Plate tectonics, Heliobacter pylori, and the physics of nerves

Tuesday’s New York Times included an obituary for Jack Oliver . In 1964 he and two of his former graduate students, Bryan Isacks and Lynn Sykes, set up seismographs in Tonga and Fiji. The trio detected unexpectedly deep earthquakes that originated in the subduction of the Pacific plate beneath the Indo-Australian plate. The discovery provided further, and ultimately conclusive, evidence that Earth’s continents drift about on the surface of the globe.

Remarkably, the idea of continental drift was proposed decades earlier, in 1912 by Alfred Wegener. Although Wegener could point to circumstantial evidence to support his idea—the coasts of Africa and South America look as though the two continents were split then pushed apart, for instance—he couldn’t tell skeptics what processes powered the titanic motions.

The detailed mechanism behind continental drift remains uncertain. But by 1965, evidence from paleomagnetism, ocean bathymetry, and seismology vindicated Wegener’s then 53-year-old theory. Born in Berlin in 1880, Wegener might have lived to savor his victory over the skeptics. Unfortunately, he died in Greenland in 1930 while conducting meteorological experiments.

Barry Marshall and Robert Warren didn’t have to wait to prove their skeptics wrong. In the early 1980s, they succeeded in growing colonies of a bacterium, Heliobacter pylori, outside its usual habitat, the human stomach. To vindicate their theory that H. pylori causes stomach ulcers, Marshall drank a beaker of the bacteria. Several days later he became ill with gastritis. Twenty years later he and Warren shared a Nobel Prize.

Skepticism is a core value of science. Qualifying for inclusion in the body of scientific knowledge is, and should be, a rigorous, if not a difficult, process. Wegener’s skeptics were justified in pointing out the lack of a mechanism and direct evidence. Unlike Marshall and Warren, Wegener couldn’t satisfy his critics in a direct, straightforward way.

Which brings me to a paper that I received recently from Bill Brownell of Baylor College of Medicine’s department of otolaryngology. Written by Thomas Heimburg of the Niels Bohr Institute, the paper bears the title “The Physics of Nerves.” Its abstract reads

In Heimburg’s model the membrane carries the neural signals as a local change of state from liquid to solid. The membrane’s electric potential still changes as ions pass back and forth through the membrane, but the potential’s role is to mediate the signal-carrying phase change, rather than to carry the signal itself. To support his case, Heimburg evokes various experiments that evince mechanical and thermodynamic changes in nerves as they transmit signals.

Heimburg argues that his model can also explain the still-unresolved mechanism of chloroform and other general anesthetics. Biochemical evidence has ruled out the possibility that general anesthetics work in the same way as neurotoxins—that is, by binding to, and thereby jamming, ion channels. Rather, anesthetics work, he argues, by lowering the membrane’s melting point and putting the all-important phase change out of reach.

Even though my background is astronomy, not biophysics, I’m skeptical of Heimburg’s model. Sperm whales can function at the sea surface, where the pressure is 1 atm; at depths of 3 km, where the pressure is 300 atm; and at all depths in between. Given that melting temperature depends on pressure, Heimburg’s model would seem to require some sort of continuous adaptation to ambient pressure.

As for general anesthetics, a computer simulation published in 2002 suggests that they do indeed alter the fluidity of the membrane, but in a way that makes the embedded ion channels floppier and therefore less effective.

My skepticism aside, a cell membrane, being made up of interacting particles, has mechanical and thermodynamic propertiees. It is not implausible that nature has found a way to adjust and exploit them.