Physiology of sleep & consciousness

Definitions:

• SLEEP
• reversible behavioral state of unresponsiveness & perpetual dissociation from the environment
• CONSCIOUSNESS:
• Perception of sensations, voluntary initiation & control of movement, capabilities associated with higher mental processing

Stages of consciousness

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Sleep

• Not a period of absolute rest
• some parts of the brain are more active during sleep
• Dreams
• necessary
• Experiments where people have been awakened as they enter the REM sleep phase become moody & depressed, and disoriented; and may have hallucinations
• Purpose of sleeping
• Restorative
• especially slow wave sleep
• Conservation of energy
• Maintaining adequate function of the immune system
• Mentally practise & refine behaviour
• e.g. escaping from predators without actually having to perform them
• REM sleep
• we dream to forget
• reverse learning helps us forget trivial things
• and to sort out our emotions
• Physiological changes in sleep
• Decreased reaction to external stimuli
• Reduced activity of the sympathetic system
• Parasympathetic dominance
• Reduced heart rate, BP, respiratory rate
• especially in Non-REM sleep
• Reduced metabolic rate
• Somnolent Metabolic Rate (SMR)
• is lower than BMR
• Reduced muscle tone
• Extensor  Babinski response
• unmasking of extensor dominance
• Changes in EEG
• Increased growth hormone production during deep sleep
• decreased in REM sleep

Sleep cycles

• First cycle:
• descends from stage 1-2-3-4
• resurfaces from 3-2-1
• enters REM-descends intom2-3-4 and so on
• Subsequent cycles become shorter
• The cycle ends with REM sleep
• In adults, sleep begins with NREM
• in infants it begins with REM
• REM-onset sleep is seen in adults in
• jet lag
• chronic sleep deprivation
• acute withdrawal of REM-suppressing drugs
• endogenous depressinarcolepsyon
• Retrograde amnesia
• poor recall of midnight dreams
• not remembering clock ringing

• Awake
• Alert
• Beta wave
• Resting
• Alpha wave
• Sleep
• Stage 1
• Theta wave
• Stage 2
• Spindles
• K complex
• Stage 3
• Delta wave appears
• Stage 4
• Delta wave
• Also seen in deep sleep/ anaesthesia / brain damage in awake adults
• Synchronisation
• REM Sleep
• Ponto-geniculo-occipital (PGO) spike

Difference between deep sleep and REM sleep

Changes in pattern of sleep with age

• With age
• REM sleep decreases
• deep sleep decreases
• Total sleep time decreases

Regulation of sleep

• Sleep is regulated by
• an interaction of homeostatic and circadian processes
• The homeostatic mechanisms
• keep track of how long we have been awake and asleep
• how tired we are!
• The circadian process
• determines the optimal time for sleep
• They influence sleep duration and the relative contribution of the two major types of sleep:
• non-rapid eye movement (NREM)
• rapid eye movement (REM) sleep

The circadian process

• Circa=about , dian=day
• The suprachiasmatic nucleus (SCN) dictates the sleep-wake cycles over a roughly 24-hour period
• the SCN receives inputs via the retino-hypothalamic fibres
• is  influenced by external cues (e.g daylight)
• but the rhythm is still present in volunteers kept in caves or in people blind from birth
• Light initiates sleep in nocturnal animals & waking in diurnal animals (including humans)
• Melatonin & melanopsin may play a role
• Phase shifts can occur with rapid crossing of time zones
• resulting in ‘jet lag’

The ultradian process

• that is characterized by the alternation of the two basic sleep states non-rapid-eye-movement (NREM) sleep and REM sleep
• influenced by SCN input

• Nucleus reticularis pontis oralis – have 4 nuclei
• PGO-on
• causes PGO spikes associated with REM sleep
• REM-on (stimulatory)
• atonia of muscles during REM sleep
• REM-on (inhibitory)
• inhibits the 2 nuclei inhibiting PGO-on
• REM-waking on
• eye movements
• muscle twitches
• Hypothalamus – have 1 nuclei
• NREM-on

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Sleep disorders

• Transient relief with “sleeping pills”
• benzodiazepines
• but daytime performance may be compromised

Insomnia

• Definition
• chronic inability to maintain the amount/quality of sleep to function normally in the day
• due to medical or mental conditions
• Right amount of sleep
• sleep requirements vary from 4-9 hours between individuals
• Margaret Thatcher & Napoleon need(ed) only 4 hours of sleep
• Fatal familial insomnia
• autosomal dominant inheritance
• Features
• worsening insomnia
• impaired ANS & motor functions
• dementia
• death
• Types of insomnia
• Onset sleep insomnia
• difficulty in falling asleep initially
• associated with anxiety disorders
• Maintenance sleep insomnia
• wake up middle of the night
• difficulty in remaining asleep
• with frequent awakening at night
• may get back to sleep
• associated with
• pain disorders
• medical illness
• Terminal (late) insomnia
• persistent early morning awakening
• cannot get back to sleep
• characteristic of clinical depression

Sleep deprivation

• Marked by brief psychosis but non-permanent psychological effects
• Features
• irritability
• stimuli misperception
• decreased waking alpha activity
• disorientation
• lack of attentiveness
• Neurological consequences
• tremors
• seizures

Narcolepsy

• Definition
• a chronic neurological disorder caused by the brain’s inability to regulate sleep-wake cycles normally
• excessive daytime sleepiness (EDS)
• Narcolepsy results from
• disease processes affecting brain mechanisms that regulate REM sleep
• For normal sleepers a typical sleep cycle is about 100 – 110 minutes long, beginning with NREM sleep and transitioning to REM sleep after 80 – 100 minutes
• But, people with narcolepsy frequently enter REM sleep within a few minutes of falling asleep
• Three other major symptoms frequently characterize narcolepsy
• Cataplexy
• the sudden loss of voluntary muscle tone
• Vivid hallucinations
• during sleep onset or upon awakening
• Brief episodes of total paralysis
• at the beginning or end of sleep 

Somnambulism (sleep walking)

• more common in
• children
• males
• may last a few minutes
• walk with eyes open
• to avoid collision
• but cannot remember anything
• Occur during arousal from slow wave sleep
• somnambulism
• noctural aneuresis
• bed-wetting
• night terrors
• Nocturnal emission
• Wet-dreams
• normal in male (sometimes females) of reproductive age

Sleep apnoea

• Definition
• stopping of breathing for short periods
• typically between 10 and 90 seconds) during a normal nightly sleep
• Can be repeated hundreds of times during a sleep period
• Almost always accompanied by
• snoring, which is usually quite loud
• Patient is  unaware of this happening, in normal circumstances
• 3 types of sleep apnoea
1. Obstructive sleep apnoea (OSA)
• Fairly common
• upper airway collapses as the throat muscles relax during normal sleep
• cerebral hypoxia results in waking up
• may be repeated every 15 minutes
• May result in
• excessive daytime tiredness and a lack of concentration
• Counter-measure
• Relieve the obstruction
• sleep on the side; reduce weight
2. Central
• Rare
• Neurological dysfunction
• the brain simply "forgets" or "delays" to send the signal to breathe
• More difficult to treat
• some drugs may help
3. Mixed
• Extremely rare

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Reticular Activating System (RAS)

• The reticular formation resides in the brain stem
• midbrain, pons & medulla
• Comprised of the
• motor system (reticulospinal system)
• sensory systems (RAS)
• the vital centres
• The RAS stimulates the whole brain to cause arousal (wakefulness)
• its output is suppressed in sleep & coma
• stimulated by
• input from the cortex (corticofugal fibres)
• OREXINS
• peptides released by hypothalamic neurons just before waking

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Consciousness

• Various definitions
• mind
• awareness
• ability to respond
• Can it be equated to activity in the EEG?
• Sometimes, people under general anaesthesia feel the pain and are conscious of their surroundings
• but cannot convey this to the outside world , trapped in their own body
• Perception of sensations, voluntary initiation & control of movement, capabilities –> associated with higher mental processing
• The brain’s ability to adjust its activity and consciousness levels can be impaired in several ways:
• When people are severely deprived of sleep
• When and immediately after a seizure occurs
• When both cerebral hemispheres are suddenly and severely damaged
• When the reticular activating system malfunctions
• When blood flow or the amount of nutrients (such as oxygen or sugar) going to the brain decrease
• When toxic substances impair the brain (liver & kidney failure)
• Drugs
• morphine
• diazepam
• antipsychotic drugs
• Periods of impaired consciousness can be short or long. The level of impairment can range from slight to severe:
• Lethargy
• slight reduction in alertness or clouding of consciousness
• People tend to be less aware of what is happening around them and to think more slowly
• Obtundation
• moderate reduction in alertness or clouding of consciousness
• Stupor
• excessively long or deep sleep-like state
• A person can be aroused from it only briefly by vigorous stimulation
• such as repeated shaking, loud calling, pinching, or sticking with a pin.
• Coma
• state of complete unresponsiveness
• A person cannot be aroused at all
• A person in a deep coma lacks even the most basic responses, such as avoidance of pain
• although reflexes may be present

Coma (read coma lecture notes)

• In coma, brain oxygen utilisation is below normal resting levels
• in sleep, the brain is active and oxygen consumption is comparable to the waking state
• Caused by
• trauma
• tumours
• metabolic disorders
• hypoglycaemia (hyperosmolar coma)
• drug overdose
• liver and kidney failure
• fluids & electrolytes imbalance
• hypernatraemia
• hypokalaemia
• water intoxication
• myxoedema
• Depression of the reticular activating system (RAS)
• Clinically assessed using Glasgow Coma Scale (GCS)
• Four Score Coma Scale
• eye & motor responses
• brain stem reflexes
• respiration.
  Total unresponsiveness to sensory stimuli for an extended period

Source: anatomyguru.in