Nervous system: Action Potential

Question 1

Action potential: step 1

Answer 1

Threshold initiation:
Signals from the dendrites and cell body reach the axon hillock.

As the axon hillock depolarises, voltage-gated
channels for sodium open rapidly, increasing membrane permeability to sodium.

Sodium diffuses down its
concentration gradient into the cell.

If the stimulus at the axon hillock is great enough, the neuron depolarises by
about 15 mV to a point called ‘threshold’ (-55 mV). At threshold, an action potential is generated.

Question 2

Step 2

Answer 2

Depolarisation:
More sodium voltage-gated channels open.

This causes more sodium to flow into the cell, which in turn causes the cell to depolarise further and opens more voltage-gated sodium channels.

This positive feedback loop produces the rising phase of the action potential.

This ends with the inactivation of the Na+ voltage-gated channels and the opening of K+ voltage-gated channels.

Question 3

Step 3

Answer 3

Repolarisation:
Potassium diffuses out of the cell as the potassium voltage-gated channels open.

With less sodium
moving into the cell and more potassium moving out of the cell, the membrane potential becomes more negative, moving back towards the resting value.

Repolarisation restores the electrical
balance.

Question 4

Step 4

Answer 4

Hyperpolarisation:
Excessive potassium continues to diffuse out of the cell, causing the membrane potential to
become more negative than the resting membrane potential.

All the potassium channels are closed and the sodium-potassium pump redistributes the ions to their original, resting state levels.