may also be a link between auditory cortex damage/deficits and schizophrenia, as damage in this area is linked to major symptoms of schizophrenia (e.g., hearing voices and auditory hallucinations).
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