SN7
SHORT NOTE
5M
Describe the normal electroencephalogram and the stages of sleep.
Definition
Electroencephalogram (EEG) = the record of the spontaneous electrical activity of the cerebral cortex, obtained from electrodes placed on the scalp.
- Amplitude 50–100 µV — far smaller than the ECG
- Arises chiefly from summated postsynaptic potentials of cortical pyramidal cells, not from action potentials
Normal RHYTHMS
| Wave | Frequency | Where and when seen |
|---|---|---|
| Alpha (α) | 8–13 Hz | Awake, relaxed, eyes closed; parieto-occipital. Disappears on opening the eyes — alpha block |
| Beta (β) | > 13 Hz | Alert, eyes open, mental activity; frontal |
| Theta (θ) | 4–7 Hz | Children; drowsiness and light sleep in adults |
| Delta (δ) | < 4 Hz | Deep sleep; infants. Abnormal in an awake adult |
Stages of Sleep
| Stage | EEG | Features |
|---|---|---|
| NREM 1 | Theta | Drowsiness; easily woken |
| NREM 2 | Sleep spindles and K complexes | The largest proportion of sleep |
| NREM 3–4 | Delta — slow wave sleep | Deepest; GH secretion peaks; sleepwalking and night terrors occur here |
| REM | Low voltage, fast — resembles the awake EEG ("paradoxical sleep") | Dreaming; rapid eye movements; complete loss of muscle tone; irregular pulse and respiration; penile erection |
- A cycle lasts 90 minutes; there are 4–6 cycles a night
- REM occupies about 20–25% of adult sleep, but 50% in the newborn
- REM periods lengthen towards morning; deep NREM predominates early in the night
Uses of EEG
- Diagnosis and classification of epilepsy — the chief use
- Localisation of space-occupying lesions (largely superseded by imaging)
- Assessment of depth of anaesthesia
- Sleep studies; investigation of coma and encephalopathy
- Confirmation of brain death — an isoelectric (flat) EEG
Applied Aspects
- Absence (petit mal) seizures — the classical 3 Hz spike-and-wave pattern
- Grand mal — high-voltage fast spikes in the tonic phase
- REM sleep deprivation → irritability and REM rebound on the following night, indicating that REM serves a genuine need
- Narcolepsy — sleep begins directly with REM; associated with cataplexy and sleep paralysis
All questions in Nervous System (neurophysiology)
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- SN7Describe the normal electroencephalogram and the stages of sleep.