Limbic System Notes

The Limbic System: HOME

Introduction

  • LU: Leids Universitair
  • MC: Medisch Centrum
  • Lecturer: Egbert Lakke (E.A.J.F.Lakke@lumc.nl)
  • Subject: Anatomy and Embryology

The Limbic Lobe (Broca, 1878)

  • Cortical areas surrounding the callosal body and brainstem.
  • Distinct from surrounding cortices: allocortex.
  • Components:
    • Septal area
    • Cingulate gyrus
    • Parahippocampal gyrus
    • Hippocampus

Papez Circuit (1937): Theory of Emotion

  • Includes hypothalamus and parts of the thalamus.
  • Links the limbic lobe to emotional and motivational behavior.

Allocortex and Emotions

  • Emotions are necessary for survival.
    • Fear triggers escape from predators.
    • Aggression defends territory, offspring, and food sources.
  • Emotions facilitate quick, complex survival decisions.
  • Close ties with the HPA (Hypothalamic-Pituitary-Adrenal) system.

The Limbic System (MacLean, 1952): The Triune Brain

  • Additions to the limbic system:
    • Accumbens nucleus
    • Orbitofrontal cortex
    • Amygdala
  • Emphasizes the importance of emotions.
  • Overrules stereotyped brainstem behavior.

Limbohypothalamic Complex

  • Key structures within the complex include:
    • Gyrus cinguli
    • Orbitofrontal cortex
    • Olfactory bulb
    • Thalamic nuclei
    • Habenulae
    • LMA (likely Lateral Medullary Area)
    • Corpus mamillare
    • Prepiriform cortex
    • Hypophysis
    • Amygdala
    • Hippocampus
    • Parahippocampal gyrus

The "Modern" Limbic System: HOME

  • HOME represents the key functions:
    • Homeostasis:
      • Endocrine system
      • Autonomic nervous system
    • Olfaction
    • Memory
    • Emotions and Drives
  • A multifunctional system, not defined exactly.

Homeostasis

  • Hypothalamus:
    • Visceromotor output of the limbic system.

Hypothalamus (HeaL)

  • Homeostasis:
    • Endocrine system
    • Autonomic nervous system
  • Limbic output system:
    • Coordination of visceral and motor responses. (HeaL)

Hypothalamus and Pituitary

  • Hypothalamus controls the pituitary.
    • Dictates activity level based on CNS information.
  • Pituitary controls the endocrine glands.
    • Maintains activity level using feedback systems.

Autonomic Nervous System

  • Visceral motor nuclei in the hypothalamus.
  • Preganglionic neurons.
  • Autonomic ganglia and autonomic nuclei in the brainstem & spinal cord feed into visceral effectors, like:
    • Smooth muscle
    • Cardiac muscle
    • Glands
    • Adipocytes
  • Ganglionic spinal neurons.

Sympathetic and Parasympathetic Nervous Systems

  • Sympathetic Nervous System
    • Sympathetic outflow to smooth muscle of hair follicles, sweat glands, and peripheral blood vessels.
    • Sympathetic outflow to organs of the head and trunk.
  • Parasympathetic Nervous System
    • Includes cranial nerves (VII, IX, X) and sacral spinal nerves.
    • Ganglia such as ciliary, pterygopalatine, submandibular, and otic.
    • Vagus nerve (X) innervates heart, bronchi, lungs, stomach, spleen, pancreas, intestines, etc.
  • Illustrative Diagram displaying:
    • Sympathetic chain ganglia
    • Celiac ganglia
    • Superior and inferior mesenteric ganglia
    • Pelvic ganglia
    • Preganglionic and postganglionic nerve fibers

Olfaction

  • Nasal Cavity:
    • Nares.
    • Coarse dust filter.
    • Vestibulum (= Nostril: nose picking).
    • Respiratory segment (Turbinates).
    • Olfactory segment.
    • Nasopharynx.

Olfactory Epithelium

  • Small area high up on medial and lateral walls of the nasal cavity.
  • Olfactory receptor cells with cilia embedded in mucus.
  • Odorants must first dissolve in mucus.
  • Fila olfactoria: The 1st cranial nerve.

Primary Olfactory Cortex

  • Piriform cortex.
  • Amygdala.
  • Entorhinal cortex.
  • Direct link to the hippocampus, but hippocampus appears to play no role in olfaction.

Memory

  • Memory Systems:
    • Sensory Memory
    • Short-Term/Working Memory
    • Long-Term Memory
      • Explicit (Conscious/Declarative):
        • Episodic (Events)
        • Semantic (Facts)
      • Implicit (Unconscious/Non-declarative):
        • Procedural (Skills)

Memory Types

  • Declarative Memory:
    • Medial temporal lobe; diencephalon.
    • Facts and Events.
  • Procedural Memory:
    • Striatum.
    • Skills and habits.
  • Nondeclarative Memory:
    • Classical conditioning.
      • Skeletal musculature (Cerebellum).
      • Emotional responses (Amygdala).

Declarative Memory: The Hippocampus

  • Allocortex.
  • Limbic system.
  • Key Structures:
    • Hippocampus
    • Rhinal sulcus
    • Lateral ventricle
    • Thalamus
    • Entorhinal cortex
    • Perirhinal cortex
    • Parahippocampal cortex

Hippocampal Circuitry

  • Entorhinal cortex to dentate gyrus: Perforant path (1).
  • Dentate gyrus to CA3: Mossy fibers (2).
  • CA3 to mammillary bodies: Fornix.
  • CA3 to CA1: Schaffer collateral (3).

Papez Circuit and Memory

  • Papez circuit is involved in memory, rather than emotions.
  • Clinical relevance:
    • Korsakoff's syndrome
    • Henry Molaison (patient H.M.)
  • Circuit Components and function:
    • Neocortex $\rightarrow$ Cingulate gyrus (Emotional coloring)
    • Cingulate cortex $\rightarrow$ Hippocampus (Emotional experience)
    • Hippocampus $\rightarrow$ Hypothalamus (Emotional expression)
    • Hypothalamus $\rightarrow$ Anterior nuclei of thalamus $\rightarrow$ Cingulate Cortex

Long Term Potentiation (LTP)

  • Tetanic stimulation of Schaffer collaterals induces LTP.
  • Parameters: 50-100 Hz, 0.5 sec pulse, 1 pulse/min, 15-30 mins.
  • Input specific.

Emotion (and Motivation)

  • Bilateral amygdalectomy $\rightarrow$ Kluver-Bucy syndrome:
    • Docility (diminished fear/aggression; lack of emotional response).
    • Hyperorality (compulsion to examine orally).
    • Hypersexuality (tendency to seek stimulation from unusual or inappropriate objects).
    • Visual agnosia (ventral stream lesion).
    • Hyperphagia (overeating, eating inappropriate stuff).
    • Hypermetamorphosis.
  • Amygdala: the emotional center of the limbic system.

Amygdala and Fear

  • Coarse visual processing in the thalamus alerts the amygdala.
    • Initiates startle reflexes (ANS; Fight, Flight, Freeze - FFF response).
  • Precise visual processing in visual cortices may suppress the amygdala.
    • Example: Recognizing a stick is harmless.

Brain Pathways

  • Path of the Calm Brain
  • Path of the Hijacked Brain

Amygdala and Emotion

  • Emotions are necessary for survival.
    • Fear triggers escape from predators.
    • Aggression defends territory, offspring, and food sources.
  • Emotions are used to make quick, often complex (survival) decisions.
  • Close ties with the HPA system.
  • Amygdala likened to a spider in the web of emotional control and expression.

Amygdala and Hypothalamus in Aggression

  • Aggressive behavior is initiated via the amygdala.
  • Hypothalamus refines the type of aggression expressed:
    • Predatory aggression vs. Affective aggression.
      • For food vs. For love.
      • Non-ANS activity vs. ANS activity.
      • Lateral hypothalamus vs. Medial hypothalamus.
  • Amygdala establishes links between stimuli and their emotional value.

Diffuse Modulatory Systems and Arousal

  • Amygdala output targets:
    • RN (Raphe Nuclei)
    • VTA (Ventral Tegmental Area)
    • PPTg (Pedunculopontine Tegmental nucleus)
    • LC (Locus Coeruleus)
    • SNc (Substantia Nigra pars compacta)
    • CUN (Cuneiform nucleus)
    • PAG (Periaqueductal Gray)
    • motX (Motor cortex)
    • SOL (Solitary nucleus)
    • PB (Parabrachial nucleus)
  • Involved in:
    • Control of mood & emotion
    • Reward-based reinforcement
    • General brain excitability

The Limbic System: Evolutionary Perspective

  • (Sub)cortical structures in ventral and medial hemispheres.
  • Evolutionarily ancient:
    • Entire hemisphere in reptiles (MacLean).
    • Survival!

The Limbic System: Function

  • A group of interconnected cortical and subcortical structures dedicated to linking visceral states and emotion to cognition and behavior.
  • Functions:
    • Integration of olfactory, visceral, and somatic impulses.
    • Control of "survival activities" of individual and species.
    • Emotional behavior.
    • Retention of recent memory.

The Limbic System: Components and Connections

  • HOME (Homeostasis, Olfaction, Memory, Emotions and Drives)
  • Key components:
    • Limbic cortex
    • Septal area and basal forebrain
    • Thalamus and epithalamus
    • Basal ganglia
    • Olfactory bulb and olfactory cortex
    • Hippocampal formation
    • Amygdala
    • Hypothalamus
    • Brainstem
  • Connections to other cortical areas, and spinal cord, cranial nerve, and neurohumoral pathways.