Seizures

Seizures

Epilepsy is the tendency to have repeated seizures that start in the brain. We know that a seizure is an event that causes a change in the person’s mood, feelings, awareness or movements. But what goes on in the brain to cause a seizure to happen? In this article we look at how a disruption in brain activity can cause seizures.

To understand seizures we need to remember a little about how neurones communicate.

Brain cells are called neurones and they link together through synapses.

Neurones work by sending messages to each other and around the body. This communication happens when an electrical discharge in one neurone causes a neurotransmitter to be released. The neurotransmitter then travels through the synapse to the next neurone.

Neurotransmitters are either excitatory (making a neurone active and it sends the message on) or inhibitory (which stops any further activity).

So, what causes seizures?

Generally speaking, when brain cells are ‘over-excited’, seizures can happen. Sometimes neurones send too many messages and this disrupts normal communication, which disturbs how the brain works. When this occurs, a seizure can happen. But why does this over-excitation happen and how does it cause seizures?

At the moment there is no theory that completely explains why all seizures happen. It is likely that lots of different things may come into play to cause seizures to start. There may be more than one process that causes seizures, and more than one process may happen at the same time. These processes may depend on the type of seizures or epilepsy that someone has, and what causes partial seizures may be different to what causes generalised seizures.

A closer look at seizures

Neurones normally communicate by using electrical and chemical signals or messages that cause ‘depolarisation’. But sometimes neurones might send out an ‘abnormal’ message. For example, a neurone could send out an excitatory message that causes a larger-than-normal depolarisation. This in turn could make other neurones send out abnormal messages.

If just a single neurone behaves like this and sends out an unusual message, it is unlikely to cause a seizure. But if lots of neurones, from the same part of the brain, send out unusual messages together, then this could cause a seizure to happen. For the messages to cause a seizure:

• each neurone must be excited
• they must be connected to many other neurones within just a few synapses, and
• the message must be large enough to cause the other neurones to act in the same way.

Under these conditions, the message could spread very quickly. If the unusual message causes disrupted activity, this may affect just part of the brain (and cause a partial seizure) or the disrupted activity could spread to affect the whole of the brain (and become a generalised seizure). The part of the brain where the disruption starts is called the ‘epileptic focus’.

What might cause neurones to behave differently from normal?

It could be that:

• if neurones are damaged, this might change how they normally work and affect how they send signals
• if there are too many or not enough neurotransmitters, this affects how the neurones can communicate, and could cause seizures
• some neurones have ion channels which are slower than normal, that is the messages are not ‘turned off’ as quickly as they would normally
• some neurones may have different synaptic receptors that receive the neurotransmitters, and this could make it harder for the neurone to stop or ‘switch off’ the message, so the neurone continues to send the message even after it is not needed anymore.

Once a seizure has started, how does it stop?

Again, there is no clear answer to this. For most people, when seizures start, they last for a certain length of time and then stop by themselves. It may be that the brain has a mechanism that recognises a seizure and releases chemicals that inhibit or stop the seizure from carrying on. What this mechanism or substance is, is still not known.

These are just some facts about how neurones work and their possible involvement in how and why seizures happen. Although there is no definite answer yet, there is a lot of research going on to try and find the answers.

Glossary

Depolarisation: a vital part of how neurones communicate (when the balance of ions inside and outside the neurone changes).

Epileptic focus: the part of the brain where the seizure activity starts.

Excitatory/inhibitory: neurotransmitters that either excite or inhibit a neurone so that either the message is carried on or it is stopped.

Messages: signals that send information from one neurone to another and around the body.

Neurones: the individual cells that make up the nervous system.

Neurotransmitters: chemicals that help to send messages from one neurone to another.

Synapses: the point where two neurones connect.

© Epilepsy Society
July 2006