15-883 Homework 1

Computational Models of Neural Systems

In this assignment you will experiment with a Matlab simulator for a spiking neuron based on the Hodgkin-Huxley equations. The simulator code is located in the matlab/hh directory in the course home page. If you have access to AFS, you can access the code directly at /afs/cs.cmu.edu/academic/class/15883-s01/matlab/hh. You can also download the code from the web page as a zip file.

Instructions

To start up the simulator, cd to the directory containing the code. Then type matlab to run Matlab. When you see Matlab's ">>" prompt, type run.
Click on the blue "S1" button to present an excitatory stimulus to the neuron. Click on the "S2" button to present an inhibitory stimulus. The red line is the membrane voltage.
The yellow and green lines represent the states of the activation and inactivation gates of the voltage-gated sodium channel. The blue line represents the activation gate of the voltage-dependent potassium channel. Watch how these gates changing state results in a "spike".
Click on the "Membrane" button to call up a window where you can modify ion concentrations and membrane properties.
Click on the "Channels" button to call up a window where you can turn channels on and off, and alter channel parameters.
To exit Matlab, type quit.

Questions

Hand in one or two sentence answers to each of the following:

Clicking on S1 causes the neuron to spike. Clicking on S1 followed by S2 blocks the spike. How closely must the inhibitory stimulus follow the excitatory one to prevent a spike from occurring?
Click on S1 to cause a spike; then click on S1 again shortly afterward. Why doesn't the neuron spike again?
Clicking on S2 once causes a temporary hyperpolarization; then the membrane potential returns to its normal resting value. Click on S2 several times in quick succession, to keep the membrane hyperpolarized for 3 msec. What happens? Why does this happen?
Go to the "Channels" window and turn off the fast (voltage-gated) sodium channel by unchecking the box. What happens when you stimulate the neuron by hitting S1? Why?
Turn the fast sodium channel back on, and turn off the delayed rectifier (voltage-gated potassium channel). Now what happens when you click S1? Why?
Turn both voltage-gated channels back on. Click S1 to generate a normal spike. Go to the "Membrane" window and reduce the external sodium concentration to 100 mM. Stimulate with S1 until you get a spike; what is the peak amplitude? Change the external sodium concentration to 550 mM. How does the spike amplitude change?

Answers due Monday, January 29

Dave Touretzky

Last modified: Mon Jan 22 02:48:49 EST 2001