A seizure starts when a group of brain cells fires too rapidly and in unison, creating an electrical surge that disrupts normal brain function. In the first fractions of a second, individual neurons become abnormally excitable and begin recruiting their neighbors into the same rapid-fire pattern, creating a wave of synchronized activity that can stay in one area or spread across the entire brain. Understanding this process, from the earliest chemical shifts to the warning signs you might actually feel, helps make sense of what’s happening during a seizure and why they can look so different from person to person.
What Happens Inside a Single Neuron
The very first event in a seizure is something called a paroxysmal depolarizing shift. Normally, a neuron fires a quick electrical pulse and then resets. During this shift, the neuron’s internal voltage swings dramatically, jumping 20 to 70 millivolts above its resting state and staying elevated for hundreds of milliseconds instead of the usual few. That prolonged surge causes the neuron to fire a rapid burst of signals rather than a single pulse.
This burst happens because of a chain reaction at the chemical level. The brain’s main excitatory signaling molecule, glutamate, activates receptors on neighboring neurons. One type of receptor in particular is essential for triggering the burst. Without it, the abnormal firing pattern doesn’t start at all. Other receptor types amplify the burst and help sustain it, while calcium channels on the cell surface determine how long each burst lasts and how many bursts cluster together.
How the Signal Spreads to Neighboring Cells
A single misfiring neuron doesn’t cause a seizure. A seizure is a network event that requires large numbers of neurons firing in lockstep. This recruitment process, called hypersynchrony, happens through several routes: direct chemical signaling between neurons, tiny electrical fields that influence nearby cells, and even physical connections called gap junctions that let electrical current flow directly from one neuron to the next.
Once enough neurons are recruited, the abnormal activity becomes self-sustaining. Each firing cell excites its neighbors, which excite their neighbors in turn. The result is a cascade of synchronized electrical discharge that overwhelms the brain’s normal activity in that region.
The Chemical Balance That Keeps Seizures in Check
Your brain constantly balances excitation against inhibition. Glutamate pushes neurons to fire. A chemical called GABA does the opposite, calming neurons down and preventing them from firing too easily. GABA is the brain’s primary inhibitory signal, and it works by controlling how excitable neurons are across the cortex, the hippocampus (a memory-related structure deep in the brain), and the thalamus (a relay station near the brain’s center).
When this balance tips toward too much excitation or too little inhibition, the threshold for a seizure drops. This can happen in several ways. Sometimes GABA signaling weakens because the transporters that regulate GABA levels outside cells malfunction. Paradoxically, even chronically elevated GABA levels can backfire: they initially calm neurons down, but over time they cause the brain to reduce its own inhibitory signaling, which can actually make certain circuits more excitable. In one well-studied example, overactive GABA signaling in the thalamus causes neurons there to become deeply inhibited and then rebound into synchronized bursting, a pattern that generates the characteristic spike-and-wave discharges seen in some generalized seizures.
Ion Channels Set the Threshold
Neurons control their firing through tiny protein gates called ion channels, which let charged particles (ions) flow in and out of the cell. Potassium channels are especially important because they help reset a neuron after it fires and maintain the resting voltage that keeps the cell quiet between signals. When these channels don’t work properly, due to genetic mutations or metabolic changes, the neuron sits closer to its firing threshold and is more easily triggered.
Sodium channels play the complementary role, driving the rapid voltage spike when a neuron does fire. Mutations that make sodium channels open too easily or stay open too long can cause neurons to fire repetitively instead of once. Even metabolic conditions matter: elevated blood sugar and high energy stores inside cells can suppress a specific type of potassium channel, tipping the balance toward a more excitable state. These channel-level defects explain why some people have a lower seizure threshold from birth, while others develop one after brain injury or metabolic stress.
Where Seizures Begin in the Brain
Not all seizures start the same way, and where they originate determines what they look and feel like. The current medical classification, updated by the International League Against Epilepsy, divides seizures into four main classes: focal, generalized, unknown, and unclassified.
Focal seizures begin in one specific area of the brain. The temporal lobes, located on either side of the brain near the ears, are the most common starting point. Among people with drug-resistant epilepsy who are evaluated for surgery, roughly two-thirds have seizures originating in the temporal lobe. The frontal lobe is another frequent source, and because it has extensive connections to other brain regions, seizures starting there tend to spread rapidly and widely.
Generalized seizures, by contrast, involve both sides of the brain from the very beginning. These often arise from circuits connecting the thalamus and the cortex. The thalamus acts as a relay hub, and its neurons can switch between a calm, steady firing mode and a highly synchronized bursting mode. When that switch flips abnormally, the burst pattern propagates through the thalamus’s widespread connections to the cortex, causing the entire brain to seize at once. This is why generalized seizures typically cause loss of consciousness so quickly: the disruption is brain-wide from the start.
Warning Signs Before a Seizure
Many people experience warning signs hours or even days before a seizure occurs. This early phase, called the prodrome, isn’t part of the seizure itself but reflects subtle shifts in brain excitability building toward a tipping point. Common prodromal experiences include mood changes like unusual irritability or anxiety, physical sensations such as fatigue or muscle aching, sleep disruption, and cognitive fogginess or difficulty concentrating. Not everyone recognizes these patterns, but people who track them over time can sometimes anticipate when a seizure is more likely.
Closer to the seizure, some people experience an aura, which is actually a focal seizure in its own right, just one that stays small enough that the person remains fully conscious. What the aura feels like depends entirely on which part of the brain is involved. It might be a sudden wave of nausea, a strange smell with no source, flashing lights or visual distortions, tingling or shaking in one part of the body, or an intense feeling of déjà vu. These sensations reflect the abnormal electrical activity firing in a localized brain region before it has a chance to spread. For some people, the aura is the entire event. For others, it’s a brief warning, lasting seconds to a minute, before the electrical storm recruits enough brain tissue to cause a larger seizure with impaired awareness or convulsions.
From Spark to Storm
Putting it all together, a seizure follows a progression from microscopic to whole-brain. It begins with individual neurons that are sitting too close to their firing threshold, whether because of ion channel defects, a chemical imbalance between excitation and inhibition, or damage to brain tissue. A trigger, which can be as ordinary as sleep deprivation, stress, or flickering light, pushes those vulnerable neurons past their threshold. They fire in prolonged bursts, recruit their neighbors through chemical and electrical signaling, and the synchronized activity expands outward.
If the brain’s inhibitory systems contain the spread, the result is a focal seizure, perhaps experienced only as a brief aura. If those systems fail, the activity cascades through connected circuits, potentially reaching the thalamus and spreading to the entire cortex within seconds. The whole process, from the first abnormal burst to a full convulsive seizure, can unfold in under 30 seconds. It’s a rapid escalation from a small electrical spark in a handful of cells to a storm that temporarily overwhelms the brain’s normal function.