March 14, 2007

synapse again . . .

In continuing with the topic of neuronal communication, here's a recent press release about research that exposes a possible flaw in our understanding of neuron activity.

Okay, so again, our current understanding is that neurons communicate with each other through synaptic transmission of neurotransmitters. Well, recent research suggests that neurotransmitter release is not in fact exclusively synaptic.

Instead, researchers at the University of Bonn claim that neurons release neurotransmitters all along the length of the axon, thereby exciting neighboring cells. This seems to have been demonstrated in their research concerning the "white matter" of rat brains. The white matter consists of only axons and ancillary cells. There are no dendrites or synapses present. Since there are no synapses present, one would expect not to find neurotransmitters in this matter. Well, as it turns out, they do observe the presence of a neurotransmitter here:

"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 their speculation turns out to be correct, it will show that neuronal communication in the brain is a lot more chaotic than what was originally thought. Our original understanding will have to be revised to include non-synaptic transmission of neurotransmitters. Interesting stuff.

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