It ain’t sound…

March 22nd, 2007

… if no one can hear it. Everything is relative, right? So, waves of pressure or compression might roll along at a given frequency, but unless you get those waves to bump up against your eardrum, to you, the observer, there is no sound. They are just waves, rolling along. And, even then, if they don’t fall within the frequency range that your ear is set up to perceive, you won’t hear a thing.

The thing that got me thinking about this is a paper that came out recently postulating that the old model of electrical conduction in nerves might not be correct. The paper hypothesizes that nerves might instead produce “sound waves” that compress the cell membrane of the nerve, and thus cause changes in the membrane itself that produce the voltage change that we measure as the Action Potential. It wasn’t until the end of the paper that they say, ‘oh yeah, we didn’t really mean sound waves at all.’


“It should be noted that we use the term sound propogation in a general sense that includes all changes of the thermodynamic variables that accompany a mechanical compression…” Once I started reading the paper, I realized that they weren’t talking about sound as noise persay. But, the use of the terminology did open them up for misunderstandings with the general media. Hence, the headline from Live Science: “Controversial New Idea: Nerves Transmit Sound, Not Electricity”.

Now, there are a couple of things wrong with this headline. First, they aren’t saying that nerves don’t transmit electricity. What the paper does suggest is that the current model doesn’t account for other thermodynamically based observations, and that there might be a mechnical component involved as well. It is possible that the stimulation of ion channels initiates internal changes that create coincident mechanical and electrical forces.

Second, the headline is wrong that this is a new idea, Hodgkin and Huxley, the originators of the currently accepted model of the nerve pulse, suggested that the pulse might be a mechanical wave in 1945. But, the model they came up with didn’t have room for thermodynamic properties. This is just the first time that anyone has succinctly come up with a way to incorporate thermodynamics into the physics of the nerve pulse.

The third thing that bothers me about the headline is the drama that is insinuated. Controversy? Of course there is controversy. This is science. Of course, this model still needs to be rigorously tested before it will become widely accepted within the community, but it does tie the story up nicely.

I think the interesting question now is for the researchers to come up with a link between ion channel activation and the initiation of the soliton (mechanical compression wave) within the axon. Then, what propogates what? Does the soliton produce the electrical pullse, which goes on to stimulate ion channels at the synapse, or does the electrical impulse created by the ion channels produce the soliton? Do the electrical and mechanical pulses depend on one another for propogation down lengthy axons? Just a few interesting questions that have yet to be addressed.

I really love it when situations like this arise… when you think something is known irrefutably, and then evidence appears to suggest that there is an entire world yet to explore beyond what is known. I might be alone in this persspective, but really love the challenge of having to open my world-view a little wider.

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