February 03, 2011
Electric Fields Speed Neural Communication

The standard model for how neurons transmit messages involves a wave of depolarization due to ions flowing thru channels in neuron membranes. Then when the wave of depolarization reaches a synapse neurotransmitters are released to travel across a gap from the axon and bind to a receptors of the dendrite of a different neuron on the other side of the gap. But some Caltech researchers believe that electrical fields generated by neurons also impinge upon other neurons and alter their behavior.

Pasadena, Calif.—The brain—awake and sleeping—is awash in electrical activity, and not just from the individual pings of single neurons communicating with each other. In fact, the brain is enveloped in countless overlapping electric fields, generated by the neural circuits of scores of communicating neurons. The fields were once thought to be an "epiphenomenon, a 'bug' of sorts, occurring during neural communication," says neuroscientist Costas Anastassiou, a postdoctoral scholar in biology at the California Institute of Technology (Caltech).

New work by Anastassiou and his colleagues, however, suggests that the fields do much more—and that they may, in fact, represent an additional form of neural communication.

This has a couple of interesting implications. First off, signaling via electrical fields would speed up neural communications. Atoms and molecules move much more slowly than electrical fields.

Second, alterations in electrical fields due to, say, cell phones or electric motors or other sources of electro-magnetic radiation have a much higher chance of altering cognitive processes if those neurons accept signals via variations in electric fields.

"In other words," says Anastassiou, the lead author of a paper about the work appearing in the journal Nature Neuroscience, "while active neurons give rise to extracellular fields, the same fields feed back to the neurons and alter their behavior," even though the neurons are not physically connected—a phenomenon known as ephaptic coupling. "So far, neural communication has been thought to occur at localized machines, termed synapses. Our work suggests an additional means of neural communication through the extracellular space independent of synapses."

Hundreds of millions of years of evolution produced a lot of design optimizations for the brain.

As Stephen Smith's lab at Stanford showed last fall the human mind is already stunning in its complexity measured only by considering neural synapses.

In particular, the cerebral cortex — a thin layer of tissue on the brain’s surface — is a thicket of prolifically branching neurons. “In a human, there are more than 125 trillion synapses just in the cerebral cortex alone,” said Smith. That’s roughly equal to the number of stars in 1,500 Milky Way galaxies, he noted.

...

Observed in this manner, the brain’s overall complexity is almost beyond belief, said Smith. “One synapse, by itself, is more like a microprocessor —with both memory-storage and information-processing elements — than a mere on/off switch. In fact, one synapse may contain on the order of 1,000 molecular-scale switches. A single human brain has more switches than all the computers and routers and Internet connections on Earth,” he said.

My guess is that the quantity of information that flows across synapses is many times the amount that flows via electric fields. Synapses localize information flow and therefore allow larger total quantities of information to be transmitted and stored.

Share |      Randall Parker, 2011 February 03 10:24 PM  Brain Structure


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