Hodgkin-Huxley Axon

### Introduction

This applet gives an introduction to the spread of excitation on an axon based on a spatially extended Hodgkin-Huxley model.

### Credits

This applet was written by Thomas Pollinger.

### Notes

Notice that the applet may only be viewed with a browser that has a Java virtual machine for Java 1.1 built in.

### Spike Propagation on a Hodgkin-Huxley Axon

This applet simulates the propagation of action potentials on axons made up of Hodgkin-Huxley like compartments put in a linear chain. This applet is quite similar to the previous one on the Hodgkin-Huxley model. If you have problems getting started, look at the description of the input fields there. Notice that the axon is stimulated at its front end (first compartment) by means of an injected current.

The graph below the input fields shows the membrane voltage of four compartments, the 1st, the 4th, the 36th and the 40th (last) compartment to outline the time course of an action potential at different locations along the axon.

Spike Propagation
A spike will be artificially produced in the first compartment by means of current injection. The compartments are linked to each other at their sides. This is represented schematically by the axial resistance Ra. The higher this resistance, the less current can flow from one compartment to its neighbours.
1. Consider the applet above you.
2. Click on "run" to start the simulation.
You can see two green graphs that represent the membrane voltage for the compartments 1 and 4 and two blue graphs for the compartment 36 and 40. The spike moves along the axon as you can see in the time shift between t hese spikes.
• Try to estimate the speed of the action potential for the obtained plot. Notice that the compartment width is 1mm.
1. Change the axial resistance from 0.03 to 0.015
2. Start a new simulation
3. Observation? Speed of spike for this new axial resistance?

4. Change axial resistance to 0.03
5. Click on "clear".
6. Start simulation
7. Change axial resistance to 0.06
8. Begin simulation
9. Observations?
Now, try to run a simulation with the axial resistance set to 0.01. You will discover that the green graphs are delayed at the beginning. This is due to small injection current so that the membrane currents are not strong enough to accumlate the sodium ions inside the axon to go beyond the threshold. Take a stronger injection current (for example 5). What can you observe?