The Limbic System

What is the limbic system?

In the human brain, the limbic system is “the regulator.”

As one of the oldest-known brain networks, the limbic system has been found to regulate many core brain functions, including response, reaction,1 behavior, emotion, memory, and learning.2

The limbic system has developed throughout the course of evolution to regulate core functions in mammals and humans. These processes often take the form of:

    • The response to a stimulus, such as a new smell, sound, or sight
    • The level of reaction or motivation, be it self-protection or reward-seeking
    • The behavior that ensues
    • The emotion associated with that response or behavior
    • The memories formed based on the experience
    • Any learning or takeaways from the experience
Limbic-1

Early discovery

 

The limbic system was first named le grand lobe de limbique3 or the “great limbic lobe” by French neurosurgeon, Paul Broca, in 1878.4 Broca dubbed system as such because he found the limbic lobe to encompass several lobes4 that integrated with other core brain processes, such as olfactory and vision senses. It wasn’t until 1937 that American neuroanatomist, James Papez, identified the role the limbic system also plays in governing emotion.4

The role of the "regulator"

 

Whenever we see something move or notice a new smell, it is an innate response to an unknown stimulus. Our brains immediately set to work figuring out the source of the new experience.

The limbic system works alongside our other brain networks to evaluate the world around us and regulate our responses to external conditions. As one of the oldest areas of the brain, the limbic system developed in the earliest mammals as a means of response to stimuli and protection from predators.

Since the limbic system was not designed to live in polite society, this critical part of the brain now plays a slightly different role in regulating reponse.

For instance, when you’re seated in a restaurant, each time a server carries a tray of food into the dining area, your brain immediately notices the smells wafting to your table.

As a key regulator of your olfactory system and response to smells, the limbic system governs your reaction to these new sensations and helps you identify each plate of food. Yet, it is also busy regulating your behavioral inhibition and emotional response.

As soon as you realize the tray of food is headed to a nearby table, the limbic system gives you the self-control to wait for your own meal.

To make this decision, the limbic system processes past emotion and learned memory of societal norms to suppress the primitive motivation to scavenge food off someone else’s plate.

Historically, the limbic brain would have regulated these responses for survival and self-preservation. Today, the limbic system regulates our day-to-day reactions with self-referential processing to help us coexist in our society — and in the case of restaurant etiquette, it keeps you seated in your chair until your food arrives.

Where is the limbic system in the brain?

 

The limbic system generally includes the amygdala, thalamus, hypothalamus, hippocampus, as well as numerous functionally and anatomically-related areas that are considered paralimbic structures.5 These areas include the prefrontal-limbic system,2 anterior cingulate cortex (ACC), medial temporal network, parahippocampal gyrus, olfactory lobe, and the ventral tegmental area (VTA).2

 

LIMBIC_AXI AXIAL VIEW
LIMBIC_SAG SAGITTAL VIEW
CORONAL VIEW CORONAL VIEW

Loss of function in the limbic system

 

As a major player in memory, emotion, behavior, and control of the senses, loss of function in the limbic system typically manifests as an intensification or reduction of core brain activity.6

Temporal lobe epilepsy7 is a result of pathology or disease in the limbic system, and it may cause experiential phenomena8 when the limbic system becomes overactive during a seizure.

Limbic encephalitis9 is a rare autoimmune condition characterized by inflammation in the limbic structures and other parts of the brain. The condition often causes a loss of short-term memory, confusion, and seizures.

Since the limbic system is one of the networks that plays the greatest role in encoding long-term memories, it is often implicated in dementia. Distinctive lesions10 in the area of the limbic structures frequently contribute to Alzheimer’s disease.

With regard to mental illness, the limbic system’s direct role in behavior and self-referential processing can contribute to numerous neuropsychiatric disorders. These include anxiety, bipolar disorder,11 schizophrenia, autism, and others.12 Psychopathic disorders that exhibit aggressive or impulsive behavior13 are also thought to stem from irregular emotional processing in the limbic system.

Connections to other brain networks

 

As a primitive survival tool, the limbic system controls fundamental and core brain functions, such as alertness, motivation, and learning.2 As the human species developed throughout history, the limbic system evolved to play a role in more complex behaviors and functions (as is the case in our restaurant analogy). 

To operate cohesively with the other important areas of the brain, the limbic system has key integrations with paralimbic areas, such as the orbital frontal cortex, intermodal frontal lobe, and the fusiform gyrus.

The limbic system also has connections to other brain networks for regulating key functional processes. The limbic system modulates:

    • The sensorimotor network for sensory perception, spatial reasoning, and motor commands
    • The language subnetwork for language processing
    • The olfactory system for analyzing smells6 
    • The central gustatory system for the sense of taste14
    • The default mode network for memory, self-referential processing,15 and behavioral decision-making

While each brain network has key characteristics and processes, it is important to remember that every network modulates the interaction of other networks. None exist in a vacuum. Many connections in one network will activate connections in another network, and the limbic system may change or regulate how that activation works.

In addition to regulating the brain’s core processes, the limbic system can also determine how connections fire within other brain networks. And while it won’t directly impact another network’s functional connectivity, it changes the context in which other networks operate.

Though the limbic system has been an integral part of the human brain throughout our evolution, we are still uncovering connections between the limbic system and other brain networks. These discoveries will continue to give insight into how the limbic system regulates key processes and visceral emotions in our brain.

Reference list

 

  1. Swenson RS. Limbic system. In: Swenson RS, ed. Review of Clinical and Functional Neuroscience. Hanover, NH: Geisel School of Medicine; 2006. https://www.dartmouth.edu/~rswenson/NeuroSci/chapter_9.html. Accessed December 15, 2020. 
  2. The limbic network: influence over motor control, memory, and learning. Neupsykey website. https://neupsykey.com/the-limbic-network-influence-over-motor-control-memory-and-learning/ Published April 22, 2020. Accessed December 7, 2020. 
  3. Broca P. Anatomie comparée des circonvolutions cérébrales: le grande lobe limbique et la scissure limbique dans la série des mammifères. In: Mason G, ed. Revue d'Anthropologie. 2nd ed. Paris, France. 1879;2:385-498. https://babel.hathitrust.org/cgi/pt?id=uc1.b3282461&view=1up&seq=9. Accessed December 29, 2020. 
  4. Rajmohan V, Mohandas E. The limbic system. Indian J Psychiatry. 2007;49(2):132-139. doi.org/10.4103/0019-5545.33264
  5. Koikegami H, Hirata Y, Oguma J. Studies on the paralimbic brain structures. Psychiatry Clin Neurosci. 1967;21:151-180. doi.org/10.1111/j.1440-1819.1967.tb01290.x
  6. Isaacson R. The Limbic System. Germany: Springer US; 2013. 
  7. Concha L, Beaulieu C, Gross DW. Bilateral limbic diffusion abnormalities in unilateral temporal lobe epilepsy. Ann Neurol. 2005;57:188-196. doi.org/10.1002/ana.20334
  8. Gloor P, Olivier A, Quesney LF, Andermann F, Horowitz S. The role of the limbic system in experiential phenomena of temporal lobe epilepsy. Ann Neurol. 1982;12:129-144. doi.org/10.1002/ana.410120203
  9. Limbic encephalitis. National Institutes of Health website. https://rarediseases.info.nih.gov/diseases/8742/limbic-encephalitis. Accessed December 28, 2020. 
  10. Hopper MW, Vogel FS. The limbic system in Alzheimer's disease. A neuropathologic investigation. Am J Pathol. 1976;85(1):1-20. https://pubmed.ncbi.nlm.nih.gov/135514/. Accessed December 22, 2020. 
  11. Leow A, Ajilore O, Zhan L, et al. Impaired inter-hemispheric integration in bipolar disorder revealed with brain network analyses. Biol Psychiatry. 2013;73(2):183-189 doi.org/10.1016/j.biopsych.2012.09.014.
  12. Dutta SS. Limbic system and behavior. News Medical Life Sciences website. https://www.news-medical.net/health/Limbic-System-and-Behavior.aspx. Updated August 23, 2018. Accessed December 8, 2020. 
  13. Hesselink JR. The temporal lobe & limbic system. University of California, San Diego website. http://spinwarp.ucsd.edu/NeuroWeb/Text/br-800epi.htm. Accessed December 8, 2020. 
  14. de Araujo IE, Simon SA. The gustatory cortex and multisensory integration. Int J Obes (Lond). 2009;33 Suppl 2(Suppl 2):S34-S43. doi:10.1038/ijo.2009.70
  15. Alves PN, Foulon C, Karolis V. et al. An improved neuroanatomical model of the default-mode network reconciles previous neuroimaging and neuropathological findings. Commun Biol. 2019;2:370. doi.org/10.1038/s42003-019-0611-3