Know Your Brain: Limbic System
Where is the limbic system?
The word limbic comes from the Latin limbus, which means "border." The term was originally introduced in the 1600s by the physician Thomas Willis to refer to an arc of brain tissue that forms a border around the brainstem and other structures deep in the center of the brain. The renowned neuroscientist Paul Broca used Willis' terminology a couple centuries later when he referred to the limbic areas as another lobe of the brain, which he called "the great limbic lobe." Twentieth century neuroscientists began to associate the structures in Broca's limbic lobe with emotions, and eventually a circuit involving these structures and others became known as the limbic system (for more details on the history of the limbic system, see below).
The limbic system, then, is not a unitary structure but consists of a collection of brain regions. There is a lack of consensus as to what structures should be included in the limbic system, but areas that are often included are the amygdala, cingulate cortex, hippocampus and its neighboring regions, hypothalamus, septal nuclei, mammillary bodies, prefrontal cortex, certain nuclei of the thalamus, and tracts such as the fornix and medial forebrain bundle.
What is the limbic system and what does it do?
Historical perspectives on the limbic system
Neuroscientists have long been trying to identify parts of the human brain that are responsible for producing emotional responses. In 1937, the prolific neuroscientist James Papez made an important contribution to this effort when he published a paper that described a new brain circuit for emotion. It included much of the limbic lobe, a region of the brain identified by Paul Broca close to 60 years earlier. Papez's model became known as the Papez circuit, and it included the hippocampus, mammillary bodies and the rest of the hypothalamus, cingulate cortex, certain nuclei of the thalamus, and connections between these regions.
Within a few years after Papez proposed the existence of the Papez circuit for handling emotions, researchers Heinrich Kluver and Paul Bucy found that the removal of parts of the temporal lobe---including a structure called the amygdala---had a dramatic effect on the emotional responses of monkeys. Specifically, monkeys that had their temporal lobes removed became very placid and didn't display any fear (along with a list of other behavioral changes). The constellation of behavior that resulted from Kluver and Bucy's experiments became known as Kluver-Bucy syndrome, and it helped to establish areas of the temporal lobe such as the amygdala as having potential importance in emotions.
In the 1940s and 1950s, a young researcher at Yale named Paul MacLean published several papers exploring Papez's emotion circuit. MacLean argued for the addition of several structures to the circuit, such as the amygdala (due in part to the influence of Kluver and Bucy's work), prefrontal cortex, and septal nuclei. MacLean also was the first to use the term limbic system. In addition to providing the limbic system with its name, MacLean elaborated on the functions of the system. He hypothesized that the collection of structures integrates sensory information about the outside world with internal information from the body; that integration, MacLean proposed, leads to emotional responses.
MacLean's research on the limbic system was highly influential, and led to the conceptualization of the limbic system as the primary emotion circuit of the brain. Over the years, researchers have incorporated additional areas into the limbic system. Indeed, the list of structures generally considered to be part of the system is not always consistent from source to source.
More recently, the idea of one "system" for emotion in the human brain has fallen out of favor. Emotions are highly complex experiences, and neuroscience research has failed to support the hypothesis that they are all managed by one unitary system. Additionally, while many of the regions included in the limbic system may be involved with emotional responses, they are also typically involved in much else besides emotion. This makes it a bit imprecise to link these structures primarily with emotion.
For these reasons, some neuroscientists have suggested doing away with the term "limbic system" altogether. For those who continue to use the term, it typically is not only linked to emotions but also to a variety of functions such as: emotion, memory, executive functions, pleasure and reward, olfaction (smell), and more.
The amygdala and emotion
After Kluver and Bucy found that removal of part of the temporal lobe could cause Kluver-Bucy syndrome, another researcher named Larry Weiskrantz observed that many aspects of Kluver-Bucy syndrome could be elicited just by removing the amgydalae (which are found in the temporal lobes). This, combined with MacLean's inclusion of the amygdala in the limbic system, led later researchers to focus more on the role of the amygdala in emotional responses.
This subsequent research found the amygdala played an especially large role in fear responses. For example, studies found that lesions or damage to the amygdala would prevent rodents from learning to fear a pain-inducing stimulus like a mild electrical shock. Additionally, human cases have since been identified where amygdala damage is linked to a diminished ability to experience fear.
Thus, the amygdala earned a reputation as a part of the brain that plays a critical role in fear. But more recent research has led to an expanded role for the amygdala in emotions. We now know, for example, that the amygdala is not only active during fear, but also when creating positive memories. Indeed, it has even been linked to the enduring memories associated with addiction, and researchers now believe it is involved with orchestrating emotional responses (both positive and negative) to important stimuli in the environment.
The amygdala can play such an integral role in orchestrating emotional responses because it has extensive connections to various other parts of the brain that are involved in emotions. For example, the amygdala forms connections with the hypothalamus, a part of the brain that can initiate autonomic nervous system responses that are linked to emotion (such as the fight or flight response that occurs when confronted with a fear-inducing stimulus).
Additionally, the amygdala is interconnected with areas of the cerebral cortex that are involved in processing sensory information. In this way, the amygdala is privy to information regarding moment-to-moment sensory experiences, which again is critical information for the regulation of emotional responses.
The amygdala also forms connections with regions such as the thalamus, basal ganglia, brainstem, and more, which enable the amygdala to act as a hub in a network that links together structures in the cerebral cortex and those in the limbic lobe of the brain. And it interacts with regions like the hippocampus to form memories associated with emotional experiences.
Other regions of the limbic system
While a thorough examination of all of the limbic system structures would be outside the scope of this article, I will list some other structures typically included in the limbic system along with a short summary of their related functions. It is important to remember, however, that these short summaries will not do justice to the complex roles these regions truly have.
As mentioned above, the hypothalamus plays a key role in the regulation of autonomic responses, which are important to emotion. It also regulates the release (from the pituitary gland) of a number of hormones, some of which contribute to emotional responses.
The mammillary bodies are a structure found at the base of the hypothalamus; they are best known for their role in memory. They form connections with the hippocampus and the thalamus, and are thought to facilitate memory consolidation through these connections.
The cingulate cortex is a section of the cerebral cortex that sits just above a large bundle of nerve fibers called the corpus callosum. Among other things, the cingulate cortex is thought to be important to a number of functions related to emotion such as the regulation of overall affect and mood.
The hippocampus (aka hippocampal formation) is found in the temporal lobe, and is best known for its critical role in memory consolidation. As part of this functionality, it is also involved in the formation of emotional memories, which are often stronger and longer-lasting than memories without a strong emotional component. The hippocampus has also been implicated in the regulation of emotion, perhaps due to its ability to coordinate the recall of important contextual information about whatever it is in the environment that might spark an emotional response.
The anterior nucleus of the thalamus is interconnected with the amygdala, cingulate cortex, hippocampus, and orbitofrontal cortex. Through these connections it is thought to be involved with memory, emotional responses, and executive functions.
The septal nuclei form connections with structures such as the hippocampus, amygdala, hypothalamus, and a dopamine-rich area in the brainstem called the ventral tegmental area (VTA). Through these connections, the septal nuclei are thought to play roles in learning, memory, social behavior, emotion, motivation, reward, and addiction.
Areas of the prefrontal cortex are often included as part of the limbic system as well. The prefrontal cortex has a long list of functions, but in relation to the limbic system it may be involved with the higher-order cognitive processing of emotions, memory, executive functions, other complex cognition associated with emotions, and more. For instance, the prefrontal cortex is thought to play a role in assessing the emotional and motivational value of stimuli in the environment, as well as with exerting top-down control of emotions when deemed necessary.
References (in addition to linked text above):
Dalgleish T. The emotional brain. Nat Rev Neurosci. 2004 Jul;5(7):583-9. doi: 10.1038/nrn1432. PMID: 15208700.
Roxo MR, Franceschini PR, Zubaran C, Kleber FD, Sander JW. The limbic system conception and its historical evolution. ScientificWorldJournal. 2011;11:2428-41. doi: 10.1100/2011/157150. Epub 2011 Dec 8. PMID: 22194673; PMCID: PMC3236374.
van Groen T, Kadish I, Ver Hoef L, Wyss JM. The Limbic System. In: Conn MP, ed. Neuroscience in Medicine, 3rd. ed. Totowa, NJ: Humana Press.
