Membrane Potentials And Action Potentials

Question 1

What is the Nernst equation?

Answer 1

Used to calculate (theoretically) the equilibrium potential (E)

Look it up

Question 2

What is the GHK equation?

Answer 2

Goldman-Hodgkin-katz

A more accurate way of calculating membrane potential (Em)

Takes into account P (permeability or Chanel open probability)

The membrane potential is almost wholly caused by the movement of K+ out of the cell. But a tiny amount of Na+ can move into the cell just because nothing is perfect. This is what gives the difference between the theoretical and actual value

Question 3

What is depolarisation?

Answer 3

Membrane potential becomes more positive

Question 4

What is repolarisation?

Answer 4

Membrane potential decreases towards resting potential

Question 5

What is overshoot?

Answer 5

Membrane potential becomes positive

Question 6

What is hyper polarisation?

Answer 6

Membrane potential decreases beyond resting potential

Question 7

What are graded potentials?

Answer 7

Different depending on type or strength of stimulation

Can cause depolarisation or hyperpolarisation at different levels

They determine what happens next

Do they initiate or prevent action potentials? Depends

Question 8

When do action potentials occur?

Answer 8

When a graded potential reaches a certain threshold

After enough sodium channels have been opened

Results in an all or nothing event

Occurs on neurones and muscles but also some endocrine tissues

Question 9

What happens to the ion channels In different states?

Answer 9

Opened by depolarisation

Inactivated by sustained depolarisation

Closed by hyper/repolarisation

All changes in membrane potential during an action potential are due to ion channels NOT ion pumps

Question 10

What are the 5 phases of an action potential?

Answer 10

1. Resting membrane potential - Permeability for potassium is much grater than that for sodium. -70mV
2. Depolarising stimulus - this depolarises the membrane towards the more positive end and towards the threshold
3. Upstroke - a lot of VGSCs open causes an influx of Na+. Some VGKCs open slowly some K+ leaves cell. Around +40mV
4. Repolarisation - VGSCs start to close. VGKCs open and stay open so more K+ leaves the cell. Sodium channel inactivation also occurs so that no more sodium ions can move through. A new action potential cannot be triggered - refractory period
5. After-Hyperpolarisation - the VGKCs remain open. As membrane potential moves closer to K+ equilibrium some of the VGKCs start to close. Causes a hyperpolarisation. Then the membrane potential returns to is resting value

Question 11

What is the relative refractory period?

Answer 11

Occurs just after the absolute refractory period

This is the dip In the graph.

Some of the VGSCs have recovered from inactivation

This requires a stimulus that is much stronger than usual

Question 12

What factors effect the propagation velocity and distance?

Answer 12

Internal and membrane resistance

Diameter

Myelinated or not

Larger diameter, myelinated neurons propagate fastest and farthest

The speed of conduction is about 120m/s

Question 13

What decreases speed of conduction?

Answer 13

Reduced diameter (re growth after injury)

Reduced myelination(multiple sclerosis and diphtheria)

Cold, anoxia, compression, drugs (some anaesthetics)