Note to future instructors: It might be a good idea to use NEURON for this simulation, in the future. Ted Carnevale has a page that relates to this.
Since we haven't the resources to examine and to manipulate actual squid axons, in this exercise you'll use a computer program to simulate the membrane electrical properties of an axon. As you learned in class, propagation of the action potential depends principally on the rapid activation of sodium channels followed by slow inactivation of sodium channels and activation of the inward rectifying potassium channel. The actions of these channels alter the ionic composition of the intracellular medium and thus change the membrane potential. This change is described by the Hodgkin-Huxley Equation:
When gCl and gNa are zero, and thus the membrane is permeable only to potassium, the Hodgkin-Huxley Equation reduces to the Nernst Equation:
The program that you'll be using models the membrane potential using a set resting value for the potassium conductance in this equation. Voltage-gated ion channels are modelled by making gNa and gK non-linear, time-dependent functions of the membrane potential. The controls allow you to supply up to two distinct electrical stimuli. The first of these stimuli may be -5mV to 100mV in amplitude and 0 to 8ms in duration. The second may be 0 to 100mV and 0 to 1ms. The delay between the onsets of the two stimuli can be up to 10ms. By manipulating these parameters and observing the behaviour of the membrane, you should be able to draw some conclusions about the ways in which important properties of neural action are implemented by electrochemical membrane properties. You should pay attention not only to the membrane potential but also to the separate sodium and potassium conductances graphed in the lower panel of the display. (If this were a live preparation instead of a simulation, note that we could use TTX and TEA to separate these conductances.)
As you learned in class, every excitable membrane has a threshold potential. Depolarisation beyond the threshold results in an action potential, while lesser depolarisations result only in temporary and localised ionic currents. Set the duration of stimulus #1 at 0.1ms. By manipulating the amplitude of stimulus #1, determine the difference between the resting potential and the threshold potential of the membrane being modelled by this program. What happens when you increase the amount of injected current so that the membrane depolarises beyond the threshold?
Leave stimulus #1 at 0.1ms duration, and set it at the largest current that doesn't bring the membrane to threshold. Set stimulus #2 to 0.1ms duration and threshold amplitude. Set the inter-stimulus delay to 5ms. Now run the simulation. What happens? How can you explain this? What happens when you increase the amplitude of the stimulus? Record your observations for each step in amplitude.
Return the amplitude of stimulus #2 to its original setting. Increase the inter-stimulus delay in increments of 0.1ms. Record your observations for each delay step. Explain what mechanism might produce the effects that you see.