Question 1
Paper 2Medium16 marksA stimulus initiates an action potential at site A of an axon. Recording sites B and C are further along the same axon. The threshold potential is −55 mV.
| Recording site | Distance from A / mm | Time at which peak potential is reached after the stimulus / ms | Peak potential / mV |
|---|---|---|---|
| A | 0 | 1.0 | +30 |
| B | 4 | 4.2 | +30 |
| C | 8 | 7.4 | +30 |
State the threshold potential for an action potential in this axon.
Calculate the propagation speed of the action potential between B and C. Show your working and include a unit.
Describe how the time at which the peak potential is reached changes with distance from A. Use the data to support your answer.
Explain how local currents arising at A can bring the adjacent region of the axon to threshold and initiate an action potential there. Include the movement of sodium ions inside and outside the axon.
The experiment is repeated with the same stimulus, but with a reduced concentration of sodium ions outside the axon. An action potential is still initiated at A in the reduced-sodium condition. The maximum potential reached at B and whether an action potential is detected there are recorded.
| Condition | Maximum potential at B / mV | Action potential detected at B |
|---|---|---|
| Normal external sodium-ion concentration | +30 | Yes |
| Reduced external sodium-ion concentration | −58 | No |
Using the data and the threshold potential, explain why reducing the external sodium-ion concentration affects propagation to B.