Biological Physics: Energy, Information, Life

444 Chapter 12. Nerve impulses[[Student version, January 17, 2003]]

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above-threshold

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stimuli

hyperpolarizing

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ab

Figure 12.2: (Experimental data; sketch graph.) (a)Response of a crab axon to long (15ms)pulses of injected
current. The vertical axis shows the membrane potential at a point close to the stimulus, measured relative to its
resting value. The lower traces record the behavior upon hyperpolarizing stimuli, the upper traces to depolarizing
stimuli. The threshold value of the stimulus has arbitrarily been designated as strength 1.0; the curves are labeled
with their strength relative to this value. The top curve is just over threshold, and shows the start of an action
potential. [Data from Hodgkin & Rushton, 1946.] (b)Sketch of the response to an above-threshold, depolarizing
stimulus at three distances from the stimulation point (compare Figure 12.1). The time courses (shapes) of the pulses
quickly assume a stereotyped form; each is shifted in time from its predecessor, reflecting a constant propagation
speed. Note how the potential drops below its resting value after the pulse, then rises slowly. This is the phenomenon
of afterhyperpolarization; see also Figure 12.6b.

up. Figure 12.2b, drawn with a coarser vertical scale, shows schematically how the potential hits a
peak (typically changing by over 100mV), then rapidly falls.
The action potential is the behavior we have been calling a “nerve impulse.” Experiments like
the one sketched in Figure 12.1a show several remarkable features of the axon’s electrical response
(orelectrophysiology):

• Instead of being graded, the action potential is an all-or-nothing response. That is, the
  action potential arises only when the membrane depolarization crosses a threshold;
  subthreshold stimuli give electrotonus (with no response far from the stimulating
  point). In contrast, above-threshold stimuli create a travelingwave ofexcitation,
  whose peak potential is independent of the strength of the initial stimulus.


• The action potential moves down the axon at a constant speed (see Figure 12.2b), which
  can be anywhere from 0.1–120ms−^1 .This speed has nothing to do with the speed at
  which a signal moves down a copper wire (about a meter every threenanoseconds,
  abillion times faster).


• When the progress of an action potential is measured at several distant points, as in
  Figure 12.2b, the peak potential is found to be independent of distance, in contrast to
  the decaying behavior for hyperpolarizing or subthreshold stimuli. A single stimulus