Enumerate the components of the limbic system and describe its functions.

Components

The Papez Circuit

Hippocampus → Fornix → Mammillary body → Mammillothalamic tract → Anterior thalamic nucleus → Cingulate gyrus → Back to the hippocampus

• Originally proposed as the anatomical basis of emotion; now regarded as more important for memory

Functions

• Emotion — fear, rage, pleasure, anxiety
• Motivation and drives — the "four F's": feeding, fighting, fleeing and reproduction
• Memory — hippocampus is essential for consolidation of declarative memory
• Olfaction — the only sensation with direct limbic connections, which is why smells evoke emotion and memory
• Autonomic and endocrine control, through the hypothalamus
• Reward and punishment — the mesolimbic dopamine pathway from the ventral tegmental area to the nucleus accumbens

The Amygdala

• The centre for fear and aggression, and for attaching emotional significance to a stimulus
• Stimulation → rage and fear; ablation → placidity and loss of fear
• Klüver–Bucy syndrome — bilateral temporal lobe and amygdala damage → placidity, hyperorality, hypersexuality, visual agnosia and loss of fear

Applied Aspects

• Sham rage — decortication leaves the hypothalamus unrestrained → violent but undirected rage on trivial stimuli
• Temporal lobe epilepsy — arises in limbic structures; may produce olfactory hallucination, déjà vu, fear and automatisms
• The reward pathway is the substrate of addiction — all drugs of dependence raise dopamine in the nucleus accumbens
• Depression and anxiety disorders involve limbic dysfunction, particularly of the amygdala and cingulate cortex
• Post-traumatic stress disorder — an over-active amygdala with impaired prefrontal regulation
• Prefrontal leucotomy, once used for intractable psychiatric illness, severed limbic–frontal connections; abandoned because of the severe personality change it caused