Membrane potentials

Question 1

What is a membrane potential

Answer 1

The charge differential between the positively charged extracellular fluid and the negatively charged cytosol

Question 2

What is the neuronal resting potential

Answer 2

-70mV

Question 3

What is the cardiac resting potential

Answer 3

-90mV

Question 4

How is a membrane potential formed

Answer 4

A high intracellular concentration of potassium and a high extracellular concentration of sodium

Question 5

How is the membrane potential gradient maintained and how does it work

Answer 5

The ATP driven sodium-potassium pump

For every 3 Na+ pumped out, 2 K+ is pumped

Question 6

How is the membrane potential increased

Answer 6

There are leaky potassium channels, so there is a net flow of potassium out of the cell therefore a net negative charge inside the cell

Question 7

What is the equilibrium potential - E ion

Answer 7

The equilibrium potential of a membrane permeable to one ion

Question 8

How can the E ion be calculated

Answer 8

The Nernst equation

E ion = 62 mV * Log[ion]outside / [ion]inside

Question 9

Why is calculating an E ion from a single ion not completely accurate

Answer 9

The neuronal cell membranes have many more k+ channel then Na+ channels therefore there is a slight compromise between the Ek and Ena
The cardiac cell membranes have even more K+ channels then Na+ channels therefore the resting potential is closer to the equilibrium potential of potassium

Question 10

What does the Goldman equation calculate

Answer 10

The resting potential of a cell taking into account multiple ion permeabilities
E= RT/F * ln (PNa[Na+]out + PK[K+]out +PCl[Cl-]in) / (PNa[Na+]in + PK[K+]in +PCl[Cl-]out)

Question 11

What is hyperpolarisation

Answer 11

When gated K+ channels open, the K+ ions diffuse out making the inside of the cell more negative

Question 12

What is depolarisation

Answer 12

When gated Na+ channels open, the Na+ ions diffuse in making the inside of the cell more positive

Question 13

How does an action potential form

Answer 13

1. Resting state - lots of open potassium channels, a few open sodium channels and an active sodium potassium pump. voltage gated ion channels will be shut
2. Depolarisation - depolarising stimulus will cause sodium channels to open. Na+ will flow out into the cell making it more positive, this will cause other sodium channels across the membrane to open creating a huge influx of sodium into the cell
3. Falling phase - The sodium channels will suddenly plug up, however the voltage gated potassium channels will open causing an efflux of of potassium down the concentration and electrical gradient
4. Undershoot - The cell hyperpolarises and goes more negative then the resting state, causing an inactivation of the voltage gated potassium channels
5. Reforming the resting potential - The sodium potassium pump will recreate the resting voltage. During the refractory period after an action potential, a second action potential cannot be initiated since the proteins involved are inactivated

Question 14

How is an action potential generated

Answer 14

Na+ flows inward across the membrane at one location
The depolarisation of the action potential spreads to the neighbouring regions, reinitiating the action potential there
To the left of this region, the membrane is depolarising as K+ flows out
The depolarisation process is repeated in the next region of the membrane
Local currents of ions therefore propagate the action potential along the length of the axon

Question 15

How does an axons diameter affect the speed of an action potential

Answer 15

The speed of the action potential increases as the axons diameter increases

Question 16

How are myelin sheaths formed

Answer 16

Made by oligodendrocytes in the CNS and Schwann cells in the PNS

Question 17

What do myelin sheaths do

Answer 17

Causes the action potentials to jump further therefore speeds them up

Question 18

Whats grey matter

Answer 18

Cell bodies in the brain which are unmyelinated

Question 19

Whats white matter

Answer 19

Axons in the brain which are myelinated

Question 20

Where are action potentials formed

Answer 20

Only at the nodes of ranvier at gaps between the myelin sheaths where voltage Na+ channels are found

Question 21

What are the phases of the cardiac action potential

Answer 21

• Phase 4: Diastole
• Phase 0: Depolarisation
• Phase 1: Rapid Na+ inactivation
• Phase 2: plateau phase
• Phase 3: Rapid repolarisation
• Phase 4: diastole

Question 22

First Phase 4

Answer 22

Diastole
resting potential created from sodium potassium pumps
Potassium and sodium voltage gates ion channels are closed

Question 23

Phase 0

Answer 23

Depolarisation

Stimulus from SAN causes voltage gated sodium channels to open causing rapid depolarisation

Question 24

Phase 1

Answer 24

Rapid Na+ inactivation

Voltage gated potassium channels begin to open

Question 25

Phase 2

Answer 25

Plateau phase
Voltage gated calcium channels open causing Ca2+ to flow in therefore there is a balance between efflux and influx of positive charge
Calcium induces calcium release from sarcoplasmic reticulum
Chloride channels open causing influx of Cl- ions

Question 26

Phase 3

Answer 26

Rapid repolarisation
Calcium and chloride voltage gated channels close
Potassium channels are still open so potassium keeps leaving the cell

Question 27

Last Phase 4

Answer 27

Diastole

Resting potential of -90mV restored by sodium potassium pump

Question 28

What happens to the heart when the frequency of action potentials increase

Answer 28

Increases the heart rate and the plateau phase shortens due to quicker inactivation of Ca2+ channels

Question 29

What does the action potential do in a neurone

Answer 29

Chemical synapse
Causes voltage gated calcium channels on the pre synaptic terminal to open, causing vesicles full of neurotransmitter to release
These act as chemical ligands that active ligand gated ion channels in the post synaptic terminal
This allows ion flow in the next neurone

Question 30

What does the action potential do in a cardiac cell

Answer 30

Gap junction proteins between presynaptic and post synaptic terminals allows ion flow to the next cell very fast