The Embryo at the Wound 65
are charged particles into which a salt breaks up when dissolved in
water; all salts dissociate in water into positive and negative ions, such
as the positive sodium and negative chloride ions of table salt.) Bernstein
postulated that the membrane could sort most of the negatives outside
and most of the positives inside the fiber. The membrane was polarized
(with like charges grouped on one side), having a transmembrane poten-
tial, because the negative charges, all on one side, could potentially flow
in a current across the membrane to achieve a balance on both sides.
This was what happened in a short segment of the membrane whenever a
nerve was stimulated. Part of the membrane became depolarized, revers-
ing the transmembrane potential. The nerve impulse was actually a dis-
turbance in the potential traveling along the membrane. As the area of
disturbance moved along, the membrane quickly restored its normal
resting potential. Thus the nerve impulse wasn't an electrical current,
even though it could be measured electrically.
Bernstein's hypothesis has been confirmed in all important respects,
although
it
remains
I
hypothesis
because no one has yet found what
gives the membrane the energy to pump all those ions back and forth.
Soon it was broadened, however, to include an explanation of the current
of injury. Reasoning that all cells had transmembrane potentials, Bern-