The limbic system is not a single organ but a collection of structures that work together, performing essential survival functions, most notably functions involving emotions and memories.
The limbic system is composed of four main parts:
Triune Brain Theory
During the 1960's scientists divided the brain into three parts:
The reptilian brain was theorized to be the center for instinctive reactions. The mammalian brain responsible for somatosensory and emotional experience, and the homo sapiens brain for intellectual and executive functioning.
The limbic system theoretically hosts many of the functions originally assigned to the mammalian brain.
Modern science largely dismisses the triune brain theory. Lisa Feldman Barrett PhD pulls no punches in her criticism, "the triune brain idea and its epic battle between emotion, instinct, and rationality is a modern myth" (2020, location 265).
Barrett warns that, "anything you read or hear that proclaims the human neocortex, cerebral cortex, or prefrontal cortex to be the root of rationality, or says that the frontal lobe regulates so-called emotional brain areas to keep irrational behavior in check, is simply outdated or woefully incomplete" (location 260).
Robert Sapolsky reminds that, "there really aren’t 'centers' in the brain 'for' particular behaviors" (2018, location 487).
Daniel Siegel explains "the brain as a whole functions as an interconnected and integrating system of subsystems" (2020, location 623).
So, as we look at the functions of the limbic system, we must remember that while these specified regions of the brain have significant involvement in listed functions, they remain interconnected to the entire biological system, impacted and influencing through bidirectional communications with many other subsystems.
Functions of Limbic System
The limbic system acts as a control center for conscious and unconscious functions, regulating much of what the body does. In many ways, the limbic system bridges the gap between psychological and physiological experiences.
Processing of sensory input begins in the limbic regions of the brain. Dr. Robert DeMoss explains that the limbic system "provides us with an immediate, 'orienting response.'" He describes the limbic system as the general area "that supports an emotional response" (1999, page 173).
The limbic system is involved in lower order emotional processing of input from sensory systems. The limbic system activates biological systems before involvement of the cortex (speaking in microseconds).
Early theories hypothesized that the limbic system housed the emotions. Continued research has discovered the limbic system serves more functions than previously believed. These structures are also involved in processing and regulating of emotions, formation and storage of memories, sexual arousal, and learning. In addition, the limbic system is thought to be crucial to the body’s response to stress because it being highly connected to the endocrine and autonomic nervous systems.
The structures of the limbic system and the cortex are intricately and complexly connected. Most sensory information flows to the cortex as well, where it is decoded, and integrated with language, and comprehended against the backdrop of previous knowledge.
The limbic system and the cortex communicate, integrating through assimilation and accommodation. The limbic system and cortex are involved in bidirectional communication rather than the cortex simply reigning in irrational emotional reactions. Sapolsky explains, "the frontal cortex and limbic system stimulate or inhibit each other, collaborate and coordinate, or bicker and work at cross-purposes" (2018, location 544).
Specific Functions Linked to the Limbic System
Research has linked the limbic system to:
1. Reward, Motivation, and Addiction
Research suggests that feelings of motivation and reward originate in the ventral tegmental area (VTA) of the brain. The ventral tegmental area is a group of neurons that connects to the nucleus accumbens in the basal ganglia. The ventral tegmental neurons release dopamine, a neurotransmitter that supports feelings of pleasure, and a foundational ingredient of reward.
In a healthy brain, dopamine motivates learning, meeting new people, and trying new experiences. Drug and alcohol abuse act on dopamine delivery and reception, and over time, the chemically induced changes can become addictive, depleting the brain’s dopamine stores, making it difficult to feel pleasure without drugs.
2. Learning and Memory
The amygdala and hippocampus work together to help the brain interpret the emotional content of memories. Both help the brain form new memories, store those memories, retrieve them, and make sense of their emotional content. The hippocampus is particularly important in long-term memory formation. It also supports spatial memory and spatial reasoning.
The amygdala attaches emotional meaning to memories and helps the brain organize and prioritize memories based on the potency of the emotion. Research suggests, for example, that the cingulate gyrus focuses the brain’s attention on emotionally significant events.
The brain makes new neurons from stem cells in the hippocampus, suggesting the hippocampus and the feelings and memories intricately connected to the hippocampus change with new experiences.
Alzheimer’s and other dementias attacks the hippocampus. This explains why dementia compromises learning when old memories remain intact.
3. Fight and Flight Response
The limbic system prepares the body to respond to environmental threats by activating the fight or flight response. The limbic system may also initiate a freeze response, giving the organism time to orient to vast amounts of incoming stimuli.
When the amygdala perceives a threat, it activates the limbic system. In response, the adrenal glands release hormones that raise blood pressure and heart rate, improve blood flow to muscles and organs, and elevate breathing rate. The biological changes prepare the organism for survival.
The fight or flight response can be life-saving, eliminating precious seconds wasted on cognitive processing. Over time, however, when a body is invaded by chronic stress, the constant activation of the limbic system damages the body. Long-term release of epinephrine and other hormones can damage blood vessels, cause high blood pressure, and change appetite.
See Burnout for more on this topic
4. Hormones Affecting Autonomic Functions
The body communicates through chemical messengers (hormones), sending signals in response to environmental elements. Environments include internal and external events. A threatening shadow or a menacing thought can activate the release of chemical messengers that readies the body to protect.
The hypothalamus releases hormones that inpacts a wide range of functions:
These messages also help the body maintain a homeostatic balance by regulating the autonomic nervous system.
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Barrett, L. F. (2020). Seven and a Half Lessons About the Brain. Mariner Books
DeMoss, R. T. (1999). Brain Waves Through Time. Basic Books
Sapolsky, R. M. (2018). Behave: The Biology of Humans at Our Best and Worst. Penguin Books; Illustrated edition.
Siegel, D. J. (2020). The Developing Mind, Third Edition: How Relationships and the Brain Interact to Shape Who We Are. The Guilford Press; Third edition