In a classic series of papers from the early 1950's, A.L. Hodgkin and A.F. Huxley performed a painstaking series of experiments on the giant axon of the squid. Based on their observations, Hodgkin and Huxley constructed a mathematical model to explain the electrical excitability of neurons in terms of discrete Na+ and K+ currents. A Java version of their Nobel prize winning model (as described in J. Physiol., 1952, 117: 500-544) is presented below:
Source code
The model simulates an electrical signal called an action potential that passes through
the axon of a neuron. Action potentials allows neurons to
communicate with one another and with muscle cells. This electrical
communication makes possible all of our brain's activity and all muscle
movement. After you start the model and hit the stimulate button,
enough current to raise the voltage +15 mV is injected into the axon. The first
time you do this, you'll observe an action potential. If you hit the stimulate button
again immediately after the action potential has fired, you'll notice that another
action potential does not occur. If you wait a bit longer, however, and again hit the
stimulate button, an action potential will again fire. This demonstrates the
"refractory period". After a neuron fires, it needs to
"rest" before it can fire again. If you experiment a bit with the
stimulate button, you'll notice that if you hit it quickly many times in a row
you can overcome the refractory period and cause the neuron to fire.
The first 4 equations above are the differential
equations which the program repeatedly steps through. You can go look up what
all the symbols mean in Hodgkin and Huxley's 1952 paper (J. Physiol.,
117:500-544, p. 518)!
What does the model
show?
What
equations does the model use?
Where can I learn more?
Known
bugs
E-mail the author
What does the model show?
What equations does the model use?
Where can I learn more?
The classic book on this stuff is "Ionic
Channels of Excitable Membranes" by Bertil Hille. The first few chapters
cover the Hodgkin-Huxley model in detail. If you are looking for something a
little more introductory, I am partial to "Molecular Biology of the
Cell" by Alberts, Bray, Lewis, Raff, Roberts and Watson. The book covers
all of cell biology (not just neurons and the like) so it is big and expensive,
but it very well written and has pretty pictures. Alternatively, you could just
go to
graduate school!
A much more comprehensive suite of models in Java is online at http://pb010.anes.ucla.edu/
Like all Java applets, the model may not run from behind a firewall. And obviously, the applet will not run if the browser is not Java-enabled.
On some browsers scrolling up and down can cause the model to become scrambled. I've notice this happening on applets at http://java.sun.com as well, so it seems to be a Java problem. Hitting the refresh button on the browser will reload and unscramble the applet.
If you have any comments, criticisms or bugs to report, I would be very grateful for feedback. You can e-mail me at anthony.fodor@gmail.com.