NEUROBIOLOGY OF SLEEP

NEUROBIOLOGY OF SLEEP Study Notes

Learning Objectives

• Understanding Sleep Stages: Describe the distinctions among various sleep stages, including the behaviors exhibited and the corresponding brain electrical patterns.

• Regulation of Sleep: Explain the major brain structures and pathways involved in regulating sleep.

• Systems Promoting Wakefulness: Identify the brain areas, neurotransmitters, and triggers that facilitate wakefulness.

• Systems Promoting Sleep: Identify the brain areas, neurotransmitters, and triggers that facilitate sleep.

• Flip-Flop Switch Concept: Define and elaborate on the Flip-Flop switch and the principle of mutual inhibition in sleep regulation.

• Regulation by S or C Process: Discuss how sleep is regulated by the circadian (C) and sleep homeostatic (S) processes.

• Sleep Disorders: Analyze and discuss 3-5 various sleep disorders.

Relevant Reading

• Textbook Reference: Chapter 14, Biological Rhythms & Sleep (Covers Learning Objectives in sections 14.3-14.6).

• Comic Reference: "The Allnighter" by Jorge Cham (2012) illustrates the experience and consequences of pulling an all-nighter.

Sleep Control Mechanisms

• C Process (Circadian Clock): This process induces wakefulness, suggesting a natural tendency to be awake during daylight hours.

• S Process (Sleep Homeostat): Builds up sleep pressure during wakeful hours and triggers the desire to sleep after prolonged wakefulness.

• Analysis of Sleep as Behavior: Historically, sleep has been viewed as similar to a state of pseudo-death, as described by J.W. Waterhouse in his work "Sleep and His Half Brother Death" (1874).

Measuring Brain Activity - EEG

• Electroencephalography (EEG): Invented in 1924, EEG is a method that measures voltage changes from neuronal firing. It is limited to recording from surface cortical areas due to scalp electrode placement.

  • Significance: Recognized as a crucial invention in clinical neurology by David Millet.

  • Neuronal Activity:

  • Low Amplitude, Fast Frequency: Represents neurons not firing in synchrony.

  • High Amplitude, Slow Frequency: Indicates neurons firing in unison, associated with higher levels of sleep.

Stages of Sleep

• Active Brain During Sleep (Discovered in 1957): Sleep is categorized into 4 distinct stages per cycle of approximately 90 minutes:

  • Stage 1: Light sleep.

  • Stage 2: Disengagement from consciousness, characterized by decreased body temperature and slowed bodily functions.

  • Stage 3: Deep sleep, often referred to as slow-wave sleep.

  • REM Sleep: Also known as paradoxical sleep, where brain activity is similar to wakefulness but the body is effectively paralyzed.

EEG Patterns by Sleep Stages

• Sleep Spindles: 12-14 Hz patterns indicative of certain sleep states.

• K Complexes: Sharp waves that can indicate reactions to external stimuli during sleep.

• Theta Waves: 4-7 Hz bands, generally found in stages of light sleep.

• Alpha Waves: Ranging from 8-13 Hz, associated with relaxed wakefulness.

• Beta Waves: Fast frequencies (13-30 Hz), present during alertness.

• Delta Waves: 0.5-4 Hz, significant during deep sleep.

Sleep Architecture Over Time

• Changes Overnight: Earlier in the night, NREM sleep and stage 3 are more prevalent, while REM sleep becomes more prominent as the night progresses.

• Changes Through Lifespan:

  • Sleep duration decreases with age.

  • Proportion of REM sleep also decreases with age.

The Importance of Sleep

• Consequences of Sleep Deprivation: Results can include:

  • Decreased attention.

  • Increased irritability and difficulty in concentrating.

  • Cognitive disorientation.

  • Hallucinations in extreme cases.

  • Weakened immune system leading to illness.

  • Notably, death can occur due to extreme sleep deprivation, as observed in rat studies.

Leading Theories on Sleep Functions

• Energy Conservation: Sleep may serve to conserve bodily energy.

• Niche Adaptation: It allows species to engage in activities during times they are most adapted to.

• Body Restoration: Optimal number of sleep hours is crucial for maintaining health and physiological repair.

  • The glymphatic system, responsible for clearing waste products, operates more efficiently during sleep.

• Memory Consolidation: Sleep strengthens synapses linked to recent learning while pruning less useful connections.

Sleep Disorders

• REM Behavior Disorder: Involves the intrusion of REM sleep into wakefulness, potentially leading to dangerous behaviors.

• Narcolepsy: Characterized by symptoms like sudden sleep attacks, cataplexy, hypnagogic hallucinations, and disturbances in normal sleep patterns.

• Parasomnias: Various sleep disorders that occur during sleep, including:

  • Sleepwalking.

  • Night terrors.

  • Sleep-related eating disorders.

• Breathing and Movement Disorders: Such as obstructive sleep apnea and restless leg syndrome.

• Disrupted Sleep Rhythms: Conditions like non-24h disorder, shift work syndrome, and jet lag that affect the timing of sleep.

Regulation of Sleep

Brain Systems Involved in Sleep Regulation

1. Forebrain System: Responsible for displaying slow-wave sleep by itself.

2. Brainstem System: Activates the forebrain and facilitates wakefulness.

3. Pontine System: Triggers REM sleep.

4. Hypothalamic System: Influences the other three systems to decide the brain's state at any given time.

Neurotransmitters and Sleep

• Locus Coeruleus (LC): Located in the brainstem, it produces norepinephrine (NE) and is linked to wakefulness. Activation of LC is associated with alert states.

Optogenetics in Research

• Optogenetics Techniques: Genetically introduced opsin actuators allow researchers to manipulate neural activity. For instance:

  • Channelrhodopsin-2 (ChR2) can be stimulated to alter ion flow and neuronal activity.

  • Halorhodopsin (NpHR) inhibits neural activity.

Hypothalamic Contributions

• Anterior Hypothalamus (vlPOA): Produces GABA, inhibiting the ascending arousal pathways (AAP) during sleep and contains "sleep neurons".

• Lateral Hypothalamus: Produces orexin (hypocretin), a key player in promoting wakefulness and facilitating transitions between sleep states.

  • Narcolepsy: Often linked to the loss of orexin signaling.

Mutual Inhibition and the Flip-Flop Switch

• Functionality:

  • Waking state involves activation of orexin neurons and wake neurons while sleep neurons (vlPOA) are off.

  • Only one system can be active at a time due to mutual inhibition.

Regulation of Sleep Cycles

• Sophisticated Neural Circuits:

  • A brief analysis of how different neurotransmitters and neural pathways interact to control transitions between wakefulness and sleep, including the regulation of both REM and NREM states.

Circadian Mechanisms and Adenosine Role

• Interaction of Two Processes: The C process (circadian rhythm) and S process (sleep homeostat) significantly interact throughout the day and night influencing sleep behavior.

• Adenosine: A purine nucleoside released by astrocytes that is implicated in sleep pressure. Its levels increase during wakefulness and decrease during sleep. Investigations reveal that adenosine promotes sleep when introduced and blocking its receptor (e.g., using caffeine) leads to sleep disturbances by inhibiting orexin signaling.

Melatonin Regulation

• Role of Melatonin:

  • Produced by the pineal gland, melatonin's synthesis is inhibited by the SCN during daylight and increased during darkness, influencing sleep onset.

• Implications for Sleep Disturbances: Jet lag is attributed to melatonin’s delayed adjustment to new light-dark cycles.

Summary of Sleep Regulation

• Sleep is a complex and active process influenced by interactions between multiple neural circuits. Disruptions in these circuits may lead to various sleep disorders affecting overall health.

Upcoming Activities

• To-Do Lists: Engage with the assigned reading, work on collaborative projects, draft proposals, and conduct peer evaluations as planned for the upcoming week.