Lecture 19 Sleep

Lecture Summary: Sleep and Its Functions

Goals

  • Define sleep and explore why it is essential.
  • Understand the Two-Process Sleep Model, incorporating Circadian Rhythms (C) and Homeostatic Sleep Processes (S).
  • Examine the neural basis of sleep, including brain regions and neurotransmitters involved.
  • Discuss the relationship between sleep and memory consolidation.
  • Analyze the metabolic, endocrine, emotional, and cognitive effects of sleep deprivation.
  • Explore REM sleep, its role in dreaming, and its functional hypotheses.

What is Sleep?

  • Sleep is a recurring state characterized by:
    • Reduced sensory input and environmental interaction.
    • Muscular inhibition (paralysis during certain phases).
    • Altered brain activity measurable via EEG (electroencephalography).
  • Sleep phases in mammals, birds, and reptiles include:
    • NREM (Non-Rapid Eye Movement): Stages 1-4, with progressively deeper sleep culminating in slow-wave sleep (SWS).
    • REM (Rapid Eye Movement): EEG similar to wakefulness, with vivid dreaming and muscular paralysis.

Universality of Sleep

  • Nearly all organisms demonstrate rest-activity cycles tied to light-dark rhythms (circadian rhythms).
  • No species has been found to entirely forgo sleep without compensatory mechanisms or severe health consequences.

Why Do We Sleep?

  • Survival Dependence:
    • Sleep deprivation leads to death in extreme cases (e.g., Fatal Familial Insomnia, experimental studies on rats).
    • Rats deprived of REM or total sleep develop:
      • Increased food intake with weight loss.
      • Impaired thermoregulation and immune failure.
      • Death due to infections.
  • Biological Necessity:
    • Sleep is necessary for the brain to:
      • Remove toxins (e.g., glymphatic system clearing beta-amyloid and tau proteins linked to Alzheimer’s disease).
      • Replenish glycogen in glial cells.
      • Normalize synaptic activity to prevent saturation and energy overuse.
  • Metabolic Conservation:
    • Sleep conserves energy (46 calories/hour vs. 102 while awake and sedentary).

The Two-Process Sleep Model

Circadian Rhythms (C)

  • Governed by the Suprachiasmatic Nucleus (SCN) in the hypothalamus, which acts as the master clock.
  • Light entrainment:
    • SCN receives input from melanopsin-containing retinal ganglion cells via the retino-hypothalamic pathway.
    • Melatonin production signals darkness and facilitates sleep.
    • Orexin production signals wakefulness and mobilizes energy.

Homeostatic Sleep Process (S)

  • Regulated by the buildup of adenosine, a byproduct of ATP metabolism:
    • Adenosine agonists (e.g., barbiturates) promote sleep.
    • Adenosine antagonists (e.g., caffeine) delay sleep onset.
  • Adenosine accumulates throughout the day, increasing sleep pressure.

Neural Basis of Sleep

Key Historical Discoveries

  • Moruzzi and Magoun (1949): Identified the Ascending Reticular Activating System (ARAS) as crucial for wakefulness.
  • Walter Hess: Demonstrated that electrical stimulation of the thalamus induces sleep.

Key Brain Regions and Systems

  • Suprachiasmatic Nucleus (SCN):
    • Synchronizes circadian rhythms with light-dark cycles.
    • Regulates melatonin and orexin systems.
  • Thalamus:
    • Generates up and down states in NREM sleep.
    • Disconnection from sensory input during down states aids memory consolidation.
  • Brainstem:
    • Initiates REM sleep via acetylcholine neurons in the Pons.
    • Produces PGO waves, critical for REM sleep and dreaming.

REM Sleep

  • EEG during REM resembles wakefulness but with sensory disconnection and motor paralysis.
  • Functions and Hypotheses:
    • Memory Consolidation:
      • Reinforces procedural and declarative memories.
      • Synaptic remodeling (weakening and strengthening) during REM and SWS.
    • Development:
      • High REM activity in infants may aid brain circuit maturation.
    • Dreaming:
      • May simulate coping strategies or prune unnecessary memories.

Sleep and Memory

  • Sleep improves both declarative and procedural memory.
  • Hippocampal replay during SWS trains cortical areas in memory consolidation.
  • REM sleep fine-tunes memories, potentially by reducing “noise.”

Consequences of Sleep Deprivation

Metabolic and Endocrine Effects

  • Increased cortisol levels and sympathetic activation.
  • Insulin resistance (prediabetic states) and reduced testosterone in males.
  • Dysregulation of hunger hormones:
    • Decreased leptin (satiety hormone).
    • Increased ghrelin (hunger hormone).

Cognitive and Emotional Effects

  • Impaired memory consolidation.
  • Increased likelihood of emotional volatility and unethical behavior.
  • Heightened susceptibility to false confessions and paranoia.

Key Terms

  • REM Sleep: Sleep phase characterized by rapid eye movements and vivid dreaming.
  • NREM Sleep: Includes slow-wave sleep, crucial for memory consolidation.
  • Glymphatic System: Facilitates toxin clearance during sleep.
  • Adenosine: A neuromodulator promoting sleep via accumulation.
  • Suprachiasmatic Nucleus (SCN): The brain’s master clock, regulating circadian rhythms.

Conclusion

This lecture examined the intricate processes underpinning sleep, emphasizing its necessity for memory, energy conservation, and overall health. Despite significant advances, the core functions of sleep continue to be a topic of ongoing research and discovery.

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