The Limbic System

Overview

• The limbic system is a set of structures in the brain involved in regulating emotion.
• There is no consensus among experts on which structures make up the entire limbic system.
• Key mnemonic: HAT HIPPO - Hypothalamus, Amygdala, Thalamus, Hippocampus.

Main Structures

1. Thalamus

• Located as a pair of structures.
• Functions as a sensory relay station for senses such as sight, hearing, taste, and touch.
• Directs sensory inputs to the appropriate areas in the cortex and brain.
• Does not process the sense of smell directly.
• Important for how sensory inputs connect to emotions.

2. Amygdala

• Sometimes called the "aggression center."
• Stimulation of the amygdala can invoke feelings of anger, violence, fear, and anxiety.
• Destruction of the amygdala leads to a "mellowing" effect.
• Kluver-Busey Syndrome: Bilateral destruction leading to symptoms like hyperorality, hypersexuality, and disinhibited behavior.
• Benzodiazepines (similar to alcohol) affect the amygdala, leading to similar effects as alcohol.

3. Hippocampus

• Plays a key role in converting short-term memory into long-term memory.
• Damage affects the ability to form new memories but does not affect old memories.
• Important in invoking emotions related to memories.

4. Hypothalamus

• Located below the thalamus; very small, less than 1% of brain volume.
• Regulates various body functions and the autonomic nervous system (ANS).
• ANS involves fight or flight vs. rest and digest.
• Controls the endocrine system by triggering hormone releases like epinephrine (adrenaline).
• Regulates basic drives such as hunger, thirst, sleep, and sex.

Conclusion

• The limbic system includes the thalamus, amygdala, hippocampus, and hypothalamus.
• These structures are pivotal in emotional regulation and memory processing.