A scientist studies one axon at rest and then fires it with a stimulus…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A scientist studies one axon at rest and then fires it with a stimulus. Predict what happens in each case, and give the reason.
- A poison blocks the voltage-gated Na+ channels. A stimulus is applied.
- A drug blocks the K+ channels that open during the spike, and a stimulus is applied.
- A poison stops the sodium-potassium pump. The axon is fired a few times at once.
- The fluid outside the axon is changed so that it holds a high concentration of K+. What happens to the resting potential?
Need a reminder? The key ideaShow
At rest the membrane is more permeable to K+ than to Na+, and the pump moves out and in. The outside is positive and the inside is negative. A stimulus makes the membrane freely permeable to Na+, the polarity reverses, and this is the action potential. K+ then leaves and the resting potential returns. For one ion, , and at 310 K.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) Without Na+ channels, Na+ cannot rush in. The membrane is not depolarised, and no action potential forms.
Step 2. (b) Na+ enters and the potential rises as usual. The potential cannot return quickly, because the K+ cannot leave. The fall is slow and the spike is longer.
Step 3. (c) The first spikes still occur, because the gradients built by the pump are large and take time to run down. After many spikes the gradients fade, and the axon stops firing.
Step 4. (d) The K+ gradient is smaller, so less K+ leaves the cell. The inside is less negative, and the resting potential moves towards zero.
Answer: (a) no spike; (b) a long spike with a slow fall; (c) a few spikes, then failure as the gradients fade; (d) a less negative resting potential.
Watch out. The pump does not make the spike. It keeps the gradients that the spike uses.
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