happens unless a depolarization large enough to reach about -50 mV
takes place. When this occurs, things change abruptly and dramati-
cally. The membrane potential very rapidly becomes more and more
positive, up to a peak of about +30 mV, at which point it decreases and
becomes negative again, all the way back down to where it was at rest.
The rapid reversal and then return to resting potential takes place
very quickly, in about 1 millisecond, one thousandth of a second!
j
an)rhiviiritiy|N °o
Membranepotential(mV)(^) PRUrrpPrrrryprurrryp rere]
1 2 3 4
Time (milliseconds)
Figure 5.6. Action potential.
Direct measurement of the voltage changes across an axon mem-
brane during an action potential were carried out by Alan Hodgkin
(1914-1998) and Andrew Huxley (1917-2012) and reported ina
short paper in the journal Nature in 1939. They worked with axons
from squids, axons with a diameter large enough (0.5 millimeter) to
allow insertion of a small voltmeter electrode within a functioning
axon. Working with minimal equipment, Hodgkin and Huxley made
a major contribution to modern cellular and molecular neurophysiol-
ogy.