Limbic System Lecture

Limbic System Overview

  • Lecturer: Rola Mahmoud, MD

  • Position: Associate Professor of Neurology, Associate Program Director, Neurology Residency, University of Missouri-Kansas City

  • Affiliation: Kansas City Physician Partners, University Health

Objectives

  • Review limbic system function and key structures

  • Overview of limbic circuitry

  • Discuss specific limbic system structures

  • Review important clinical disorders of the limbic system

Limbic System Description

  • Definition: Network of brain structures located on the medial and inferior aspects of the brain.

  • Regulation of:

    1. Homeostasis: Autonomic and neuroendocrine control (Hypothalamus/pituitary).

    2. Olfaction: Olfactory cortex.

    3. Memory: Hippocampal formation.

    4. Motivation and reward: Ventral tegmental area.

    5. Emotions and drives: Amygdala.

Importance of the Limbic System

  • Involvement in behavioral and emotional responses essential for survival, such as:

    • Feeding

    • Reproduction

    • Caring for young

    • Fight or flight responses

  • Structures are located deep within the brain, beneath the cerebral cortex and above the brainstem.

Components of the Limbic System

  • Limbic cortex:

    • Parahippocampal gyrus

    • Cingulate gyrus

    • Medial orbitofrontal cortex

    • Temporal pole

    • Anterior insula

  • Diencephalon:

    • Thalamus: Gateway to cerebral cortex, relays sensory information (anterior nucleus, dorsomedial nucleus).

    • Hypothalamus: Involved in maintaining homeostasis (mamillary bodies, ANS nuclei regulating thirst, hunger, mood).

  • Basal ganglia: Involved in pleasure, reward processing, habit formation (nucleus accumbens), and motor control.

  • Hippocampus: Central to memory formation.

  • Amygdala: Involved in emotional responses.

  • Olfactory cortex and septal nuclei: Related to pleasure, reward, reinforcement, and further cognitive processes.

Limbic Circuits

  • Description: Complex network characterized by multiple reciprocal connections.

  • Major Limbic Pathways: Refer to Table 18.4 in original material for visual representation.

    • Fornix: Connects various subcomponents of the limbic system.

    • Cingulum: Pathway associated with emotional behavior.

    • Further pathways connecting various limbic structures and their inputs/outputs are discussed.

Hippocampal Formation

  • Function: Memory formation and consolidation (conversion from short to long term).

  • Anatomical Details:

    • Medial and dorsal continuation of parahippocampal gyrus.

    • Located within the medial temporal lobe.

    • Largest anteriorly—Hippocampal head.

    • 3 components:

      1. Dentate gyrus: Afferent pathway.

      2. Subiculum: Efferent pathway.

      3. Hippocampus (Ammon’s horn): Efferent pathway.

  • Common Misidentification: People often refer to hippocampal formation simply as the hippocampus.

  • Development: Anatomical resemblance to a seahorse due to folding during development.

Functions of the Hippocampus

  • Location: Temporal horn of the lateral ventricle, positioned in CSF.

  • Functions:

    1. Memory Storage:

      • Episodic memory: Personal experiences recollection.

      • Spatial maps: Navigation and environment understanding (notably larger in London taxi drivers).

      • Neurogenesis: Site for the birth of new neurons from adult stem cells, facilitating learning and brain plasticity.

Fun Facts About Memory and the Brain

  • The Egyptians were pioneering in brain medical understanding, describing conditions such as migraines, epilepsy, and strokes.

  • Earliest known references to the human brain date back to the Edwin Smith Surgical Papyrus (approximately 1700 BC).

H.M. Case Study

  • Landmark case of amnesia post bilateral medial temporal lobe resection (including hippocampal formation):

    • Result: Inability to learn new facts or recall new experiences (anterograde amnesia).

Memory Classification from H.M.

  • Long-term Memory Types:

    1. Declarative Memory (Explicit):

      • Conscious recollection of facts and personal experiences (semantic and episodic memory).

      • Amnesia linked to bilateral medial temporal or diencephalic lesions.

      • Involvement of Papez circuit essential for encoding.

      • Dominant hemisphere lesions: More verbal memory deficits; non-dominant: Visual-spatial memory deficits.

    2. Non-declarative Memory (Implicit):

      • Semi-automatic learning of skills/habits, less accessible to consciousness.

      • Procedural memory as a key demonstration of this type.

      • Associated brain areas include basal ganglia, cerebellum, and motor cortex.

      • Non-associative learning examples: Involving cerebellum, brainstem, etc.

Memory Conversion Dynamics

  • Transition from short-term to long-term memory:

    • Involves working memory: Ability to hold multiple pieces of information (requires dorsolateral prefrontal cortex).

Memory Lost in Early Life

  • No recall for events 1-3 years of age, often termed benign senescent forgetfulness.

  • Psychogenic amnesia: Loss of autobiographical details (name, birthplace).

Limbic System Disorders

  • Herpes Simplex Virus Encephalitis:

    • Symptoms: Fever, headache, changes in consciousness, seizures.

    • High mortality if untreated, particularly devastating for infants.

    • Treatment: IV acyclovir.

  • Limbic Encephalitis: Autoimmune; can have various antibody responses affecting limbic structures.

    • Common antibody: Anti-NMDA receptor antibody.

  • Transient Global Amnesia (TGA):

    • Abrupt onset of memory issues with full recovery in 4-12 hours.

    • No known cause, presents similar to migraine or seizure.

    • Memory for the event will not be recalled post-event.

    • 15% chance of future episodes.

  • Wernicke-Korsakoff Syndrome:

    • Often seen in alcoholics; caused by vitamin B1 (thiamine) deficiency.

    • Characterized by acute neurological symptoms (Wernicke’s encephalopathy) like confusion, ataxia, and ophthalmoplegia.

    • Long-term condition can lead to Korsakoff’s syndrome, involving profound anterograde amnesia and confabulations due to damage in mammillary bodies.

Papez Circuit

  • Critical for episodic memory encoding; connects hippocampus to prefrontal cortex facilitating memory integration with cognitive processes.

  • Illustrates reciprocal links between limbic components and neural pathways involved in emotion, memory, and cognition.

Amygdala’s Functions and Connections

  • Involved in emotional responses, feeding, motivation, and behaviors.

  • Structure: Almond-shaped nuclear complex adjacent to hippocampus.

    • Two main nuclear groups:

      1. Corticomedial: Olfactory-related emotions.

      2. Basolateral: Other emotions and behaviors.

  • Functions:

    • Emotional significance of memories, fear conditioning, and implicit learning.

    • Relevant to anxiety disorders, phobias, PTSD, and theory of mind (ToM) disturbances.

Amygdala and Behavioral Influences

  • Links emotional state with behaviors (feasting when sad, sexual behaviors).

  • Control neurotransmission pathways related to hunger and sexual arousal (hypothalamus connections).

  • Involved in the mesolimbic pathway linking to reward and substance abuse.

Autonomic Nervous System Control

  • Controlled by interactive processes involving amygdala, hypothalamus, and brainstem.

  • Sympathetic (fight or flight) and parasympathetic (rest and digest) outcomes discussed.

Kluver-Bucy Syndrome

  • Rare condition characterized by symptoms such as inappropriate behaviors, memory loss, and placidity due to damage to bilateral amygdala.

Olfactory System Relations

  • Smell's role in generating sensations and taste.

  • Olfactory receptors: Hundreds of receptor types allowing for diverse odor interpretations.

  • Primary olfactory cortex receives direct sensory input, bypassing thalamic relay.

Disorders of Olfaction

  • Olfactory Neuroblastoma: Tumor arising from olfactory epithelium causing nasal symptoms.

  • Kallman Syndrome: Genetic disorder often diagnosed during puberty featuring anosmia and hypogonadism.

Effects of Limbic System Lesions

  • Can lead to:

    1. Memory challenges (new memory formation and consolidation issues).

    2. Behavioral and psychiatric abnormalities.

    3. Seizures originating from limbic structures initiated by emotional triggers.

Psychiatric Disorders and the Limbic System

  • Complex due to circuit dysfunctions:

    • Schizophrenia: Cognitive deficits related to limbic and frontal lobe dysfunction.

    • OCD: Serotonergic dysfunction linked to neural networks.

    • Anxiety: Neurotransmitter dysregulation affecting various cortical and limbic areas.

    • Depression: Involvement of several neurotransmitter systems related to mood regulation.

Assessment Questions

  • Designed to test knowledge and understanding of limbic system functions, structures, and associated disorders.

Acknowledgments

  • The lecture's materials, case studies and clinical examples are derived from Neuroanatomy Through Clinical Cases textbook by Blumenfeld and further reference points to clinical and laboratory sources.

Conclusion

  • Institute continual learning and review of clinical cases to solidify grasp of limbic system functionality and related disorders.

Additional Resources

  • Recommended videos and online resources to further understand limbic system pathways, functions, and disorders.

Thank You!

  • Acknowledgment to audience and invitation for questions.