Limbic System Lecture

Module 5 Overview of Important Brain Structures

• Focus on functional anatomy rather than physiology.

Limbic System and Cerebral Cortex

1. Identify important structures.

2. Summarize their function.

3. Summarize their inputs (type of information and approximate origin).

4. Summarize their outputs (where do they send information and why?).

Pain Pathways and the Limbic System

Ascending Pain Pathways

• Fast Pain (Sharp, localized):
    - Major fiber type: Aδ fibers
    - Function: Immediate pain sensation.

• Slow Pain (Dull, chronic):
    - Major fiber type: C fibers
    - Role: Strongly influences the limbic system by evoking emotional responses.

Limbic System Functions

• Major functions are related to:
      - Self-preservation
      - Species preservation

• Functions include:
      - Emotional state of mind and feelings associated with sensory information.
      - Regulation of autonomic and endocrine responses to emotional stimuli.
      - Setting levels of arousal.
      - Involvement in motivation and reinforcing behaviors.
      - Critical for long-term declarative memory.
      - Direct connections to the olfactory system and indirect connections from all other sensory modalities.

• Limbic System also known as the paleomammalian system.

Historical Perspectives on the Limbic System

Paul Broca (1824-1880)

• Defined initial structures of the limbic system:
    - Cingulate gyrus
    - Parahippocampal gyrus
    - Hippocampal formation

Components of the Limbic System

• Papez Circuit:
    - James Papez (1937) noted this system includes:
      - Hypothalamus
      - Anterior thalamic nucleus
      - Cingulate gyrus
      - Hippocampus
      - These components elaborate central emotional functions and emotional expression.

Expanded Limbic System

Key Structures

• Anterior commissure

• Stria medullaris

• Cingulate gyrus

• Septal nuclei

• Thalamus

• Corpus callosum

• Habenular nucleus

• Fornix

• Olfactory bulb

• Additional structures: Rhinal sulcus, Dentate gyrus, Mammillary body, Amygdaloid complex
  

Basic Summary of Connections

• Involved in:
      - Autonomic nervous system (ANS) control
      - Endocrine function
      - Memory and emotion connection
      - Motivation and reward systems

Hypothalamus: Autonomic and Endocrine Control Center

• Receives information regarding hormonal systems status.

• Integrates data from both central and peripheral nervous systems.

• Functions include:
      - Direct sensing of hormone levels.
      - Coordinates responses via autonomic nervous system and endocrine systems.

Basal Ganglia and Dopamine Pathways

Components

• Substantia nigra pars compacta and ventral tegmental area (VTA)
    - Important for dopamine pathways related to reward and motivation.

• Key Pathways:
      - Nigrostriatal pathway: Projects from SN to striatum; influences action selection.
      - Mesolimbic pathway: Projects from VTA to nucleus accumbens (NAc); involved in reward and reinforcement.

Implications of Narcotics

• Narcotics can elevate dopamine levels in the NAc leading to addiction.

Neural Pathways and Hunger Responses

1. Indirect and Direct Pathways:
       - Facilitate movement
       - Affect feelings associated with hunger through different dopamine receptors.

2. Dopamine Receptors:
       - D2: Gi coupled
       - D1: Gs coupled

Reward and Hedonistic Responses

• Rewarding stimuli can create:
      - Unconscious desire to engage in activities (reinforcement or “wanting”).
      - Conscious pleasure from the reward (hedonism or “liking”).

Pleasure and “Liking” Experience

• Involves numerous anatomical hotspots with activities correlating with pleasure and distaste in imaging studies, highly sensitive to opioids (Berridge & Kringelbach, 2015).

Memory, Motivation, and Emotion in the Limbic System

• Memory Types:
      - Declarative memory managed chiefly within hippocampus and prefrontal cortex.
    - Types of memory influenced by emotional context include:
      - “What, where, when, and how we feel about it.”

The Hippocampus and Memory Function

Important Features

• Hippocampal Amnesia:
    - Example of Henry Molaison (1926-2008), who underwent bilateral medial temporal lobectomy:
        - Loss of long-term semantic and episodic memory with intact working memory.
        - Relatively intact spatial memory.

Memory Types

• Explicit memory (conscious)

• Implicit memory (unconscious)

• Main roles of the hippocampus include:
      - Spatial learning and navigation
      - Episodic/event memory
      - Associational recollection

Mechanisms of Memory Formation

1. Inputs received:
       - “What?”, “Where?”, “When?”, and “Feel?” related to emotional context.

2. Memory Encoding:
       - Based on the strength of feelings associated with experiences.

The Hippocampus Structure and Circuitry

Specific Components

• Trisynaptic Circuit: Includes the CA regions (CA1, CA2, CA3) and Dentate gyrus interconnected via perforant path from the entorhinal cortex.

• Molecular Mechanisms:
      - Long-Term Potentiation (LTP) facilitated through high levels of NMDA-mediated Ca2+ entry.
      - NMDAR-dependent LTP and LTD implications for synaptic changes in learning.

Amygdala and Emotion

Functional Overview

• Location: Anterior medial temporal lobe, dorsal to anterior tip of hippocampus.

• Key Roles:
      - Emotion processing
      - Drive regulation

• Input Sources:
      - Direct and indirect sensory inputs, including from higher cortical areas.

• Output Destinations:
      - Influences responses across various brain structures, including motor areas and the hypothalamus.

Emotional Responses and Conditioning

• Fear Conditioning: Utilizes visual and sympathetic responses, with the amygdala enhancing reaction to perceived threats.

The Neuroanatomy of Fear Conditioning

• The amygdala processes visual stimuli relevant to situational threats, strongly linked to physiological responses such as heart rate and blood pressure adjustments.