12.1. The problem of nerve impulses[[Student version, January 17, 2003]] 443
a voltage measuring devices
stimulus sourcex=λaxon x=2λaxonb
t,0 0.5 1.0 1.5 2.0
msx=λaxonx=2λaxon
x=3λaxon
v
=∆V−V0 ,arbitrary unitsFigure 12.1:(Schematic; sketch graphs.) (a)Schematic of an electrophysiology experiment. Stimuli at one point
on an axon (shown as a cylinder) evoke a response, which is measured at three places. (b)Responses to a short, weak,
depolarizing pulse of current. The vertical axis represents the potential relative to its resting value. The response
to a hyperpolarizing pulse looks similar, but the traces are inverted. The pulses observed at more distant points are
weaker and more spread out than those observed up close, and they arrive later. The distance unitλaxonis defined
in Equation 12.8 on page 452.
Figure 12.1a shows a schematic of an experiment to examine the passage of nerve impulses.
Measuring devices situated at various fixed positions along an axon all measure the time course
of the membrane potential ∆V after the axon is stimulated. The axon could be attached to a
living cell, or isolated. The external stimulus could be artificially applied, as shown, or could
come from synapses to other neurons. Figure 12.1b sketches the results of an experiment in which
astimulating electrode suddenly injects some positive charges into the interior of the axon, or
removes some negative charges. The effect of this change is to push the membrane potential at one
point to a value less negative than the resting potential (that is, closer to zero); we say that the
stimulusdepolarizesthe membrane. Then the external current source quickly shuts off, allowing
the membrane to return to its resting potential.
The sketch graphs in Figure 12.1b show first that a potential change at one point spreads to
nearby regions, but that the response is weaker at more distant points. Moreover, the spread is
not instantaneous, but rather takes time. Another key point is that for weak depolarizing stimuli,
the peak height is proportional to the stimulus strength: We say that the response isgraded(see
Figure 12.2a). The response to a stimulus of the opposite sign—tending to driveVmore negative
orhyperpolarizethe membrane—is qualitatively the same as that for weak depolarization. We just
get Figure 12.1b turned upside down.
The behavior shown in Figure 12.1b is calledelectrotonus,or“passive spread”. Passive spread
is not a “nerve impulse”—it dies out almost completely in a few millimeters. Something much
more interesting happens, however, when we try larger depolarizing stimuli. Figure 12.2a shows
the results of such an experiment. These graphs depict the response at asinglelocation close
to the stimulus, for stimuli of various strengths. The bottom nine traces correspond to small
hyperpolarizing or depolarizing stimuli. They again show a graded response.
The axon’s response changes dramatically, however, when a depolarizing stimulus exceeds a
threshold of about 10mV.Asshown in the top two traces in Figure 12.2a, such stimuli can
trigger a massive response, called theaction potential,inwhich the membrane potential shoots