Biomedical Engineering Reference
In-Depth Information
12.6 THE HODGKIN-HUXLEY MODEL OF THE
ACTION POTENTIAL
Alan Lloyd Hodgkin and Andrew Fielding Huxley published five papers in 1952 that
described a series of experiments and an empirical model of an action potential in a squid
giant axon. Their first four papers described the experiments that characterized the changes
in the cell membrane that occurred during the action potential. The last paper presented the
empirical model. The empirical model they developed is not a physiological model based
on the laws and theory developed in this chapter but a model based on curve fitting using
an exponential function. In this section, highlights of the Hodgkin-Huxley experiments are
presented along with the empirical model. All of the figures presented in this section were
simulated using SIMULINK and the Hodgkin-Huxley empirical model parameterized with
their squid giant axon data.
12.6.1 Action Potentials and the Voltage Clamp Experiment
The ability of nerve cells to conduct action potentials makes it possible for signals to be
transmitted over long distances within the nervous system. An important feature of the
action potential is that it does not decrease in amplitude as it is conducted away from its
site of initiation. An action potential occurs when
V
m
reaches a value called the
threshold
potential
at the axon hillock (see Figure 12.1)
.
Once
V
m
reaches threshold, time- and
Na
þ
and
K
þ
gates that drive
voltage-dependent conductance changes occur in the active
V
m
, and finally to the resting potential. These changes in
conductance were first described by Hodgkin and Huxley (and Katz as a coauthor on one
paper and a collaborator on several others). Figure 12.19 illustrates a stylized action
potential with the threshold potential at approximately
toward
E
Na
, then back to
E
K
40 mV.
+60
Action Potential
Threshold Potential
−
60
Resting Potential
2
4
6
8
10
Time (ms)
FIGURE 12.19
Stylized diagram of an action potential once threshold potential is reached at approximately
5 ms. The action potential is due to voltage and time-dependent changes in conductance. The action potential rise
is due to
Na
þ
, and the fall is due to
K
þ
conductance changes.
