Emotion and the Brain – Limbic & Autonomic Systems

Lesson Objectives

  • Understand main structures of the limbic system ("HAT HIPPO": Hypothalamus, Amygdala, Thalamus, HIPPOcampus)
  • Explain how these structures influence unconscious bodily functions and, most importantly, emotion
  • Evaluate consequences when any of the four structures are damaged

The Limbic System: Overview

  • Collection of evolutionarily older fore-brain structures wrapped around the thalamus
  • Governs instinctive drives (hunger, thirst, reproduction), memory formation, and the entire spectrum of human affect
  • Operates largely outside conscious awareness yet shapes subjective feeling states and observable behaviour
  • Links sensory input → physiological arousal → cognitive appraisal → behavioural output

Key Structures of the Limbic System

Hypothalamus

  • Maintains internal stability (homeostasis)
    • Regulates:
    • Hunger/thirst
    • Body temperature, circadian rhythms
    • Pain/pleasure perception
    • Hormonal drives: anger, aggression, sexual behaviour
  • Hub for rewards & punishments → establishes motivational salience
  • Initiates endocrine responses via the pituitary; critical in activating the sympathetic branch during emotion

Hippocampus

  • Seahorse-shaped structure buried in medial temporal lobe
  • Primary site for declarative (explicit) memory formation
    • Encodes experience → distributes to cortical long-term storage
  • Couples contextual details (where/when) with affective tone supplied by amygdala
  • "Tags" physical stimuli with remembered emotions → enables learned approach/avoidance

Amygdala

  • Almond-sized nuclei anterior to hippocampus
  • Central to basic emotions: especially fear, threat detection, but also pleasure/reward
  • Coordinates physiological (autonomic) & behavioural (fight, flight, freeze) response packages
  • Generates and stores new emotional memories; modulates strength of hippocampal encoding
  • Interfaces with:
    • Olfactory bulb → direct smell–emotion pathway
    • Hypothalamus → hormonal stress responses (e.g., cortisol, adrenaline)

Thalamus

  • Bilateral relay station atop brainstem
  • All sensory information except smell synapses here first
  • Functions
    • Filters/weights incoming data according to relevance
    • Sends parallel projections to amygdala (rapid, crude affective appraisal) & to higher cortex (slower, detailed analysis)
    • Regulates sleep–wake cycles and certain memory processes

Information Flow After "HAT HIPPO" Does Its Job

  • Sensory stimulus → Thalamus (triage) →
    • Fast track: thalamus → amygdala → immediate autonomic & motor response
    • Longer track: thalamus → cortex → conscious evaluation → feedback to amygdala
  • Amygdala instructs Hypothalamus to mobilise sympathetic nervous system (SNS)
  • Hippocampus binds emotional tone to explicit memory → enables future prediction & regulation

Case Study: Patient H.M.

  • 2727-year-old with intractable seizures (for 1010 years)
  • Surgical removal: bilateral hippocampi + amygdalae
  • Outcomes
    • Seizures ↓ dramatically (surgery goal achieved)
    • Severe anterograde amnesia (could not form new explicit memories)
    • Could not remember new faces, facts, or events moments after occurrence
    • Diminished capacity to form new emotional associations
  • Lessons Learned
    • Hippocampal damage → profound explicit memory loss
    • Amygdala damage → selective deficit in remembering emotional qualities of new experiences even when general memory spared

Injuries to Limbic Structures & Emotional Effects (Research Prompts)

  • Hypothalamic lesions (tumour, stroke)
    • Disrupted hunger/thirst regulation → anorexia or hyperphagia
    • Extreme rage bursts or apathy (sham rage in animals)
  • Amygdala lesions (surgical, degenerative disease)
    • Reduced fear recognition, inappropriate trust, flattened affect
    • Possibly heightened risk-taking; difficulty attaching emotional significance to stimuli
  • Thalamic stroke
    • Emotional lability; impaired sensory-emotion linkage → misinterpreting bodily states
    • Sleep-wake disturbances affecting mood regulation
  • Hippocampal sclerosis (epilepsy, hypoxia)
    • Inability to consolidate new memories → “living in the present”
    • Emotional regulation deficits due to lost contextual recall (e.g., overreaction to minor stressors)

Autonomic Nervous System (ANS) & Emotion

  • ANS = involuntary motor system controlling glands & smooth muscle
  • Divided into two complementary branches that toggle during emotional episodes

Sympathetic Nervous System (SNS)

  • Prepares body for "fight-or-flight"
    • Hypothalamus → adrenal medulla → releases epinephrine (adrenaline)
    • Physiological signatures: ↑ heart rate, ↑ blood flow to skeletal muscles, bronchodilation, analgesia
  • Adaptive in danger but chronic activation linked to stress disorders

Parasympathetic Nervous System (PNS)

  • Promotes "rest-and-digest" recovery state
    • Vagal activation → ↑ gastric activity, ↓ cardiac output, energy storage
  • Works in tandem with SNS; emotional states often involve push–pull between the two

Textual Map of the Human Nervous System

  • Central Nervous System (CNS): Brain & Spinal Cord
  • Peripheral Nervous System (PNS)
    • Somatic (voluntary muscle control)
    • Autonomic (involuntary)
    • Sympathetic (arousing)
    • Parasympathetic (calming)

Integrative Summary & Key Takeaways

  • Emotional processing is an emergent property of distributed circuits; the limbic system provides the core machinery
  • Damage to any node disrupts specific facets of emotion: appraisal (thalamus), generation (amygdala), physiological execution (hypothalamus), or contextual memory (hippocampus)
  • Case evidence (e.g., H.M.) illustrates double-dissociation between memory for facts vs. memory for feelings
  • Real-world relevance: understanding limbic injury guides treatment of PTSD, anxiety, eating disorders, and memory impairments
  • Ethical considerations: neurosurgical or pharmacological manipulation of limbic areas can alter personality; must weigh therapeutic benefits against identity changes