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What is sleep? 

Sleep is defined as a temporary state of unconsciousness from which a person can be aroused by sensory or other types of stimuli.

Classification

It is broadly classified into 2 types:

  1. Slow-wave sleep or Non-rapid eye movement (NREM)
  2. Rapid eye movement (REM)

Slow-wave sleep

  • It is also known as Non-rapid eye movement (NREM).
  • In this NREM, there are four stages of NREM (I-IV).
  • Deep sleep (stage III and IV), occurs during the first hour after going to sleep.
  • Muscle and vascular tone reduced
  • 10-30% decrease in blood pressure, respiratory rate, body temperature, and Basal metabolic rate.
  • Dreamless types (even if occurs, is not remembered)

Stage of Slow-wave sleep in EEG

stages of NREM sleep
  • Stage I:
    • It becomes drowsy; the EEG shows a change from beta to an alpha rhythm.
    • Immediately, light sleep begins during which alpha rhythm is replaced by high frequency and low amplitude ECG waves.
  • Stage II:
    • The amplitude of EEG waves slightly increases
    • The hallmarks of stage II are sleep spindles and K-complexes.
    • Sleep spindles: waxing and waning amplitude.
    • K-Complexes: high amplitude-sharp waves appearing irregular EEG.
  • Stage III:
    • Moderately deep sleep
    • Lower frequency and higher amplitude of rhythm of EEG waves
  • Stage IV:
    • Deep sleep stage, with delta rhythm predominant over the whole scalp.
    • The lowest frequency (maximum slowing) with large amplitude waves
    • In this stage, Very difficult to wake up and maximum time of doze.

Centers for NREM:

  • Posterior hypothalamus
  • Pre-optic area of the hypothalamus
  • Reticular formation at NTS
  • Intra laminar and anterior thalamic nuclei and
  • Possible neurotransmitter involves are serotonin & muramyl peptide.

Rapid eye movement (REM)

  • It is also known as paradoxical/ Dream sleep.
  • EEG activity is very rapid like β rhythm (high frequency and low voltage).
  • Occurs every 90 min, lasting 5-30 min, (beta waves seen in EEG)
  • Slow waves are replaced by rapid low voltage EEG activity at this stage.
  • Muscle tone is depressed. (except extra-ocular muscle)
  • Heart rate and the respiratory rate becomes irregular but the Brain is highly active, metabolism increased as much as 20% in this stage. Penile erection can occur in this type.
  • The center for REM is the pontine reticular formation and the possible neurotransmitter involve is acetylcholine.

Functions of sleep

  • It helps to restore strength, both physical and mental
  • It consolidates learning and memory.
  • Its ability to think and concentrate is enhanced with adequate sleeping.
  • An adequate amount of sleeping promotes growth, as during sleep growth hormone secretion is more
  • Dreams are produced during sleeping. However, the utility of dreams depends on the ability to remember and interpret them.

Difference between NREM and REM sleep

  NREM REM
Timing in sleep-cycle Occurs first Occurs after NREM sleep
Duration in normal adults 75% of total sleeping 25% of sleeping
Autonomic symptoms Sympathetic inhibition (low HR, BP, respiration) Sympathetic excitation (high HR, BP, respiration)
Eyeball movement No eye movement Rapid eye movement occurs
Dreams Dreams are not memorized Dreams well memorized
Muscle tone Is inhibited Is profoundly depressed
Type of sleeping Enters into deep sleeping Sleep lightens
EEG waves Slow wave-high amplitude (in stage 3 and 4) High frequency-low voltage
Mechanism Inhibition of RAS (reticular activating system) Activation of the pontine reticular formation

Theories of the sleep-wake cycle

  • Passive theory:
    • The reticular activating system(RAS) becomes fatigued during the waking day and becomes inactive at night. So, sleeping occurs as a result of the passiveness of RAS after all working days.
  • Role of circadian cycle/pineal gland:
    • The Center of the circadian rhythm is the suprachiasmatic nucleus (SCN) of the hypothalamus and it is controlled and regulated by the pineal gland.
  • Role of neurochemicals in the brain stem
  • Role of thalamocortical loop

Circadian rhythm

  • Exposure to brilliant light can either advance, delay, or have no effect on the sleep-wake cycle in humans depending on the time of day when it is experienced.
  • During the usual daytime, it has no effect, but just after dark it delays the onset of the sleeping period, and just before dawn it accelerates the onset of the next sleep period.
  • Generally, they become entrained, that is, synchronized to the day-night light cycle in the environment of the humans being.

Role of neurochemicals in the brain stem

In the hypothalamus the preoptic neurons release GABA & posterior hypothalamic neurons release histamine.

Also, orexin is produced in hypothalamic neurons and appears to be important in switching between sleep and wakefulness.

Role of thalamocortical loop

The neural circuit between the thalamus and cortex has been proposed to be an important component of the pattern generator of the sleep-wake cycle.

It generates this rhythmic activity and the inhibitory reticular neurons are believed to the main elements in the network.

The neurons exhibit high-frequency rhythmic activity and sleeping activity is of low-frequency rhythm.

Sleeping disorders

  • Narcolepsy:
    • It is a chronic neurological disorder that is caused by the brain’s inability to regulate the sleep-wake cycle normally.
    • Characterized by a sudden loss of voluntary muscle tone, brief episodes of paralysis at the beginning or end of sleep.
    • Sudden onset of REM sleeps at the beginning of sleep.
  • Somnambulism ( walking during sleeping)
  • Nocturnal enuresis (bed-wetting in the night)
  • Obstructive sleep apnea(OSA)
  • Periodic limb movement disorder(PLMD)
  • Bruxism (nocturnal grinding of teeth)
  • Hypersomnia (Excessive Sleeping time)

Electroencephalography(EEG)

The EEG helps to record the electrical activity of the brain with the help of surface electrodes placed on the scalp. Its recording depending on the summations of EPSP and IPSP.

It is sometimes of value in localizing pathologic processes. When a collection of fluid overlies a portion of the cortex in the brain, activity over this area may be damped.

This fact may aid in diagnosing & localizing conditions like subdural hematomas in the body.

EEG waveforms

  • Alpha wave
    • Awake, mentally relaxed but inattentive
    • Amplitude= 50uv, frequency=8-13/sec
    • Best record from the parietal and occipital region
    • A similar rhythm has been observed in a wide variety of mammalian species with slight variations.
  • Beta waves
    • Awake and alert, recorded in REM sleep
    • Amplitude= 20uv, frequency=13-30/sec
    • Best recorded from the frontal and parietal region.
  • Theta waves
    • Best record from the parietal and temporal region
    • A person is in lightly sleeping, emotional stress in the adult stage, sometimes in normally awake children.
    • Amplitude= 10uv, frequency=4-7/sec
  • Delta waves
    • This is recorded during deep sleeping
    • Amplitude= 100uv &  frequency=0.5-4/sec.

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