Book T of C
Chap T of C
This is the 2007 version. Click here for the 2017 chapter 02 table of contents.
Another finding from the quiet revolution was that neurons could exchange large biomolecules such as proteins and RNA. This is important because proteins and RNA are capable of carrying enormous amounts of information. Sending nerve impulses to a neuron is like tapping out a message in Morse code (a combination of dots and dashes). Sending biomolecules, by contrast, is like passing a book through a window. The speed may be slower, but a lot more information is passed at once.
What is biomolecular transfer, and why is it important?
What are microtubules and neurofilaments? What happens inside them?
Axons and dendrites have narrow transportation channels within them called microtubules, 200-300 angstroms wide, and neurofilaments, 100 angstroms wide. An angstrom is one ten-millionth of a meter, so these tubes are tiny indeed. Microtubules are capable of "rapid and precise" delivery of transmitters, proteins, and other large molecules to specific areas within the neuron or to synapses with other neurons (Aletta & Goldberg, 1982). Within a single axon of a neuron, different rate components can be detected, each for a distinct macromolecular complex. In other words, axons are great, multi-laned highways. Each type of substance has its own lane, and each lane moves at its own speed.
Why do Penrose and Hameroff suggest that quantum effects in microtubules might be involved in consciousness?
One highly speculative theory (championed by Roger Penrose in Britain and Stewart Hameroff in the United States) holds that consciousness itself is generated by quantum effects in microtubules. Why microtubules? To Penrose, they are just small enough to be subject to quantum effects, just large enough to avoid getting lost in the thermal "noise" of the brain. Penrose wants to use quantum effects to explain consciousness because quantum effects are something special, outside the normal bounds of the physical sciences, and Penrose does not believe consciousness arises "merely" from computational processes in the brain.
Physicist E. Tegmark of the University of Pennsylvania published an article in the journal Physical Review E in February, 2000, showing that quantum changes operate on a time scale far too fast for neurons, and the effects proposed by Penrose and Hameroff could not work unless the brain was cooled to near Absolute Zero to avoid background noise and heat. "It's reasonably unlikely that the brain evolved quantum behavior," concludes one physicist (Seife, 2000).
How is a previous assumption of psychologists now known to be dramatically wrong?
Even if they do not carry out quantum computing, nerve cells are very, very complex. Once-a half-century ago-psychologists modeling nerve-cell interactions on computers were able to assume that information was passed between neurons primarily in a binary (on/off) code. This provided a handy analogy to the binary code of computers. Now it is known this handy analogy is dramatically wrong (Hopfield & Tank, 1986). Much information processing of neurons involves rapid exchange of complex molecules plus weak electrical potentials that are graded (not all-or-none, therefore not binary).
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