Limbic system

MEDU3300 Lecture Notes: Limbic System

Instructor: Jacque Pak Kan IP
Institution: School of Biomedical Sciences

Primary Objective

  • To understand the morphology, spatial relations, and connections of the limbic system.

Topics Covered

  • History of the limbic system

  • Morphogenesis of the cerebral hemisphere

  • Components of the limbic system

  • Connections and a glimpse of limbic functions

Major Learning Outcomes

  • Ability to locate and identify limbic structures in specimens and images.

  • Development of a robust background for understanding functions and dysfunctions, especially those related to memory and emotion.

Historical Context of the Limbic System

  • Paul Pierre Broca (1878):

    • Introduced the term "le grand lobe limbique" (the great limbic lobe).

    • The term "limbic" is derived from Latin "limbus" meaning border, referring to the curved rim of the cortex, including the cingulate and parahippocampal gyri.

  • James Papez (1937):

    • Elaborated on the role of the limbic system in emotion in his seminal paper "A proposed mechanism of emotion."

    • Proposed a circuit for emotion termed the "Papez circuit."

  • Paul D. MacLean (1952):

    • Coined the term “limbic system” to encapsulate Broca's limbic lobe and related subcortical nuclei, acting as a collective neural substrate for emotion.

    • Developed the evolutionary "Triune Brain Theory", describing the human brain as three brains in one:

    • R-complex (reptilian complex)

    • Limbic system

    • Neocortex

Morphogenesis of the Cerebral Hemisphere

  • The human nervous system begins to form early in embryonic development.

  • Phases of Neural Development:

    • By Day 19: Formation of the neural tube via ectoderm invagination (neurulation).

    • Day 22: Initial differentiation occurs.

    • Day 23: The anterior end of the tube expands to form the left and right hemispheres.

Growth and Structure

  • The hemispheres overgrow the remaining parts of the tube, forming the central part that will evolve into the diencephalon and brainstem.

  • The growth pattern results in a tripartite assembly comprising 2 hemispheres and a central part.

  • The formations of the temporal lobe:

    • Grows downward and forward, taking on a C-shape.

  • The cortex evolves from neuronal migration and aggregation, leading to a layer of gray matter (cortex) and deep nuclei.

Limbic System Overview

  • Primary Structures:

    • Limbic Lobe:

    • Cingulate gyrus

    • Septal area

    • Parahippocampal gyrus

    • Hippocampal formation

    • Other Related Structures:

    • Orbitofrontal cortex

    • Thalamus

    • Hypothalamus

    • Epithalamus (including habenula)

Morphological Features

  • Hippocampal Structure:

    • Dentate gyrus presents a serrated appearance along its length.

    • The hippocampus consists of distinct areas known as CA (cornu Ammonis), which are CA1, CA2, and CA3.

  • Fornix:

    • A fiber bundle emerging from the hippocampus, with the fimbria connected to the hippocampus and the fornix continuing elsewhere.

Connectivity and Function

  • The limbic system exhibits extensive interconnections with various brain parts, facilitating complex processes including:

    • Emotional Responses: Memory, learning, behavior related to reward and punishment.

    • Motor Function: Connections to the striatum (part of basal ganglia) contribute to motor activities.

    • Visceral and Autonomic Responses: Through the hypothalamus to the brainstem and spinal cord.

Memory and Learning

  • Papez Circuit:

    • Connects key areas involved in memory and emotion:

    • Entorhinal area

    • Hippocampus

    • Mammillary bodies

    • Anterior nuclei of the thalamus

    • Cingulate gyrus

    • Significant for declarative memory formation.

  • Fornix Role:

    • The fornix connects the hippocampus to the mammillary body and thalamus.

Clinical Applications and Implications

  • Alzheimer's Dementia and CA1 Area:

    • CA1 region in the hippocampus is notably affected in Alzheimer’s disease, epilepsy, and ischemia.

  • Korsakoff Syndrome:

    • Associated with atrophy of the mammillary body due to alcoholism, and presents with recent memory loss and confabulation.

  • Kluver-Bucy Syndrome:

    • Resulting from bilateral lesions in the amygdala, presenting with docility, oral examination of objects, altered dietary habits, and hypersexuality.

Emotional and Behavioral Connections

  • The limbic system’s connections are crucial for processing rewards and punishments, influencing decision-making and emotional experiences.

  • Disturbances in limbic structure/function can lead to various mental health disorders, including anxiety, depression, and schizophrenia.

References

  1. Alheid GF, Heimer L. 1996. Theories of basal forebrain organization and the "emotional motor system". Progress in Brain Research, Vol. 107.

  2. Rajmohan V, Mohandas E. 2007. The Limbic System. Indian J Psychiatry. Apr-Jun; 49(2): 132–139.

  3. Additional references and sources for images are included for further study.