Saturday, October 3, 2009

The Amazing Genetics of Nerves

I'll start with a quote from Stephen W. Kuffler, John G. Nicholls and A. Robert Martin's From Neuron to Brain (second edition, page 177; 1984 Sinauer Associates Inc.):

"... conduction velocity plays a significant role in the scheme of organization of the nervous system. It varies by a factor of more than 100 in nerve fibers that transmit different information content. In general, nerves that conduct most rapidly (more than 100 m/sec) are involved in mediating rapid reflexes, such as those used for regulating posture. Slower conduction velocities are associated with less urgent tasks such as regulating the distribution of blood flow to various parts of the body, controlling the secrection of glands, or regulating the tone of visceral organs."

Presumably much of this fine-tuning was achieved in mammals long before humans evolved. It is a great example of what can be achieved with evolution through natural selection. Apparently there is a cost to speedy nerve connections. There must be genes that can be turned on to produce extra structural elements (proteins, fats, etc) that speed up transmission velocity. There must be controlling genes that turn the structural genes on and off during nerve development, as appropriate.

Given these possibilities, there must be an overall genetic blueprint for nerve transmission velocity types. This blueprint would have been fine-tuned over time through the survival of the fittest. Having high cost, fast transmission where needed, and low-cost slower transmission where that will do, would give a slight evolutionary advantage.

The issue of transmission speed would be relatively simple comparted to creating a genetic blueprint for the orders-of-magnitude more complex human brain. Yet the construction process would be similar. A variety of brain cell types were already developed by mammals and primates. Probably (please comment if you know!) more cell types, including synapse types, evolved (from the usual mutation + selection process) for the human brain. The overall structure of the brain could be one blueprint, but it is an extremely complicated one. We know it involves axons and dendrites often both connecting to neighboring neurons and running long distances to connect to neurons after bypassing thousands, millions, or billions of closer neighbors.

This could only happen through evolution. Again, we have Charles Darwin to thank for setting us on the right path to understanding both nature and ourselves. As to machine understanding, the human brain is our best blueprint.

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