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Limbic System

 

Explanations > Brains > Parts > Limbic System

Amygdala | Hippocampus | Thalamus | Hypothalamus | Fornix | Cingulate gyrus | Mammillary bodies | See also

 

The limbic system consists of an odd collection of separate parts. 'Limbic' comes from the Latin word for 'belt'. Most parts of the limbic system are within the telencephalon.

Amygdala

The amygdala (Greek for 'almond') is an oval group of neurons about an inch long. It has a primary role in processing and memory of emotional reactions, particularly fear. There is one in each hemisphere.

The amygdala can act as 'thinking bypass', triggering automatic reactions. In a fearful situation, sensory stimuli reach the amygdala's basolateral complexes where they form associations with memories of the stimuli. Fear responses created include freezing, increased heart rate and the release of stress hormones.

Damage to the amygdala disrupts the ability of an emotionally charged stimulus to elicit an unconscious emotional response. Electrical stimulation of the human amygdala humans leads to feelings of fear and apprehension.

People who have had their amygdala removed (for example in treatment of extreme cases of epilepsy) not only lose their ability to fear, they also do not recognize anger in others.

The amygdala is also involved in the relatively slow process of forming long-term memories.

The amygdala is related to a wide range of disorders, including borderline personality disorder, depression and autism.

Hippocampus

There are two 'seahorse'-shaped (actually more like a small banana) hippocampi, one in each hemisphere. The hippocampal formation contain the hippocampus proper (also called Ammon's horn), the dentate gyrus and subiculum.

The hippocampus has particular use in short-term memory and spatial navigation. It is also involved in creating episodic memory. It manages more factual memory than the amygdala, which is more emotionally anchored.

The dentate gyrus is involved in creating new memories and is one of the few areas in the adult brain where there is neurogenesis (birth of neurons). It is also involved in stress and depression.

The subiculum is the main output of the hippocampus and has been connected with epilepsy, working memory and drug addiction.

The hippocampus becomes damaged in Alzheimer's Disease, leading to memory problems and disorientation. Damage can also cause memory loss and problems with episodic memory. It is also related to epilepsy and repeated seizures can lead to hippocampal damage.

Thalamus

The thalamus (Greek for 'inner chamber') has two lobes which are connected by a bridge of grey matter called the massa intermedia, which also enters the third ventricle.

Most sensory input to the cerebral cortex goes to the thalamus first, where it acts as a form of switchboard, converting signals into forms that can be read by the cortex.

The full thalamus consists of the perithalamus (or prethalamus or subthalamus), the signalling center zona limitans intrathalamica (ZLI) and the thalamus proper.

Various nuclei in the thalamus have particular functions, for example:

  • The anterior nuclei modulate alertness and are involved in learning and memory.
  • The ventral anterior nucleus receives inputs from the basal ganglia and helps initiate or inhibit movement.
  • The ventral lateral nucleus receives inputs from the basal ganglia and helps learning, planning and coordination of movement.

Unlike most other parts of the limbic system, the thalamus is a part of the diencephalon rather than the telencephalon.

Hypothalamus

The hypothalamus is a small but important system at the base of the brain, underneath the thalamus, on both sides of the third ventricle.

The hypothalamus controls the Autonomic Nervous System and the endocrine system. It also integrates and coordinates behavior and expression of emotions, and organizes action around survival and 'The four F's': fighting, feeding, fleeing and fornication. It has also been implicated in learning.

Stimulation of the lateral hypothalamus in cat leads to hissing, spitting and claw extension. Lesions of the lateral hypothalamus leads to placidity. Lesions of the medial hypothalamus makes the cat highly excitable and easily triggered into aggression.

The endocrine system is managed by hormones that are produced in the hypothalamus in special neurosecretory cells.

The hypothalamus is a significant site for neurogenesis, which is dereased by stress.

The pituitary gland is a pea-sized endocrine gland situated at the bottom off the hypothalamus. It affects growth, blood pressure, metabolism and sex organ functions.

Unlike most other parts of the limbic system, the hypothalamus is a part of the diencephalon rather than the telencephalon.

Fornix

The fornix (also known as the fimbria) is a bundle of fibers that connects the hippocampus with other parts of the brain.

The posterior fibers (or postcommissural fornix) at each side reach through the hypothalamus to the mammillary bodies and to the anterior nuclei of thalamus, which maps to cingulate cortex. The anterior fibers (precommissural fornix) reach to the septal nuclei and nucleus accumbens of each hemisphere.

Cingulate gyrus

The cingulate gyrus wraps around the corpus callosum and is above by the cingulate sulcus fissure. The upper part of the cingulate gyrus is called the cingulate cortex.

It acts as an integral part of the limbic system and is involved in emotion and memory.

Mammillary bodies

The mammillary bodies are two small structures (that look like breasts), located in the lower limbic system, near the anterior arches of the fornix.

They are considered to a part of the hypothalamus and act as a relay for signals from the amygdala and hippocampus to the thalamus.

See also

Brain subdivisions

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