5.3.4 Action Potentials

Question 1

What causes an action potential?

Answer 1

• Rapid movement of sodium ions and potassium ions across voltage gated channel proteins in the axon
• At resting potential these voltage gated channel proteins are closed

Question 2

What are the 5 stages that occur during an action potential?

Answer 2

1. Stimulus
2. Depolarisation
3. Repolarisation
4. Hyperpolarisation
5. Returning to the resting potential

Question 3

Stage 1 : Stimulus of action potential

Answer 3

• ELECTRICAL IMPULSE or chemical change in membrane of neurone triggers sodium ion channels in membrane to open
• Sodium ions diffuse into neurone down electrochemical gradient
• If the stimulus is great enough to cause enough sodium ions to be transported from outside to inside the axon through facilitated diffusion via voltage gated channel proteins in the axon membrane, then the electrical potential of the axon will increase to a threshold of -55mV.
• If this threshold is reached, the resting potential can be converted to an ACTION POTENTIAL, and DEPOLARISATION can occur

Question 4

Stage 2 : Depolarisation

Answer 4

• When threshold of electric potential of axon reaches -55mV, an action potential is stimulated
• Voltage-gated sodium ion channels in axon membrane open
• Sodium ions diffuse into axon down electrochemical gradient
• Movement of sodium ions reduces potential difference across axon membrane as inside of axon becomes less negative
• This is called depolarisation
• Depolarisation triggers more channels to open causing more sodium ions to enter causing more depolarisation
• Positive feedback
• Action potential generated reaches around +30mV

Question 5

Stage 3 : Repolarisation

Answer 5

• Very shortly after electrical potential reaches +30mV (action potential stimulated), all sodium ion voltage gated channel proteins close in this section of the axon, so diffusion of sodium ions into axon stops
• Potassium ion voltage gated channel proteins in this section of the axon open, allowing diffusion of potassium ions out of the axon down the concentration gradient
• Electric potential returns to -70mV (resting potential)
• This section of the axon is repolarised
• Negative feedback

Question 6

Stage 4 : Hyperpolarisation

Answer 6

• Potassium channel proteins are slow to close, so too many potassium ions diffuse out of neurone
• This causes hyperpolarisation, where the electrical potential becomes more negative than the resting potential

Question 7

Stage 5 : Returning to resting potential

Answer 7

• Once potassium ion voltage gated channel proteins close, sodium ion channel proteins open to restore the electric potential of the axon back to the resting potential
• Sodium ion channel proteins then become responsive to depolarisation again in this section