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Saturday, 29 July 2017

EEG & sleep

                Electroencephalogram ( EEG) 

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 

    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 
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


* 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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