Download Limbic system and more Schemes and Mind Maps Neuroscience in PDF only on Docsity!
Limbic system
The limbic system is a group of brain areas involved in feeling emotions such as fear, anger and happiness; motivating behaviours driven by primitive drives, such as aggression, pleasure-seeking and sexual urges; and even learning and memory. The Latin term ‘limbus’ is translated as ‘edge’ or ‘border’. This system takes its name from the location of these regions, which are buried under the surface where the edge of the cerebrum (the largest part of the brain, split into two hemispheres) sits just above the brain stem.
Because the limbic system controls the basic emotions and urges which drive our behaviour, it is fundamental to survival. Dysfunction in any of these brain regions, or the connections between them, is believed to underlie the distress and difficulties suffered by individuals with psychiatric illnesses such as depression and anxiety. Abnormalities in the limbic system are also implicated in the symptoms of developmental disorders, such as autism. The main regions involved are introduced below before we return to consider the consequences of damage to the limbic system.
Key players of the limbic system
There is some disagreement about which brain structures are included in the limbic system but by popular consensus, it includes several structures in the cerebral cortex (such as the cingulate gyrus and the hippocampus ) and the diencephalon (the hypothalamus and thalamus ). It also includes some subcortical structures such as the amygdala and the basal ganglia.
Other brain regions, such as the orbitofrontal cortex and the olfactory bulb , also play an important role in motivation and emotion, and are sometimes considered to be part of the limbic system.
The amygdala
Located deep within the medial aspect of the temporal lobe, the amygdala plays a central role in decision-making and emotional responses. This is reflected in the wealth of its connections with other brain and limbic areas. Electrical stimulation of this area can produce uncontrollable bursts of fear, anxiety, sadness or aggression. The amygdala lends emotional colouring to our speech, and is believed to contribute to the recognition of emotions in others.
At the level of brain evolution, the amygdala is very old and arguably crucial for human survival. Damage to the amygdala prevents an individual from learning from unpleasant experiences (so-called ‘fear conditioning’). If cell activity in the amygdala is disrupted or the structure is lesioned, humans and animals do not learn to avoid painful or dangerous stimuli.
The amygdala is also believed to alert us to potential dangers in our environment, diverting attention to detected threats. It triggers the fight or flight response through its inputs to the hypothalamus and the brain stem. A famous neuropsychological case-study, S. M., is totally unmoved by stimuli that most people would find extremely threatening. With severe bilateral damage to the amygdala, she has been dubbed “the woman without fear”.
For many years, the amygdala was specifically associated with fear. However, it is now believed to be responsive to a range of emotions, including positive ones. Disruption of the amygdala also prevents humans and animals from learning positive associations. Normally pleasant stimuli, like food, cease to be rewarding and to motivate behaviour.
The hippocampus
The hippocampus and surrounding areas are crucial for memory. One of the most famous cases in neuropsychology, a man known as H.M., experienced a catastrophic disability when he underwent bilateral removal of the hippocampus. The surgery successfully alleviated H.M.’s epilepsy but left him bereft of the ability to form new long-term memories. H.M. retained some memories from his past, which suggests that older memories may be less dependent on the hippocampus. His inability to lay down new memories, however, left him adrift in a perpetual moment of immediate consciousness, ungrounded by the hours, days or weeks preceding it.
The amygdala interacts with the hippocampus to give memories their ‘emotional flavour’. It also affects how memories are laid down, making emotional memories easier to recall. Memories laid down by the hippocampus also affect the amygdala: having heard about someone else’s painful experience with the dentist, you may feel anxious yourself when going.
In cases where the hippocampus is damaged but the amygdala is spared, people learn to associate stimuli with emotions such as fear – but they have no idea why they’re frightened.
Cingulate gyrus
The cingulate is a ‘belt’ (cingulum) of grey matter which surrounds the corpus callosum, the thick fibre tract which connects the two hemispheres. This large area is normally considered to have posterior and anterior aspects with differential roles, although these are still under investigation.
The anterior part of the cingulate is connected not only to many of the cortical regions involved in complex mental processes, but also to the amygdala, the nucleus accumbens and other nodes in the limbic system. In patients with treatment-resistant forms of depression or obsessive-compulsive disorder, severing the fibres of the anterior cingulate produces a remarkable reduction in distress and emotional reactivity. As the cingulate is also involved in interpreting and experiencing pain, this procedure may also be used to treat chronic pain when all else fails.
phobic stimuli is only mentioned in conversation or seen in a picture. This dysfunction in the amygdala is impervious to rational thinking and so is beyond the control of sufferers. However, people can be helped to reduce their phobias in therapy, which results in normalising amygdala activity to the phobic object.
Abnormalities in the function of the anterior cingulate are common in several disorders, including depression, schizophrenia and post-traumatic stress disorder. The cingulate is also implicated in obsessive-compulsive disorder, as are the basal ganglia and the thalamus. In this condition, some researchers suggest that typical communication between the orbitofrontal cortex, the thalamus and the striatum (which includes the basal ganglia) breaks down, leaving behaviour distressingly out of control.
Many recreational drugs, like opiates, nicotine, amphetamines and cocaine, flood the brain’s reward/pleasure circuitry (the basal ganglia, particularly the nucleus accumbens) with dopamine. These substances evoke 2-10 times the amount of dopamine activity evoked by food or sex – hence why they produce such euphoric feelings and are so addictive. The brain tries to adapt to such an influx of dopamine by reducing dopamine activity in the limbic system. Unfortunately this has the effect of reducing any feelings of pleasure, such that the addicted individual tries to bring their dopamine levels back to normal by taking the drug; no longer necessarily chasing a high, but just to try to feel OK.
The limbic system functions differently in some developmental disorders, too, notably those characterised with difficulties in social communication and emotion processing. Autism is characterised by abnormality in key limbic regions and in the connections between them. The amygdala has historically been of particular interest due to some of the similarities between people with autism and people with brain damage to this area. Whilst people with autism certainly experience fear, they can be less aware of potential dangers, less able to recognise emotions, and may show a lack of vocal or facial emotion themselves, much like sufferers of traumatic brain injury.
People with developmental and intellectual disorders often suffer from additional psychiatric illnesses. It can be difficult to determine whether abnormalities in the limbic system, in these cases, are a feature of the developmental/intellectual disorder itself, or a feature of a mental illness.
Rachel Louise Moseley
See also:
Aggression, depression, obsessive-compulsive disorder, phobias, substance use, basal ganglia, cerebral cortex, cingulate gyrus, frontal lobes, hormones, neurotransmitters
Further reading:
Cardinal, R. N., Parkinson, J. A., Hall, J., & Everitt, B. J. (2002). Emotion and motivation: the role of the amygdala, ventral striatum, and prefrontal cortex. Neuroscience & Biobehavioral Reviews, 26 (3), 321-352.
Damasio, A. (2008). Descartes' error: Emotion, reason and the human brain. New York: Random House.
Haznedar, M. M., Buchsbaum, M. S., Wei, T. C., Hof, P. R., Cartwright, C., Bienstock, C. A., & Hollander, E. (2014). Limbic circuitry in patients with autism spectrum disorders studied with positron emission tomography and magnetic resonance imaging. American Journal of Psychiatry, 157 (12), 1994-2001.
Janak, P. H., & Tye, K. M. (2015). From circuits to behaviour in the amygdala. Nature, 517 (7534), 284-292.
Ketter, T. A., George, M. S., Kimbrell, T. A., Benson, B. E., & Post, R. M. (1996). Functional brain imaging, limbic function, and affective disorders. The Neuroscientist, 2 (1), 55-65.
Koob GF, Le Moal M (2006). Neurobiology of Addiction. London: Academic Press.
Le Doux JE (2000). Emotion circuits in the brain. Annual Review of Neuroscience , 23, 155–
Menzies, L., Chamberlain, S. R., Laird, A. R., Thelen, S. M., Sahakian, B. J., & Bullmore, E. T. (2008). Integrating evidence from neuroimaging and neuropsychological studies of obsessive-compulsive disorder: the orbitofronto-striatal model revisited. Neuroscience & Biobehavioral Reviews, 32 (3), 525-549.
Nestler, E. J., Barrot, M., DiLeone, R. J., Eisch, A. J., Gold, S. J., & Monteggia, L. M. (2002). Neurobiology of depression. Neuron, 34 (1), 13-25.
Richardson, M. P., Strange, B. A., & Dolan, R. J. (2004). Encoding of emotional memories depends on amygdala and hippocampus and their interactions. Nature Neuroscience, 7 (3), 278-285.
