Electroencephalogram ( EEG)
&
sleep
EEG Is a record of the variations in brain electrical potential (Voltage).
* Can be recorded with electrodes on the scalp surface, or on / in the brain through the opened skull.
* EEG recorded from the scalp measures the summation of dendritic postsynaptic potentials rather than action potentials
* EEG records may be bipolar or unipolar :
⃝ Bipolar records show fluctuations in the
potential difference between two cortical
electrodes.
⃝
unipolar records show the potential
difference between a cortical electrode
and a theoretically indifferent electrode
(at zero) distant from the cortex
* EEG waves (Rhythms):
⃝
Alpha wave(Basic electrical rhythm BER):
>Frequency 8–13 Hz & amplitude of 50–100 μV when recorded from the scalp.
> Most marked in parietal & occipital lobes
* Associated with ↓ levels of attention →
Recorded during physical & mental rest with eyes closed.
* Frequency ↑ by:↑in body temperature, blood glucose & glucocorticoids levels &↓PCO2 (Hypocapnia).
> Thus, Induced hypocapnia by hyperventilation is used to bring latent EEG abnormalities.
* Frequency of BER ↓ by:
- ↓ in blood glucose, glucocorticoids
- ↓ body temperature,
- ↑ arterial PCO2 (Hypercapnia), acidosis,
* Metabolic and toxic encephalopathies due to hyponatremia and vitamin B12 deficiency.
* Acute intoxication with alcohol, barbiturates, amphetamines, phenytoin, & antipsychotics.
* Beta wave:
Replaces BER when attention is focused.
> Frequency 13–30 Hz & voltage amplitude is lower than that of alpha.
> Most marked in frontal lobe
> Gamma oscillations (30–80 Hz) are often seen when an individual is aroused & focuses attention on something.
* Theta wave:
> Frequency 4–7 Hz & voltage amplitude is higher than that of alpha
> Appears mainly in children during emotional stresses
* Delta wave:
> Frequency is less than 4 Hz.(Slowest wave)
Appears in the last stages of Slow wave sleep
Clincal uses of EEG:
* Help in diagnosis of epilepsy:
1. Absence seizures (petit mal epilepsy):
Are one of the forms of nonconvulsive
generalized seizures characterized by a
momentary loss of consciousness.
They are associated with 3/s doublets,
each consisting of a typical spike and
rounded wave, and lasting about 10 s
There is fast EEG activity during the tonic
phase.
Slow waves, each preceded by a spike, occur at the time of each clonic jerk.
For a while after the attack, slow waves are present
2. Help in diagnosis & localization of brain tumor.
3. Help in diagnosis of Psychotic disorders
Sleep:
* State of unconsciousness from which the individual can be aroused.
* Types :
1.rapid eye movement (REM) sleep
2. non-REM (NREM, slow-wave sleep)
¤ REM sleep is characterized by eye movements.
¤ NREM sleep is divided into four stages
> A person falling asleep first enters stage
1, the EEG begins to show a low-voltage, mixed frequency pattern. A theta rhythm (4–7 Hz) can be seen at this early stage of slow wave sleep.
Throughout NREM sleep, there is some activity of skeletal muscle but no eye movements occur.
> Stage 2 is marked by the appearance of sinusoidal waves called sleep spindles (12–14 Hz) and occasional high voltage biphasic waves called K complexes.
> In stage 3, a high-amplitude delta rhythm(0.5–4 Hz) dominates the EEG waves.
> Maximum slowing with large waves is seen in stage 4. Thus, the characteristic of deep sleep is a pattern of rhythmic slow waves, indicating marke,
* REM sleep occupies 80% of total sleep time in premature infants & 50% in full-term neonates. There after, the proportion of REM sleep falls rapidly and plateaus at about 25% until it falls further in old age.
Children have more total sleep time and stage 4 sleep than adults.
* Mechanisms of sleep genesis :
* Nuclei in both the brain stem and hypothalamus are critical for the transitions between sleep & wakefulness.
* high-frequency stimulation of the midbrain reticular formation (the RAS) produces the EEG alerting response & arouses a sleeping subject.
* the brainstem RAS is composed of several groups of neurons which release norepinephrine, serotonin, acetylcholine.
* In the case of the forebrain neurons involved in control of the sleep–wake cycles, preoptic neurons the hypothalamus release GABA & posterior hypothalamic neurons release histamine.
* When the activity of norepinephrine- and serotonin-containing neurons (raphe nuclei) is dominant, there is a reduced level of activity in acetylcholine-containing neurons in the pontine reticular formation. This pattern of activity contributes to the appearance of the a wake state.
* In addition, an increased release of GABA and reduced release of histamine increase the likelihood of NREM sleep
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