Electro Encephalogram
Electro Encephalogram is the graphic record of electrical activity of the brain recorded from the surface. Technique is known as Electro Encephalography.
It is the recording of the potential difference as rhythm (frequency) and magnitude. Rhythm of the EEG are designated as alpha, beta, theta and delta rhythm.
Alpha rhythm
| Frequency | Amplitude | Characteristics |
| 8-13 cycles/sec | 50-100 mv | Seen in subject when awake relaxed state and closed eyes. |
Replaced by faster wave <—- alpha blocking <—-> when respond to stimuli and wave disappear (Not responding to any stimuli)
Alpha rhythm represents state of relaxed wakefulness of reflex the optimal state of brain excitability namely synchronized rhythm.
Beta rhythm
| Frequency | Amplitude | Characteristics |
| More than 20-30 cycles/sec | 25-50mv | Seen in alert cortex wakefulness and responding to a stimulus knowns as desynchronized rhythm. |
Theta rhythm (slow rhythm)
| Frequency | Amplitude | Characteristics |
| 37 cycles/sec | 50-100 mv | dominant in wakefuness is young subjects. seen in early stages of slow wave sleep from hippocampus. |
Delta rhythm
| Frequency | Amplitude | Characteristics |
| Less than 3 cycles/sec | High amplitude < 100 mv | stages of slow wave sleep |
- Lamda wave – have high amplitude (from fronto-temporal regions)
- Mu waves – from motor cortical areas
- Spike and wave pattern: indicates abnormal electrical activty of neurons seen in epileptic cases.
Mechanism of EEG
- Wave pattern is due to the Oscillation of alternating excitatory and inhibitory post synaptic potentials produced by impulses of non-specific thalamic nuclei.
- Frequency of the rhythm is by the activity of non-specific thalamic nuclei.
- At low intrinsic frequencies (8-13 cycles/sec) the EPSP’s are summated & give rise to low frequency synchronised EEG rhythm (alpha).
- When thalamic neurons are driven by the reticular activating system, the frequency becomes too high for summation and becomes desynchronized to become fast EEG rhythm – beta rhythm results.
- Normal ECG
- 8-13 cycles/sec – alpha wave
- 20-30 cycles/sec – beta wave
- 3-7cycles/sec – theta wave
- Less than 3 cycles/sec – delta wave
EEG and sleep stages
EEG studies help in understanding the different stages of sleep Each stage of sleep manifest different wave pattern in EEG. 4 stages of slow wave sleep can be distinguished with different wave pattern, frequency and amplitude.
Drowsiness
In the relaxed awake state alpha waves predominant 1st stage of slow wave sleep is characterised by fluctuations between alpha wave and low amplitude wave (3-7 cycles/sec).
- As sleep deepens into stage 2 wave pattern changes.
- In stage 3 and 4 EEG shows gamma waves (less than 3 cycles/sec) .
- In REM sleep low voltage fast waves are seen.