In this article we will discuss about:- 1. Meaning of EEG 2. Source of EEG 3. Clinical Significance.
Meaning of Electroencephalogram (EEG):
1. Electroencephalogram is recording of summated electrical activity of neurons of brain with help of surface electrodes placed on scalp.
2. During surgery, electrodes may be placed directly on cortex of brain and recording thus obtained is known as electrocorticogram.
3. Pair of electrodes is used, placed over different parts of scalp like frontal, parietal, temporal and occipital areas.
4. Recording can be bipolar or unipolar.
5. Recording obtained are summated electrical potentials of very large number of cortical neurons.
6. Amplitude of EEG recordings depends on whether discharge from neurons is synchronous in which case amplitude will be large, if asynchronous, amplitude will be small.
Rhythms:
1. α rhythm:
Recorded when person is awake, eyes closed, not concentrating or thinking.
Kluver-Bucy syndrome:
1. Visual—agnosia
2. Oral tendency
3. Hypermetamorphosis
4. Placidity
5. Omnisexuality
6. Omniphagia
a. This rhythm is also known as Burger rhythm.
b. Best recorded from occipital and parietal lobe areas.
c. Frequency will be about 8-13 cps
d. Amplitude will be about 50 microvolts
2. β rhythm:
Recorded when person opens eyes or starts thinking or working out some mathematical problems.
a. Best recorded from frontal or parietal lobe areas
b. Frequency 13-30 cps
c. Voltage will be about 20 microvolts
d. It is also known as alpha block or desynchronization
3. Theta waves:
a. Recorded in a child.
b. Occurs when a person is in light sleep
c. Frequency 4-7 cps
d. Amplitude 10 microvolts
4. Delta waves:
a. Occurs when a person is in deep sleep.
b. Frequency 0.5-4 cps
c. Amplitude will be about 100 microvolts
d. Suggests highly synchronized activity of cortical neurons.
Amplitude of EEG recordings depend on a number of factors, like:
a. Synchronized or desynchronized activity of neurons in brain
b. Fall in blood glucose level
c. Decrease of body temperature
d. Decreased Cortisol level in circulation
e. Increased tension of carbon dioxide in circulation
Source of EEG:
It is due to current flow in fluctuating dipoles formed on dendrites of cortical cells and cell bodies (Fig. 9.55).
Mechanism of desynchronization:
Stimulation of reticular formation of midbrain tegmentum, and of non-specific projection nuclei of thalamus brings about desynchronization of EEG.
Activity of ARAS can be increased by:
1. Afferent impulses coming from various ascending tracts.
2. Afferent impulses coming from organs of special senses.
3. Afferent impulses coming from superior transverse temporal gyrus and orbitofrontal cortex (cortico- fugal fibers). Through these fibers, intercortical events can bring about arousal (emotional aspect can bring about arousal).
Clinical Significance of EEG:
1. Diagnosis of epilepsy, like grand mal or petit mal
2. Brain death
3. Subdural hematoma
4. Space occupying lesions
5. Changes are also seen when blood glucose level falls/fall in steroid levels in circulation.