There are about 100 billion neurones in the human brain. Neurones have a small central body and long arms which connect them to between 1,000 and 10,000 other cells, forming a very complex set of competing networks. Some neurones are excitatory: when they fire, they increase the firing of neurones they're connected to. Some are inhibitory, transmitting molecules that dampen activity in connecting cells. This system allows the cells to pass on information, and at the same time works as a feedback mechanism.

A seizure happens when the activity of inhibitory neurones fails to balance that of excitatory ones, so there is too much firing. This could happen if there is a lack of inhibitory neurones, or if they don't work, being wrongly formed; or on the other hand if there are too many excitatory neurones or they are too easily excited. Though this system is well understood in principle, the details of how it works at a cellular level in human brains are unclear.

In a collaboration between the Karolinska Institute in Stockholm, Sweden and the Brain Mind Institute in Lausanne, Switzerland, scientists investigated these competing systems in cells from the neocortex. This is the outermost layer of the brain. Though only 2-4 mm thick, it contains six layers of cells, on average 21 billion of them. It accounts for about three quarters of the brain's volume. The neocortex is involved in the most advanced functions of the brain, such as conscious thought, spatial reasoning, generating commands to move, sensory perception and language.

Drs Gilad Silberberg and Henry Markram looked at cell firing in this layer of the brain. Eighty percent of cells here are pyramidal neurones, which are excitatory. In amongst them are small numbers of cells called Martinotti cells, first identified in 1888. The researchers found that it was these cells that were inhibitory in this part of the brain. When Martinotti cells, which connect clusters of pyramidal neurones, receive signals above a certain rate or above a certain strength, they send out signals which dampen activity in surrounding pyramidal cells.

The researchers, writing in the journal Neuron in March 2007, proposed that this is the central mechanism for regulating activity in this very important part of the brain, and consequently may be important in epilepsy. It is not known whether the distribution or function of Martinotti cells is affected in the brains of people who have seizures.

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