Exercise goal: The Action Potential Wave Form
The goal of this exercise is to have students piece together the entirety of the wave form of an action potential provided limited data on channels. Prior knowledge of equilibrium potentials, what type of forces act on the ions, and how ions will behave relative to the current membrane potential are required to be able to perform this exercise. Students perform very well on this exercise, and all groups from all 3 sections were able to completely, and accurately describe, the important parts of the wave form.
- Equilibrium potential for Na+ & K+.
- Channels which determine membrane potential:
- Leakage Channel
- Delayed Rectifier Channel
- Activation/Inactivation Gated Channel
- Identify that only Na+ & K+ are important for this exercise.
The goal is to get students to make the connection regarding:
- How channel openings affect the flow of ions into or out of a cell.
- How this effects the conductances for an ion.
- How equilibrium potential relative to membrane potential drives the flow of an ion across the membrane.
- How the equilibrium potential determines the wave form.
I begin with a discussion of how you first get an action potential. I typically draw a neuron with dendrites and an axon. I make a point of noting the axon hillock is the point where the action potential begins. I prompt the class for how one stimulates an action potential. Students typically work their way to the point, based on prior biology experience, that neurons fire in response to a signal or stimulus,eventually causing a depolarization. I then help them connect these depolarizations to the generation of an action potential (generator potential). One important point is to make sure they understand that the action potential occurs only at the axon hillock and down the axon, and that when a neuron receives a signal, it gets small depolarizations (in the case of say, a receptor which binds a neurotransmitter and lets in the flow of a + ion, like Na+).
Once they understand how an action potential forms, I move onto describing the wave form. This is where the real exercise begins. I lay out that I’m going to divide the class up into 5 groups, and ask them to explain what is happening at that phase of the wave form. I then walk them through the entire wave form to make sure they understand what it looks like, and what I am describing to them. I begin with the neuron at rest, typically at -65 mV. I then draw the wave form and mark on the Y axis -65 mV for resting, -40 mV for threshold, and 0 mV. I then label the wave form as (1) resting, (2) rising, (3) overshoot, (4) falling, (5) undershoot. After describing the wave form, I make sure they are all aware of what “threshold” means, ie, the depolarization necessary to get an AP spike.I move onto describing the 3 channels they will need to know and the characteristics of each of these channels. Note, I do not describe to them what ions flow through it however. For the leakage channel, I let them know that it is always open. The delayed rectifier, I let them know that it is slow to open and activated by a voltage change. For the activation/inactivation gated channel, I let them know it has 2 gates to the channel that open or close depending on voltage. I also note that 1 gate is fast and another is slow in to open or close. With these pieces of knowledge, the students should be able to work out what determines an action potential wave form.
Briefly, to discuss what the groups are prompted to discuss. I ask them to:
- Of the 3 channels, which ones are open and which ones are closed.
- Of the channels that are open, what ion is flowing through the channel?
- Which direction is the ion flowing and how is that effecting membrane potential? (help make the connection with driving force)
- What are the relative conductances during this phase? (ie. is conductance of sodium higher or lower than potassium)
- How does the equilibrium potential for Na+ or K+ determine that phase of the wave form?
There are a few important points to note when you discussing with them each of these points.
Important points to make:
- Resting phase:
- Make note that the leakage channels are the only ones that are open.
- Point out that the membrane potential is becoming more positive as the activation gates are opening in response to depolarization. Point out that as the conductance for Na+ increases, the membrane potential is trying to reach ENa (equilbrium potential for Na+). You can also have them do the math, say -65 – 40 = -105 mV, ie, a whole lot of sodium is moving in as it is a negative current.
- Point out that this is somewhat of a transition period, where you have inactivation gates shutting down the Na+ channels. At the same time you also have delayed rectifiers opening up to allow in K+. Make sure students understand that K+ is flowing out of the cell. (you can mention this for the falling phase as well).
- Make sure they understand that the falling phase is due to the delayed rectifier allowing K+ to flow out of the cell. Work the math out for them… At the peak, DF = +40 – -80 = 120 mV, or even at rest, DF = -65 – -80 = +15 mV, so K+ is being pushed out to reach its equilibrium potential.
- Point out that the membrane potential is more negative than it is at rest because the conductance of K+ is higher because of the slow closure of the delayed rectifiers. Also make sure students understand that this hyperpolarization is driven by Ek (equilibrium potential for K+).
Results/common problems arising/noted:
- Resting phase:
- Rising phase:
- Describing this point in time, it is good to be somewhat vague regarding at which point in the curve are all your inactivation gates shut, or when all of the delayed rectifiers turn on… In more complicated terms, it is essentially a probability curve, so as long as they understand what is occurring, in general, that is sufficient.
Common issues/understanding problems:
- Students after class ask how do you re-establish that concentration gradient, as they visualize that a whole lot of K+ has left the cell, and a whole lot of Na+ has entered the cell. Make sure they understand that, the concentrations moving across the membrane in respect to total concentration are minute, so your overall concentrations are probably going to remain the same, and that it isn’t a massive alteration in the concentration. Also make note that you have leakage gates and the Na/K+ pump which are working the entire time during an action potential.
- If there is time, you can also trace the individual current waveforms for K+ and Na+ so students understand what the flow of these ions look like. However, you could also just reserve that for a future exercise. =p