Physiology of sleep

NEET PG Physiology
#1

x- Dr. Vivek Nalgirkar
(Excerpts of the oration at the international conference at D Y Patil University, School of Medicine, Nerul, Navi Mumbai.)

 Sleep is described in many ways – ‘falling asleep’, ‘taking a nap’, ‘to pass out’, ‘so & so sleeps like a baby…’
 “Were you sleeping?..” “let’s sleep over the issue….” “I need some sleep….”
 Sleep is an essential part of our body physiology, yet it is derided when it comes at an undesirable time, in an unwanted fashion.

 Humans spend about 1/3rd of their lives in sleep. Yet, most humans know very little about sleep. However, it’s a universal need of all higher forms of life; its absence may have serious physiological bearings or consequences.
 Sleep patterns change throughout one’s life span.
 There are two extremes of the spectrum – (1) Narcolepsy (2) insomnia
 It is that part of our body physiology about which most humans do not have a great control over. (Onset of sleep, awakening from sleep)

Definition:
A temporary form of unconsciousness from which a person can be aroused by sensory and other stimuli.
(1) Physiology during sleep, (2) Physiology of sleep
• Physiology during sleep:

  1. Cardio vascular system: It is generally believed that the BP decreases during sleep (sedatives are prescribed for hypertensives), however, there may be sudden increases in BP, along with body movements, mostly seen during the K-complexes.
  2. Secretion of growth hormone:- Largest spike is seen during the first two hours (“If you don’t sleep, you won’t grow)
  3. Decreased BMR – to 90% that in the waking hours.
  4. Healing
    Sleep architecture:
    Sleep is of two types: When a person goes to sleep, he/she goes through successive stages of the slow-wave sleep which becomes deeper and eventually after about 90 minutes, there is an episode of 5 to 20 minutes of REM sleep. 5-6 such episodes may occur in a night’s sleep. Later episodes of REM sleep become longer in duration.
    What drives our sleep and its pattern?
    Circadian rhythm. It is entrained in the suprachiasmatic nucleus of hypothalamus. SCN influences the melatonin secretion by the pineal gland.

Sleep architecture is influenced by –

  • Pain and discomfort
  • Temperature
  • Physical activity
  • Noise/sound, light
  • Hunger: it is associated with wakefulness.
    REM sleep: {REM = rapid eye movement}
  • Also called paradoxical sleep; because β waves are recorded even though the person is asleep. (However, the main difference between REM sleep and wakefulness is that dream consciousness is characterized by bizarre imagery and illogical thoughts, and dreams are generally not stored in memory.)
  • In the newborn: more than half the sleep time would be in REM sleep. In young adults: 20-25% sleep duration is REM sleep. (In premature infants, it may occupy 80% duration of sleep.)
  • Ponto-geniculo-occipital (PGO) spikes are recorded in REM sleep. (PGO spikes are phasic field potentials. They are recorded from the pons, the lateral geniculate nucleus, and the occipital cortex. The appearances of these waves are most prominent just before REM sleep. The density of the PGO spikes coincides with the amount of eye movement measured in REM sleep.)
  • Associated with active dreaming.
  • Sleep paralysis: Marked reduction in muscle tone throughout the body; marked loss of tone in the muscles of neck. Also, there is locus ceruleus-dependent relative paralysis of voluntary activity.
    The only muscle groups that retain the muscle tone – extraocular muscles, middle ear muscles, diaphragm.
  • Heart rate and respiration becomes irregular.
  • Threshold for arousal by sensory stimuli is elevated.
  • Bruxism is an associated feature.
  • Penile tumescence and penile erection
  • Origin of REM sleep: cholinergic neurons of pontine reticular formation (Discharge of noradrenergic neurons in the locus ceruleus and serotonergic neurons in the midbrain raphe contributes to wakefulness, and these neurons are silent when cholinergic PGO spike discharge initiates REM sleep.) Rhombencephalon appears to be the site of origin of alert-like EEG activity during REM sleep. Hence, REM sleep is sometimes referred to as rhombencephalic sleep.
    Slow-wave sleep or non-REM sleep:
  • Divided into 4 stages: stage 1 is characterized by low-amplitude, high-frequency EEG waves (alpha waves); stage 2 shows sleep spindles and large K-complexes; stage 3 and 4 would show slower waves with large amplitude (or delta waves).
  • Slow-wave sleep can be produced by stimulation of these 3 subcortical regions:- (1) diencephalic sleep zone (in posterior hypothalamus, intralaminar and anterior thalamic nuclei) ~ low frequency stimulation (8 Hz) produces SWS; higher frequency stimulation would produce arousal, (2) medullary synchronizing zone (reticular formation at medulla) ~ sleep if the frequency of stimulus low; arousal if the frequency is high, and (3) basal forebrain sleep zone (includes preoptic area and diagonal band of Broca) ~ differs from the other two zones in that any frequency (high or low) produces slow waves and sleep.
  • Somnambulism, bed-wetting, and night terrors occur during slow-wave sleep.
  • Somniloquey
  • Serotonin agonists suppress sleep, and serotonin antagonist ritanserin increases SWS in humans.