1.08 - Membrane & Action Potentials

Question 1

Define: Potential Difference

Answer 1

A charge separation across a membrane of a cell. whenever separation of charge occurs, a potential difference exists.

Question 2

What is potential difference measured in?

Answer 2

Volts

Question 3

How is the charge separation formed?

Answer 3

it is a combination of unequal intracellular and extracellular ion concentrations and a membrane that is ‘leaky’ to ions

Question 4

What ions are more highly concentrated extracellularly and intracellularly respectively?

Answer 4

Intra: K+
Extra: Na+, Cl-, Ca2+

Question 5

What is the conductance?

Answer 5

The leakiness of the membrane to an ion. It is the measure of the ease with which an ion can cross a unit area of 1cm2 of membrane

Question 6

What factors influence the conductance?

Answer 6

p= the probability a channel is open
y = the number of ions per sec a channel can conduct
n = the number of channels

Question 7

What is the value of the resting membrane potential?

Answer 7

~-60 to -70mV

Question 8

What structure maintains the membrane potential?

Answer 8

Na/K ATPase.

It uses ATP to pump 3Na+ out of the cell and 2K+ in to the cell –> against their concentration gradient

Question 9

Define: Action Potential

Answer 9

A sudden change in resting membrane potential from -70mV to ~+20mV.

Question 10

What is the duration of an Action Potential?

Answer 10

~1-2ms

Question 11

What are the three stages of an action potential?

Answer 11

Depolarisation (upstroke)
Repolarisation (downstroke)
Hyperpolarisation (undershoot below RMP)

Question 12

What is a voltage gated channel?

Answer 12

A channel whose probability of opening varies with membrane potential

Question 13

What happens when the resting membrane potential reaches -55mV?

Answer 13

voltage gated Na+ channels open

voltage gated K+ channels open but do so more slowly

Question 14

What are the stages of depolarisation?

Answer 14

Initial depolarisaion to -55mV
Opens v-g Na+ channels
Influx of Na+
Vm becomes more positive, approaches Ena (~+60mV)

Question 15

What are the stages of repolarisation?

Answer 15

Initial depolarisaion to -55mV
Opens v-g K+ channels (slow opening –> delayed effect, longer lasting)
Efflux of K+
Vm becomes more -ve, approaching Ek

Question 16

Describe depolarisation and hyperpolarisation

Answer 16

As membrane potential approaches Ena, the electrochemical force pushing Na+ into the cell decreased
Voltage gated Na+ channels inactivate
Continued efflux of K+ repolarises the membrane.
Voltage gated Na+ inactivation is removed by depolarisation.
This hyperpolarisation must occur before another action potential can take place

Question 17

What is the purpose of the leaky channels in the action potential

Answer 17

Brings about the small change in resting membrane potential to -55mV to open the V-G Na+ channels

Question 18

What is the absolute refractory period?

Answer 18

is the period when a
second action potential cannot be initiated
Due to closed Na+ inactivation gates that must be
reset (opened) by membrane repolarisation

Question 19

What is the relative refractory period?

Answer 19

is the period where a
second action potential requires a stronger than normal stimulus to be initiated
Due to leaky K+ channels that keep Vm hyperpolarised

Question 20

Summarise the Action Potential

Answer 20

When RMP reaches -55mV voltage gated Na+ channels open
Na conductance increases
Vm shifts towards ENa
Slowly, voltage gated K+ channels open
Increased K+ conductance
Na+ channels begin to inactivate and
electrochemical force on Na+ diminishes
Combination of K+ channels opening (efflux of K+
ions) and inactivation of Na+ channels (influx of Na+ stopping) leads to repolarisation of the membrane
The after hyperpolarisation occurs because voltage-gated K+ channels are still open
Vm returns to rest when they close (only leak channels open at rest)