limbic system

limbic system, group of structures in the brain that governs emotions, motivation, olfaction (sense of smell), and behaviour. The limbic system is also involved in the formation of long-term memory. The structures of the limbic system are found deep inside the brain, immediately below the temporal lobes and buried under the cerebral cortex, just above the brainstem. The limbic system consists of several interconnected components, including the thalamus, hypothalamus, basal ganglia, cingulate gyrus, hippocampus, and amygdala. A dysfunctional limbic system is associated with several conditions and clinical disorders such as epilepsy, dementia, anxiety disorders, and autism.

Study of the limbic system

The limbic system was originally called the rhinencephalon (a combination of Greek terms meaning “nose brain”) because it was believed to be associated with one’s sense of smell. In 1878 surgeon Paul Broca described what he called “le grand lobe limbique” and applied the term limbic, from the Latin limbus (“border”), to the curved rim of the cerebral cortex. Over time, the other functions of the limbic system were gradually discovered. The role of the limbic system in emotion was studied by physician James Papez and neuroanatomist Paul Yakovlev, who published their findings in 1937 and 1948, respectively. Paul D. MacLean, in a 1952 paper, coined the term limbic system to describe Broca’s limbic lobe and related structures below the cortex. In evolutionary terms, the limbic system is considered to be among the oldest parts of the human brain, as it has been identified in fish, amphibians, and reptiles as well as early mammals. The cells of the limbic system are also less complex than those in the cerebral cortex.

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Components of the limbic system

There is no universal agreement on every component of the limbic system because of its complex interconnectedness with different parts of the brain. However, the primary components of the limbic system are:

• thalamus
• hypothalamus
• basal ganglia
• cingulate gyrus
• hippocampus
• amygdala

The interactions of the limbic system with the rest of the brain have been studied using observation and analysis of behaviour as well as via deep brain stimulation and magnetic resonance imaging, among other methods.

The thalamus is a paired structure located at the centre of the limbic system and is the confluence of many neural pathways connecting to the cerebral cortex. It serves as the main relay station for information such as sensory and motor signals from and to the rest of the brain, mainly the cerebral cortex, as well as from and to other parts of the limbic system. It transforms sensory information into a more manageable form for higher brain functions. The thalamus also maintains alertness and directs a person’s attention to sensory events. Damage to the thalamus can cause motor impairments, tremors, attention problems, insomnia, memory loss, vision loss or light sensitivity, and disorders of the motor systems. Severe damage can result in a coma.

The hypothalamus is located just below the thalamus. Its most basic function is in homeostasis (maintaining a steady internal state). It controls autonomic functions such as hunger, thirst, body temperature, blood pressure, and heart rate. It is the interface between the nervous and endocrine systems and regulates sexual activity. The hypothalamus also controls the body’s response to stress. Damage to the hypothalamus can lead to aggression, acute stress, hypothermia, hyperthermia, fatigue, weight gain or loss, and a high or low sex drive.

The basal ganglia are structures located at the top of the midbrain. They help to regulate voluntary movements, from running down a street to maintaining balance and posture. The basal ganglia are also involved in cognitive and emotional behaviour and are linked with the formation of habits and addictions. Damage to these structures can cause tremors, involuntary muscle movements, and abnormal posture and is associated with movement-based disorders such as Parkinson disease and Huntington disease.

The cingulate gyrus is part of the limbic cortex, the outermost region of the limbic lobe. This area is believed to help regulate emotions, behaviour (primarily aggression), and pain as well as control autonomic motor function. Its frontal portion links sights and smells with memories of previous emotions.

The hippocampus is the region of the brain associated with memory. It is a paired structure with two parts, each shaped like a seahorse and curving back in its respective lobe from the corresponding amygdala. The hippocampus is thought to be principally involved in storing long-term memories and in spatial processing and navigation. However, the exact process by which the hippocampus helps store memories is subject to debate. The hippocampus is also known as a site for neurogenesis, whereby new nerve cells are created from adult stem cells. Damage to this region can lead to memory impairment and anterograde amnesia. People with Alzheimer disease have shown cell loss in the hippocampus.

The amygdala is a small almond-shape structure that exists in two parts, each located in front of the corresponding hippocampus in its respective temporal lobe. The amygdala plays a key role in mediating many aspects of emotional learning and behaviour. It helps evaluate the emotional valence of situations, recognize threats, and prepare the body for fight-or-flight reactions by increasing heart rate and breathing rate. Due to its close proximity to the hippocampus, the amygdala helps modulate memory storage, especially of memories associated with strong emotions. Damage to the amygdala can result in higher levels of aggression, irritability, poor emotional control, and an inability to recognize emotions. Damage can also affect a person’s ability to recognize fear, resulting in more risk-taking.

How the limbic system affects health and well-being

The individual components of the limbic system play key roles in a person’s emotions. That is why the limbic system is sometimes referred to as the emotional nervous system. Fear conditioning is driven by an interplay between the hippocampus, amygdala, and prefrontal cortex. When a person experiences fear in a new situation to which no prior emotional experience is attached, the hippocampus activates. However, if the person feels fear in a situation because of previous experience (conditioning), the amygdala additionally activates. Failure of the hippocampus to modulate the amygdala’s activity can lead to anxiety disorders.

The body uses a fight-or-flight response in what it perceives to be life-threatening situations. The amygdala is activated by neural signals indicating stress, and it in turn activates the hypothalamus, which sends signals to the adrenal glands to facilitate the release of adrenaline into the bloodstream. The hypothalamus also suppresses the discharge once the stress has passed. In the event of continued stress, it facilitates cortisol secretion, putting the body on high alert. If this continues for long periods of time, there is a risk of psychological and physical harm.

The limbic system undergoes several changes at the time of puberty. Development of the amygdala, along with hormonal changes, may lead to feelings of rage, fear, excitement, aggression, and sexual attraction. The prefrontal cortex also exerts greater control over the limbic system during puberty, helping to better regulate impulse control and the processing of emotions. As emotion processing improves, teens get better at understanding and interpreting others.

Numerous health disorders, particularly neuropsychiatric ones, are associated with damage to or improper functioning of one or more components of the limbic system. Temporal lobe epilepsy, the most common type of epilepsy in adults, is most often caused by hippocampal sclerosis. People experiencing schizophrenia have shown smaller-size limbic system components. Dysfunction in the anterior limbic network—the thalamus and amygdala along with the prefrontal regions—may have a role in bipolar disorder. ADHD is associated with an enlarged hippocampus in children and adolescents. Deep brain stimulation has been studied as a potential treatment for limbic impairment.