Yet it is in the white matter that the scientists have made a remarkable discovery. As soon as an electrical impulse runs through an axon cable, tiny bubbles containing glutamate travel to the axon membrane and release their content into the brain. Glutamate is one of the most important neurotransmitters, being released when signal transmission occurs at synapses. The researchers were able to demonstrate that certain cells in the white matter react to glutamate: the precursor to what are known as oligodendrocytes. Oligodendrocytes are the brain's "insulating cells". They produce the myelin, a sort of fatty layer that surrounds the axons and ensures rapid retransmission of signals. "It is likely that insulating cells are guided by the glutamate to locate axons and envelope them in a layer of myelin," says Dirk Dietrich.
As soon as the axons leave the white "cable duct" they enter the brain's grey matter where they encounter their receptor dendrites. Here, the information is passed on at the synapses to the receptor cells. "We think, however, that on their way though the grey matter the axons probably release glutamate at other points apart from the synapses," Dietrich speculates. "Nerve cells and dendrites are closely packed together here. So the axon could not only excite the actual receptor but also numerous other nerve cells."
If this hypothesis is correct, the accepted scientific understanding of the way neurons communicate, which has prevailed for over a hundred years, will have to be revised. In 1897 Sir Charles Sherrington first put forward the idea that chemical messengers are
Source:University of Bonn