The three main components of the limbic system are: the hippocampus, the amygdala, the thalamus and the hypothalamus.

1. Definition of the limbic system

The limbic system is a general term for the brain tissues that evolved from the paleocortex and the old cortex in the central nervous system of higher vertebrates, as well as for the neural structures and nuclei that are in close contact with these tissues.

The major structures include the amygdala, hippocampus, thalamus, and hypothalamus.

2. Amygdala

The amygdala is a small almond-shaped structure. There is one in each of the left and right temporal lobes. The amygdala is known as the emotional center of the brain, and it is involved in assessing the emotional valence of situations (e.g., happiness, sadness, fear).

It helps the brain recognize potential threats and helps the body prepare for a fight-or-flight response by increasing the heart and breathing rate. The amygdala is also responsible for reward- or punishment-based learning. Because of its proximity to the hippocampus, the amygdala is involved in the process of memory consolidation, especially emotionally charged memories.

Emotional arousal after learning affects the strength of subsequent memories of that event, so greater emotional arousal after learning enhances one's retention of that memory. In fact, experiments have shown that applying stress hormones to subjects when tested two weeks later enhances the retention of their memories.

3. Hippocampus

The hippocampus is located deep in the temporal lobe and is shaped like a seahorse.

It consists of two horns that curve back from the amygdala. Psychologists and neuroscientists have questioned the exact role of the hippocampus, but it is generally agreed that the hippocampus plays a crucial role in forming new memories about past experiences.

Some researchers believe that the hippocampus is responsible for declarative memories in general (memories that can be articulated explicitly, such as factual and situational memories), and that damage to the hippocampus often makes it difficult to form new memories, and may also interfere with reading memories formed prior to the damage.

Although the retrograde effect usually extends for years before the brain is damaged, in some cases older memories remain intact. This is because as time passes, the hippocampus becomes less important in the storage of memories.

4. Thalamus and hypothalamus

Both the thalamus and the hypothalamus are associated with changes in emotional responses.

The thalamus is the sensory "conduit" for the rest of the brain, and its function as a structural link to the rest of the limbic system is critical. The hypothalamus is a small part of the brain located directly below the thalamus on either side of the third ventricle.

Diseases of the hypothalamus interfere with a number of unconscious bodily functions (such as breathing and metabolism) as well as with a number of so-called motivated behaviors, such as sexuality, aggression, and hunger. The lateral portion of the hypothalamus appears to be associated with pleasure and anger, while the medial portion is associated with disgust, unpleasantness, and uncontrollable laughter.