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The Nerve Impulse: Restructuring Waves and
Electrical Currents
Lev Verkhovsky (Moscow) levver@list.ru
An article “The Brain, Reimagined,” by Douglas Fox (Sci.
Amer., v.27, No 3, Fall 2018) concerns work by physicists Thomas
Heimburg and Andrew D. Jackson [T. Heimburg, A.D. Jackson, Proc.
Nat. Acad. Sci. 102, 9790 (2005)], who argue that signals in neurons
are conveyed by mechanical waves of expansion and contraction
of the cell membrane rather than by electrical spikes, or action
potentials, as described by British researchers Alan Hodgkin and
Andrew Huxley.
These authors from the Niels Bohr Institute in Copenhagen discovered
that when a nerve signal is transmitted in the axon’s membrane he is
accompanied by a shock wave that travels down the axon. Their main
statement: As the wave front advances, it squeezes the lipid
molecules, briefly changing them from fluid to liquid crystalline,
making them bulge and release heat. As the wave passes, the
molecules revert back to fluid form, narrowing and reabsorbing the
heat.
But the chief provisions of the Huxley-Hodgkin theory are firmly
established, so it is necessary not to discard it, but to supplement it. And
the Mikelsaar conjecture allows us to make such a synthesis.
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`A Hypothesis on the Structure of the Biomembrane Lipid
Bilayer` of Dr. R.-H.N. Mikelsaar (Tartu University,
Estonia) about the honeycomb-like structure of the lipid
biomembrane was published in a scientific journal
`Molecular Crystals and Liquid Crystals`, Vol. 152, pp.
229—257 (1987)
00268948708070955.pdf
Then he published an article in the Soviet popular science
magazine "Chemistry and Life" (1990, No. 4 in Russian).
I briefly once again outline the essence of the Mikelsaar hypothesis
and will give illustrations (which I did not do in my previous notes on
this topic; I took them from the article in "Chemistry and Life").
Working (= playing) with Tartu plastic atomic-molecular space-filling
models (made under his leadership) Dr. Mikelsaar discovered that the
three phospholipid (six lipid tails) molecules can form a right hexagonal
prism. Every prism is closed above by `a hat` of three polar groups
(heads of lipids) — they are bound by electrostatic interactions.
According to Mikelsaar`s hypothesis, such hexagonal trimeric units
cover all the surface of the membrane — it looks like the floor of a
room with the parquet hexagonal tiles. And it is similar to a
honeycomb.
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But inside prisms, there are cavities which must be filled with some
substance. It turned out that the three molecules of cholesterol perfectly
fit it (in the photo on the left); however, the quantity of this steroid in the
lipid layer can vary and be not enough to fill all prisms. In this case, the
prisms can contain -- and this is a clue point -- tubes of structured (ice-
like) water (they are named shafts); thus, so-called a hydrophobic lipid
membrane may contain significant amounts of water. It is important that
in the hydrophobic environment of lipid tails, this water (shafts) will
freeze not at zero by Celsius but at a higher temperature, possibly
physiological temperature. Ice`s melting will cause greater mobility of
lipids, and that`s the physical meaning of the
membrane phase transition (it is known, the high
amount of cholesterol diminishes phase transition,
now it becomes clear, why: the absence of water –
the absence of transition).
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A very interesting opportunity this honeycomb model opens for the
molecular mechanism of nerve impulses. There is opinion (Ichiji Tasaki),
that Na+ flow through the membrane occurs not by using special protein
channels, but directly through the lipid bilayer, which changes its state
under the action of a potential jump. The proposed model implements
this idea.
It is established that the initiation of sodium current into the axon is
accompanied by the shift of charged atomic groups (gate current). One
can imagine such a picture: at the potential jump on the membrane polar
heads of lipids will rise, turn at a certain angle and fall into new
positions, forming connections with the heads of neighboring prisms; in
this membrane`s domain, the highly ordered quasi-crystalline state will
arise (neighboring prisms bind to each other). The gates (caps) open and
each prism will become a channel for sodium ions — the geometry of
the holes at the top will allow Na+ (but not K+) to pass through:
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With this trigger switching, the entire lipid bilayer transcends in a
highly ordered quasi-crystalline state (each prism will be connected
with neighboring). A phase transition will take place in the water which
was fluid in the shafts – it will turn into ice, the membrane width will
increase (because lipid tails will straighten), and its area will decrease.
In this case, heat will be released: the water has frozen; when returning
to its previous state, the membrane will absorb this heat.
This means that all the effects that T. Heimburg and A. D. Jackson
observed are easily explained on the basis of Mikelsaar model. The
whole picture becomes clear: the potential jump causes a trigger effect
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and a flow of sodium ions will rush through the membrane. This will
cause a potential jump in the adjacent section of the membrane, and so
on -- structural changes and electrical impulse mutually support each
other. There are no contradictions between them.
It can be assumed that this model will also clarify the old riddle about
the mechanism of action of general anesthetics. It is possible that
anesthetic molecules, penetrating into the lipid layer, distort its
structure, due to which trigger switching does not occur.
Considered while hypothetical trigger switching of lipid heads is a
completely new effect, which is associated precisely with the
honeycomb-like structure of the membrane (again, hypothetical so far).
Now the task is to confirm or refute Mikelsaar hypothesis.
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