Electrical Properties of Cells: Action Potentials

Question 1

Define the action potential.

Answer 1

An ‘all-or-none’ self regenerating wave of electrochemical activity across the membrane of excitable cells.

It is characterised as a transient depolarisation of the membrane due to changes in ionic permeability, facilitated by the opening and closing of ion-specific voltage-gated ion channels.

Question 2

What is the significance of the Goldman-Hodgkin-Katz equation?

Answer 2

The equation incorporates the relative permeability of the membrane to other ions when calculating the membrane potential.

It is thus more accurate than the Nernst equation.

Question 3

Ionic channels open and shut stochastically - meaning?

Answer 3

Gating increases the probability that the channel will open - it does not determine if the ion channel opens or closes.

Question 4

What is the general structure of the sodium voltage-gated ion channel?

Answer 4

4 groups of 6 membrane-spanning alpha-helices.

There are 2 gates inside the channel:
A voltage-sensitive gate that opens at threshold value of -60mv
An inactivation gate (also voltage-sensitive) that is open at rest, closes at +50mv and reopens upon sufficient repolarisation.

Question 5

What is the general structure of the potassium voltage-gated ion channel?

Answer 5

6 membrane-spanning regions, each channel formed of 4.

Question 6

Class III antiarrhythmic drugs and 4-aminopyridine (4-AP) are used in the treatment of MS to block which ion channels?

Answer 6

K+ channels

Question 7

Class I anti arrhythmic drug, local anaesthetics and tetrodotoxin (TTX) block which ion channels?

Answer 7

Voltage-gated Na+ channels

Question 8

What is the difference between the Absolute Refractory Period and the Relative Refractory Period?

Answer 8

ARP - Another AP cannot be initiated as the v-g Na+ channels are either open or inactivated.

RRP - Another AP can be initiated as increasing number of v-g Na+ channels are reactivated and if the stimulus is strong enough to overcome the hyperpolarisation.

Question 9

Why are action potentials in cardiac muscle longer lasting than APs in nerve and skeletal muscle (300ms vs 3ms)?

Answer 9

There is a long plateau phase during hyperpolarisaion caused by Ca2+ channels opening and remaining open for a prolonged period of time.

Question 10

True or False: the length of APs in cardiac muscle and in the SAN of the heart is the same

Answer 10

True

300ms