Lecture 22: The Action Potential

Question 1

How does an action potential occur?

Five steps

Answer 1

A stimulus or local depolarisation triggers.

A small number of Na+ channels open, and the membrane potential DEPOLARISES towards the THRESHOLD potential.

Once threshold is reached, cell DEPOLARISES and becomes positive (up to +30mV). Once max. positive charge is reached, Na+ channels start to close.

Then, large # of K+ channels open and membrane REPOLARISES.

Then, many K+ channels close and the RMP is restored buy ATPase.

Question 2

What is the refractory period?

Answer 2

The period immediately following stimulation during which a nerve is unresponsive to further stimulation. Occurs from when threshold is reached to then RMP is restored.

Question 3

How does myelination increase the conduction velocity of a nerve?

Answer 3

It insulates the nerve, concentrating the impulse and causing it to ‘jump’ between the nodes of Ranvier.

Question 4

Why is KCl lethal?

Answer 4

The increase in K+ concentration lowers the resting electrical potential of the heart muscle cells, preventing them from repolarising and refiring.

Question 5

What is the difference between absolute and relative refractory periods?

Answer 5

Absolute: Max. depolarisation, no more action potentials can travel through cell.

Relative: Another action potential can pass through cell if very strong.