In a statement that turns conventional knowledge on its ear, Thomas Heimburg and Andrew Jackson of Copenhagen University have declared that nerves do not relay electrical impulses from the brain to other parts of the body.
"For us as physicists, this cannot be the explanation," Heimburg says in a statement on the University of Copenhagen website. "The physical laws of thermodynamics tell us that electrical impulses must produce heat as they travel along the nerve, but experiments find that such heat is not produced."
(Rubinson observes: "The reason is that the heat is produced in the creation and maintenance of a large ionic imbalance. The generation of electrical impulses (action potentials) uses the energy stored in the imbalance.")
Conventional wisdom holds that neurons, the nerve cells that transmit information, send messages electrochemically—chemicals (ions) create an electrical impulse that is passed along to the next neuron. The impulse passes from one neuron to the next in one of two ways: either through direct electrical links (electrical synapses) or via chemicals called neurotransmitters (chemical synapses), which act on external receptors, which open channels that allow smaller ions (such as sodium, potassium, and chloride) to pass through.
Heimburg and Jackson theorize that sound propagation—a purely mechanical explanation—makes more sense than electrochemical conversion. One drawback to that suggestion would be that sound propagates as a wave, spreading out and becoming weaker over distance. Ah, they say, if the transmission medium has certain properties, it is possible to create localized pulses (solitons) which propagate without losing their strength, changing shape, or spreading.
(Rubinson thinks the answer is even simpler: "Vibrations (a better term than sound since no one ever hears this) can be confined to a single element if the physical impedance of energy transfer to other elements is high. That's why most of sound bounces off the surface of a pool and a submerged swimmer hears little of what is going on above. Of course, that requires the restriction of the energy to a less dense medium.")
Heimburg and Jackson argue that the nerve membrane, composed as it is of olive-oil–like lipids, is a medium that changes its state from liquid to solid at a particular temperature trigger-point. And wouldn't you know it? That freezing point is perfectly suited to propagating concentrated sound impulses.
(You can almost hear Rubinson's eyebrow arch at this one: "Is that freezing point in the biological range? I've used olive oil, as have you, and it gets viscous in the cold—but at temperatures not found in a living mammal.)
Heimburg and Jackson have a number of publications looking at medical issues from a physics perspective, most recently a joint paper in Biophysical Journal on ""The thermodynamics of general anesthesia."
It appears that Heimburg and Jackson's focus on anesthetics is what has led them to this radical theory. In their précis, the physicists ask: "How is it possible to operate on a patient without pain? It has been known for more than 100 years that substances like ether, laughing gas, chloroform and the noble gas xenon can serve as anesthetics. These substances have very different chemical properties, but experience shows that their doses are strictly determined by their solubility in olive oil. In spite of this, no one knows precisely how anesthetics work and how the nerves are 'turned off.'"
(Rubinson found a lot to rumble with here. First, he points out, "ether, laughing gas, chloroform and the noble gas xenon" are "really CNS poisons and do not eliminate pain but, rather, eliminate consciousness so that pain and every other sensibility is blocked." He points out the claim that any substance that has to cross the lipid bilayer that makes up the cell membrane must also be soluble in lipids—even our old friend ethanol. As to the claim that we don't know " precisely how anesthetics work and how the nerves are 'turned off,'" he begs to differ: " We do know how topical anesthetics work, including those that are injected for local anesthesia: They generally block the membrane channels that let the small ions in/out of the cell. So, this entire paragraph is dreadfully misleading even if the original papers deal with it responsibly.")
Heimburg and Jackson write: "If a nerve is to be able to transport sound pulses and send signals, the membrane's melting point must be sufficiently close to body temperature. The effect of anesthetics is simply to change the melting point—and when the melting point has been changed, sound pulses cannot propagate. The nerve is put on stand-by, and neither nerve pulses nor sensations are transmitted. The patient is anesthetized and feels nothing."
(Again, Rubinson is dubious: "This paragraph seems to be pure speculation of a phenomenon that can already be accounted for by current knowledge.")
Do we buy it? Is it time to throw out everything we know about the nervous system—and trickier still, revoke a few Nobel prizes? Of course not—but we do love to watch a good fight, even if we don't have a dog in the fray. We'll be watching developments in neurophysiology with great interest.
Not to mention starting an office pool predicting when we'll see the introduction of the first lipid-augmented audio cables.
(03/14/07 Update: We received the following letter:
As a long-time reader, I was surprised and delighted to read your piece on the website ("The Nerve!" March 11, 2007) about my work with Thomas Heimburg.
"Regarding the office pool, it may interest you to know that we have already produced lipid cables (called 'tethers' in the trade). Thomas, who does the experiments, has beautiful photographs clearly showing the tethers and alternating regions of liquid and solid phase. Our next step is to introduce electrical contacts with the aim of exciting solitons in these 'artificial nerves.' Since Thomas and I are both serious Bach listeners, we will certainly attempt to be the first to transmit his music along these lipid-augmented cables!
"Of course, it's not likely that such cables will be in your local audio store soon. At the moment, they are short (about 100 microns) and quite fragile. And they would probably be both of poor audio quality and expensive.
"Our work seems to have touched a nerve (sorry), and it has received an amount of media attention that is somewhat bewildering for a pair of ivory-tower types. Your piece stands out both for the fidelity with which it presents our scientific ideas and for its accuracy in mirroring the fun we have had in developing them.
Andrew D. Jackson,
Professor of Theoretical Physics
The Niels Bohr Institute)