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# Brain rhythms and how seizures start and spread

- Mechanism
- Exam priority: medium

The EEG records the summed signals of millions of cortical cells firing together; a seizure is that synchrony running out of control.

Updated Nov 13, 2026

Also called: EEG generation, Paroxysmal depolarizing shift, Spike-and-wave

**See it in 3D:** [Thalamus](https://durgical.com/en/anatomy/thalamus/) · [Hippocampus](https://durgical.com/en/anatomy/hippocampus/) · [Temporal lobe](https://durgical.com/en/anatomy/middle-temporal-gyrus/)

## Key points

- EEG signals are summed postsynaptic potentials of aligned pyramidal cells, not action potentials; synchrony, not activity alone, makes large waves.
- Rhythms: beta > 13 Hz (alert), alpha 8–13 Hz (relaxed, eyes closed, occipital), theta 4–7 Hz, delta

## What it is

Scalp EEG waves come from synaptic currents in the dendrites of cortical pyramidal cells, which become visible only when many cells act in step; the thalamus paces much of this rhythm. A seizure starts when excitation outweighs inhibition in a group of neurons, they fire in bursts together, and the burst escapes local inhibition to spread. Antiseizure drugs work by restoring that balance.

## How it works, step by step

- Excitatory synapses on the apical dendrites of pyramidal cells create current sinks and sources; the aligned cells act like small batteries the scalp can detect.
- Thalamic reticular neurons inhibit relay neurons rhythmically; when relay neurons rebound they drive the cortex in step, pacing alpha and sleep spindles.
- In an epileptic focus, too little GABA inhibition or too much glutamate excitation lets a neuron group depolarize together (paroxysmal depolarizing shift).
- Each shift is followed by a hyperpolarization; this is what the EEG shows as an interictal spike and wave.
- If surrounding inhibition fails, for example as extracellular K⁺ rises and GABA inhibition fades, the burst recruits neighbors and becomes a seizure.
- Spread along cortical and thalamocortical links can turn a focal seizure into a bilateral tonic-clonic one.
- In absence epilepsy, hyperpolarized thalamic neurons de-inactivate T-type Ca²⁺ channels and fire bursts that lock cortex and thalamus into a 3 Hz loop.
- Seizures stop as neurons are inhibited and exhausted; the postictal state reflects this depressed activity.

### Action potential

**Rest** −70 mV **Threshold** −55 mV **Peak** +35 mV **Duration** ≈ 1–2 ms

Tap a phase (the pills above or its band) or a refractory period (ARP, RRP) at the base of the plot.

- Mainly Na⁺
- Mainly K⁺
- Rest
- Threshold
- ARP: absolute refractory period
- RRP: relative refractory period

A single neuron's spike; the EEG instead adds up slower synaptic currents from thousands of aligned cells.

## EEG rhythms

| Rhythm | Frequency | Typical setting |
| --- | --- | --- |
| Beta | > 13 Hz | Alert, focused; increased by benzodiazepines |
| Alpha | 8–13 Hz | Awake, relaxed, eyes closed; blocked by eye opening |
| Theta | 4–7 Hz | Drowsiness, light sleep; normal in children |
| Delta |
- Hyperventilation lowers PCO₂ and can provoke absence seizures; it is used to bring them out during an EEG.
- Status epilepticus is a seizure lasting 5 minutes or more, or repeated seizures without recovery; prolonged seizures exhaust inhibition and become harder to stop.
- In Latin America, neurocysticercosis is a leading cause of acquired focal epilepsy: cysts irritate the surrounding cortex.

## Memory aid

Many cells in step make a wave; too many in step make a storm.

## Test yourself

Think of the answer, then tap the question to see it.

## Why does the scalp EEG reflect synaptic potentials rather than action potentials?

Action potentials are brief and poorly aligned; slower dendritic synaptic currents in parallel pyramidal cells add up when synchronous and are large enough to reach the scalp.

## A child has brief staring spells with a 3 Hz spike-and-wave EEG. Which channel underlies this rhythm?

T-type (low-threshold) Ca²⁺ channels in thalamic relay neurons, which fire rebound bursts in a thalamocortical loop; ethosuximide blocks them.

## What is the paroxysmal depolarizing shift?

A large, prolonged depolarization of a group of neurons with a burst of action potentials, driven by glutamate and Ca²⁺ currents; it is the cell basis of the interictal spike.

## How do phenytoin and carbamazepine reduce seizure spread?

They stabilize the inactivated state of voltage-gated Na⁺ channels, which limits high-frequency repetitive firing while sparing normal firing.

## Why might hyperventilation provoke an absence seizure?

It lowers PCO₂ and raises pH, which increases neuronal excitability and favors thalamocortical burst firing.

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## Connections

## See it in 3D 3

The structures of Durgical's 3D anatomy where it happens.

- [Thalamus](https://durgical.com/en/anatomy/thalamus/)
- [Hippocampus](https://durgical.com/en/anatomy/hippocampus/)
- [Temporal lobe](https://durgical.com/en/anatomy/middle-temporal-gyrus/)

## Related topics 4

- [Wakefulness, sleep, the EEG and seizures](https://durgical.com/en/physiology/wakefulness-sleep-the-eeg-and-seizures/)
- [Synaptic transmission](https://durgical.com/en/physiology/synaptic-transmission/)
- [The main neurotransmitters](https://durgical.com/en/physiology/the-main-neurotransmitters/)
- [The nerve action potential](https://durgical.com/en/physiology/the-nerve-action-potential/)

## Related conditions 4

- [Epilepsy and seizures](https://durgical.com/en/conditions/epilepsy-and-seizures/)
- [Status epilepticus](https://durgical.com/en/conditions/status-epilepticus/)
- [Febrile seizures](https://durgical.com/en/conditions/febrile-seizures/)
- [Neurocysticercosis](https://durgical.com/en/conditions/neurocysticercosis/)

## In Biochemistry 1

- [Glutamate (glutamic acid)](https://durgical.com/en/biochemistry/molecules/glutamate/)

## In Pharmacology 4

- [Phenytoin](https://durgical.com/en/pharmacology/phenytoin/)
- [Valproic acid](https://durgical.com/en/pharmacology/valproic-acid/)
- [Carbamazepine](https://durgical.com/en/pharmacology/carbamazepine/)
- [Benzodiazepines](https://durgical.com/en/pharmacology/classes/benzodiazepines/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [EEG: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003931.htm) (MedlinePlus, US National Library of Medicine, 2025); [Focal Onset Seizure](https://www.ncbi.nlm.nih.gov/books/NBK500005/) (StatPearls (NCBI Bookshelf), 2025); [Generalized Tonic-Clonic Seizure](https://www.ncbi.nlm.nih.gov/books/NBK554496/) (StatPearls (NCBI Bookshelf), 2025).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/brain-rhythms-and-how-seizures-start-and-spread/