Limbic system

Limbic System Overview

• Higher control of several critical functions:

  • Autonomic function

  • Homeostasis

  • Reproduction

  • Emotion

  • Motivated behavior

  • Learning

Components of the Limbic System

• Limbic Lobe (Paul Broca) includes:

  • Septal area

  • Cingulate gyrus

  • Parahippocampal gyrus (with hippocampus and amygdala)

  • Uncus

  • mnemonic: Some Cats Play Hard And Unpredictable

• The Limbic System consists of the Limbic Lobe and all its connections (Papez Circuit):

  • Hypothalamus

  • Thalamus

  • Brainstem reticular formation

  • Epithalamus

  • Olfactory cortex

  • Prefrontal cortex

  • mnemonic: Hungry Tigers Bite Every Old Patient

Hippocampal Formation

• Components:

  • Part of the parahippocampal gyrus

  • Hippocampus

  • Dentate gyrus

  • Subiculum

• Afferents:

  • Entorhinal area

  • Contralateral hippocampus (via hippocampal commissure)

  • Amygdala

  • Thalamus

  • Locus coeruleus

  • Raphe nuclei

  • Ventral tegmental area

  • mnemonic: Every Hippo Always Thinks Learning Really Valuable

Functions and Efferents of the Hippocampus

• Efferents:

  • Project via the fornix to the:

    • Septal area

    • Mammillary bodies of the hypothalamus

    • Mammillary bodies project to anterior nucleus of thalamus, then to cingulate gyrus

    • mnemonic: Funny Students Make Awesome Choices

• Function:

  • Holds a map of spatial memory

  • Involved in learning and memory, especially in converting short-term memory to long-term explicit memory

• Lesions:

  • Damage results in defects in explicit memory and spatial orientation

Patient Case Study: H.M.

• Underwent bilateral removal of temporal lobe portions due to epilepsy

• Resulted in amnesia characterized by:

  • Inability to retain new information for more than a few minutes

  • Difficulty with spatial orientation

• Unaffected skills:

  • Learning, retaining, and improving performance in new motor skills despite no recollection of previous attempts

Types of Memory and Memory Loss

• Explicit (Declarative) Memory:

  • Episodic (events)

  • Semantic (facts) [associated with medial temporal lobe]

• Implicit (Non-declarative) Memory:

  • Priming (neocortex)

  • Procedural (skills, habits) [striatum]

  • Emotional responses to associative learning [amygdala]

  • Musculoskeletal responses to associative learning [cerebellum]

• Amnesia Types:

  • Retrograde

  • Anterograde

  • Global

  • Modality-specific

Amygdala Functionality

• Afferents:

  • Autonomic, sensory, auditory, and visual information converge here

• Efferents:

  • Exit via the stria terminalis to

    • Septal nuclei

    • Hypothalamus

    • Brainstem

    • Cortex

• Function:

  • Important for affective perception of sensory stimuli

  • Stimulation can evoke strong feelings of fear

• Lesion Effects:

  • Lesions result in diminished fear conditioning capacity

Klüver-Bucy Syndrome

• Caused by bilateral lesion of temporal lobes (including amygdala and hippocampal formation)

• Symptoms include:

  • Visual agnosia (psychic blindness)

  • Hyperorality

  • Hypersexuality

  • Docility

  • Apathy

  • Bulimia

  • Memory deficits

Korsakoff's Psychosis

• Associated with thiamine deficiency in severe alcoholism

• Results in anterograde and progressively retrograde amnesia

Temporal Lobe Epilepsy

• Symptoms may include:

  • Olfactory hallucinations (uncinate fits)

  • Visual hallucinations (déjà vu)

  • Gustatory and auditory hallucinations

  • Motor acts connected to seizures (e.g., lip smacking)

  • Amnesia lasting several hours, sometimes aggressive behavior

Septal Area

• Afferents/Efferents:

  • Connected to hippocampus (via fornix), amygdala (via stria terminalis), hypothalamus, midbrain, habenula (via stria medullaris thalami), thalamus, cingulate gyrus

• Function:

  • Involved in emotional behavior

  • Stimulation can result in transient rage, increased water consumption, and reward reactions