Membrane Potentials

Question 1

What does the term membrane potential mean?

Answer 1

The voltage difference across the plasma membrane

Represents a form of potential energy as it can enable the flow of ions down their electrical gradient

Question 2

What is hyperpolarisation?

Answer 2

When the inside of the cell becomes very negative due to the overshoot of K+ moving out
=> aka Recovery period causing ATP pump to restart

Question 3

How is an electrochemical gradient created within a cell?

Answer 3

The excess negative charge in cell exerts attractive force,
Opposing flow of additional K+ out of the cell
The electrical gradient counterbalances chemical [ ] gradient
=> electrochemical gradient created

Question 4

How is the plasma membranes electrical charge normally spread?

Answer 4

The lipid bilayer’s extracellular space is positively charged with respect to the intracellular cytosol

Inside of the cell is slightly negative compared to the outside

Question 5

What is a nerve impulse?

Answer 5

A self propagating wave of depolarisation with one part repolarising as the next depolarises

Question 6

How is an action potential generated?

Answer 6

1. At rest most Na+ and K+ channels are closed
2. Stimuli causes voltage gated Na+ channels to open
3. Causes influx of Na+ into cell => depolarisation
4. During influx, threshold reached and membrane
   potential continues increasing -> action potential
5. Voltage gated Na+ become inactivated
6. Voltage gated k+ open
7. K+ diffuse out of cell -> cell repolarised
8. K+ channels remain open increasing permeability to K+
   and then eventually closing
9. Resting potential restored

Question 7

Why do cardiac action potentials travel faster than neuronal?

Answer 7

Cardiac cells have a gap junction not a synapse

One cell cytosol is adjacent to the next - shorter distance for action potential to travel

Question 8

In neuronal cells, what is the resting potential value?

Answer 8

-70mV

Question 9

Why do the depolarisation curves of neurons & cardiac cells differ so much?

Answer 9

The difference in curve shapes is due to the difference in ion channels expressed in their membranes

Question 10

What is depolarisation?

Answer 10

When voltage gated Na+ channels open causing an influx of Na+ into the cell making the inside positive
=> results in an action potential

Question 11

How can we measure the membrane potential?

Answer 11

Using a voltmeter, the voltage difference can be detected

Question 12

Describe how an action potential arises in cardiac cells

Answer 12

1. At rest Na+ and K+ channels are closed
2. Na+ channels open
3. Influx of Na+ - cell becomes depolarised
4. Na+ channels deactivate and K+ channels open
5. K+ channels remain open
6. L-type Ca2+ channels open, causing Ca2+ influx
7. ryanodine receptors activated on SR causing CICR for
   muscle contraction
8. Ca2+ activates Cl- channels to open causing Cl- to
   diffuse in
9. Flow of K+ out counteracted by Ca2+ and Cl-
10. Ca2+ and Cl- channels close while K+ remains open
11. Na+/K+ pump and Na+/Ca2+ exchanger restore resting
    potential

Question 13

How can we calculate the resting potential?

Answer 13

Using the Nernst Equation

Question 14

How does a myelinated sheath increase speed of action potentials?

Answer 14

Action potentials in myelinated axons jump from nodes of Ranvier via saltatory conduction rather than travelling along the whole length of the axon
Action potentials are formed at Nodes of Ranvier

Question 15

What is the concentration of ions inside of cardiac cells during the formation of the resting potential?

Answer 15

Inside Cardiac cell:
High [K+]
Low [Na+]
Low [Cl-]

Outside Cardiac Cell:
Low [K+]
High [Na+]
High [Cl-]

Question 16

Why does the refractory period occur?

Answer 16

The refractory period is a result of a temporary inactivation of Na+ channels

Question 17

How is the resting potential maintained in a cell?

Answer 17

There is a net outflow of K+
Therefore a net negative charge inside cell
K+ channels always open at resting potential => K+ leak
Negative charge remains in cell

Question 18

State the Goldman Equation

Answer 18

EmkNa₁-xCl = (RT/F)ln(PNa[Na+]out + Pk[K+]out + Pcl[CL-]in) / (PNa[Na+]in + Pk[K+]in + Pcl[CL-]out)

where:
E = resting potential for membrane permeable to multiple ions
R = Ideal Gas constant
T = Temperature
F = Faradays constant
P = Permeability to that ion
[ ]out = extracellular concentration of that ion
[ ]in = intracellular concentration of that ion
(in/out for negative ions is flipped in the eqn)”

Question 19

What does Eion mean in the Nernst equation?

Answer 19

At equilibrium, both electrical and chemical gradients are balanced

Question 20

Why does the membrane potential change?

Answer 20

Changes in membrane potential occur due to the presence of gated ion channels in neurons and cardiac
cells that open and close in response to stimuli

Question 21

How are the Na+/K+ gradients maintained?

Answer 21

Sodium-Potassium pumps maintain the Na+/K+ gradient in cells using ATP

Question 22

How are action potentials conducted within an axon?

Answer 22

Action potentials are generated as Na+ flow inwards across the membrane at one location
Impulse travels in one direction due to hyperpolarisation

Question 23

What is the Goldman Equation used for?

Answer 23

Goldman equation used to calculate the resting potential of a cell by taking into account the permeabilities of multiple ions

Question 24

What is the resting potential of a cardiac cell?

Answer 24

-90mV

Question 25

State the Nernst Equation

Answer 25

Eion = 62mV (log[ion] outside / [ion] inside)

Question 26

What are the [ion] of neuronal cells at rest?

Answer 26

Inside neuronal cell:
High [K+]
Low [Na+]
Low [Cl-]

Extracellular
Low [K+]
High [Na+]
High [Cl-]

Question 27

What is the myelin sheath made up of?

Answer 27

In CNS - Oligodendrocytes make up the myelin sheath

In PNS - Schwann cells make up myelin sheath

Question 28

Why is the resting potential close to Eion value for K+?

Answer 28

The resting potential of a mammalian neuron is mediated by many open K+ channels with very few
Na+ channels open
=> resting potential is closer to Ek than Ena

Question 29

How is the axon structure evolutionarily adapted?

Answer 29

Invertebrates axons are insulated by a myelin sheath

- increases action potential speed

Question 30

What is the resting potential?

Answer 30

The membrane potential of a cell at rest when signals are not being sent