Researchers the University of California in Irvine (USA) have used computer modelling to investigate connectivity between brain cells in part of the hippocampus called the dentate gyrus.

The structure of brain tissue here is very sensitive to seizures: when epilepsy starts in this part of the brain, new connections form between neurons called granule cells. These new connections are excitatory, tending to increase levels of signalling and leading to seizures.

Connections between neurons
Brain cells have a central body and many long arms which connect to other brain cells, receiving and sending on information. In infancy, each neuron can have up to 15,000 connections to other neurons each, but this number falls as we get older.

The growth of these arms is not random: some neurons grow more arms, some fewer; there are patterns of connections into loops and circuits; and it's different in different parts of the brain. The patterns in epileptic tissue are different from those in non-epileptic tissue.

Computer modelling
Drs Robert Morgan and Ivan Soltesz built a series of theoretical computer models, mimicking the structure of the dentate gyrus in a rat with epilepsy. Each model featured different patterns of connections between granule cells. The excitability of each created network was then calculated.

Most connection patterns made very little difference to excitability. However, if a small number of the granule cells were given a very large number of connections each, this increased the excitability of the theoretical network remarkably, to a degree that could be considered seizure-prone. It appears that the hub cells' increased numbers of connections allow them to send round and amplify signals so much that they overwhelm the brain, leading to a seizure.

Laboratory investigation of real rat brain slices had previously identified these hyper-connected hub cells in epileptic tissue, but exactly what contribution they made to seizures hadn't been discovered.

This research was published in the Proceedings of the National Academy of Sciences USA in March 2008.

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