Chapter 12
Nerve impulses
OSolemn-beating heart
Of Nature! I have knowledge that thou art
Bound unto man’s by cords he cannot sever;
And, what time they are slackened by him ever,
So to attest his own supernal part,
Still runneth thy vibration fast and strong
The slackened cord along.- Elizabeth Barrett Browning,The seraphim and other poems
In a series of five articles published in theJournal of Physiology,Alan Hodgkin, Andrew Hux-
ley, and Bernard Katz described the results of experiments that determined how and when a cell
membrane conducts ions. In the last of these papers, Hodgkin and Huxley presented experimental
data on ion movement across electrically active cell membranes, a hypothesis for the mechanism of
nerve impulse propagation, a fit of the model to their data, and a calculated prediction of the shape
and speed of nerve impulses agreeing with experiment. Many biophysicists regard this work as one
of the most beautiful and fruitful examples of what can happen when we apply the tools and ideas
of physics to a biological problem.
Thinking about the problem in the light of this book’s themes, living cells can do “useful work”
not only in the sense of mechanical contraction, but also in the sense ofcomputation. Chapter 5
mentioned how single cells ofE. colimake simple decisions enabling them to swim toward food, but
for more complex computations, higher organisms have had to evolve systems of specialist cells, the
neurons. Just like muscle cells, neurons are in the business of metabolizing food and in turn locally
reducing disorder in an organism. Instead of generating organized mechanical motion, however,
their job ismanipulating informationin ways useful to the organism. To give a glimpse of how they
manage this task, this chapter will look at an elementary prerequisite for information processing,
namely information transmission.
One often hears a metaphorical description of the brain as a computer and individual nerve cells
as the “wiring,” but a little thought shows that this can’t literally be true. Unlike, say, telephone
wires, nerve cells are poorly insulated and bathed in a conductive medium. In an ordinary wire
©c2000 Philip C. Nelson