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Nature Enforces Nerve Speed Limit: Study

According to new Simon Fraser University research, there’s a speed limit on the information super-highways that route important messages through the nervous systems of animals. And while it may make larger mammals slower than their more petite animal kingdom cousins, it may also make them smarter.

Biomedical physiology and kinesiology professor Max Donelan and graduate student Heather More led the study, to be published online in the Proceedings of the Royal Society on June 30.

Their discovery—that the maximum speed at which nerves can conduct information in land mammals is constant regardless of the animal’s size—helps explain why large animals such as elephants are slow and lumbering, while tiny creatures such as shrews and mice are quick and agile.

“The size of an animal’s nerve is dependent on the number and diameter of cells—or axons—it contains,” says Donelan. “The speed at which each axon can conduct signals depends on its diameter, with larger-diameter axons able to transmit information faster.

“Also, if a nerve has more axons, the animal has a higher density of sensory receptors with which to respond to its environment more precisely. But since nerves cannot be infinitely large there is a compromise necessary between the number and size of axons in a nerve.”

Explains More: “Larger animals experience much longer delays in sensing stimuli and initiating movement. As body size increases, animals must trade off the ability to conduct information quickly with the ability to precisely sense and respond to the environment.

“Think of a fly landing on an elephant’s rump: it would take the elephant 100 times longer to sense and swat at the insect than it would for a tiny shrew. In order to have the same nerve speed and precision as the shrew, the elephant would need to have a sciatic nerve with the impossibly large diameter of 30 metres.”

Since large animals cannot speed up their nerve impulses, they must slow down their movements to compensate for the delayed transfer of information. (This is further evidence, notes Donelan, that dinosaurs could not have been both large and nimble.)

But he says larger animals may rely on smarts as much as speed: “We think they need to think ahead and predict any changes that will occur so they have time to adapt their movements accordingly. That’s what we want to study next.

“It could be that the nervous systems of large animals have evolved to become excellent predictive machines. A brain that is good at predicting movement may also become good at predicting other aspects of life such as social interactions.

“So understanding that prediction is required to overcome nerve speed limits is another brick in the foundation of a general theory of brain function and evolution.”

The Natural Sciences and Engineering Research Council of Canada funded the research.

Contact: Julie Ovenell-Carter/Marianne Meadahl, PAMR, Tel: 778.782.3210, Email: joc@sfu.ca

Source: Simon Fraser University

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