Sleep Physiology

Question 1

What is the function of sleep?

Answer 1

• learning - consolidation and improvement of new skills
• brain development (REM)
• repair and maintenance
• energy preservation in low-calorie evolutionary environments*
• to stay out of trouble*

*don’t explain the need for accumulation of sleep debt

Question 2

What is the structure of sleep?

Answer 2

• wakefulness
  
  • sensation and perception are vivid and externally generated
  • thought is logical and progressive
  • movement is continous and volunary
  • HR/BP/VE unstable relative to activity levels
• non-REM sleep (slow-wave sleep)
  
  • S&P dull or absent
  • thought is logical and preservative (on waking, can tell you what they were thinking)
  • movement is episodic and involuntary
  • HR/BP/VE is stable and low
• REM sleep
  
  • S&P are vivid and internally generated (dreams)
  • thought process is illogical and bizarre
  • movement is commanded but inhibited @ supraspinal level
  • HR/BP/VE are unstable
• usual progression: wakefulness –> light sleep (NR 1-2), deep sleep (NR 3-4), light sleep (NR 1-2), REM sleep, wake
• there are 4-6 sleep cycles/night, ~90-120 mintutes in length
• most deep sleep in first half, most of REM in second half

Question 3

What are circadian rhythms?

Answer 3

• biologically programmed rhythms that regulate things like sleep, hunger, libido
• can be daily (sleep), monthly (menstrual), seasonal (animals)
• sleep regulation is a combination of CR and homeostatic rhythm (pressure to sleep)

Question 4

What governs circadian rhythms?

Answer 4

• suprachiasmatic nucleus in the hypothalamus
• contains a biological clock (highly conserved)
• receives input from the retina:
  
  • rods, cones, melanopsin containing nonvisual ganglion cells
  • allows light to reset the biological clock
• receives input from the intergeniculate leaflet of the LGN (thalmus)
  
  • mediates other stimuli that reset circadian rhythms
• projects to the paraventricular nucleus, to the pineal gland which secretes melatonin
  
  • melatonin is the primary hormone involved in regulating CRs

Question 5

What are the consequences of sleep deprivation/disruption?

Answer 5

• increased risk of CVD
• increased HR variability
• <7> hours of sleep associated with earlier death
• worsening cognitive performance
• irritability
• memory lapses and impaired judgement
• severe yawning
• hallucinations
• ADHD-like symptoms
• impaired immune system
• risk of type II diabetes
• decreased reaction time
• tremors
• aches
• growth suppression
• risk of obesity
• decreased temperature

Question 6

What are the two major ascending arousal systems in the brain?

Answer 6

• act on cerebral cortex and thalamus
• cholinergic ascending system affects the thalamus
• monaminergic (NA, serotonin, histamine) bypasses the thalmus, innervates the cortex and mediates interaction between it and the hypothalamus
• the lateral hypothalamus secretes orexin (hypercretin)

Question 7

What is the function of the ventrolateral preoptic nucleus?

Answer 7

• most important nucleus for generating sleep rhythms and behaviour
• inhibits the activating systems/ascending arousal systems (cholinergic and monoaminergic)
• in the hypothalamus

Question 8

How is sleep intiated?

Answer 8

• switch between arousal systems (cholinergic and monoaminergic) and VLPO activity
  
  • VLPO activity stimualted by accumulation of adenosine in the basal forebrain throughout the day
  • dissipates during sleep, favouring the balance back to the ascending arousal systems
• VLPO promotes sleep
• mediated by orexin, which activates other arousal systems
  
  • prevents oscillation of the cycle, gives it an on-off nature

Question 9

What respiratory changes occur during sleep?

Answer 9

• breathing normally controlled by centres in the medulla & pons (wakefulness drive to breathe), peripheral chemoreceptors, emotional stimuli (limbic system), cortical centres, stretch receptors (lungs), touch/temp/pain receptors, muscle and joint receptors,
• on sleep, lose emotional stimuli (except in dreams), wakefulness drive to breathe, and higher cortical influence; other inputs are significantly downregulated
• reduced airway muscle tone
  
  • can contribute to OSA
• VE falls at sleep onset, CO2 rises until a new set-point is reached
• chemical control is the major regulator of breathing during sleep