Electrical Properties of Cells: Membrane potentials

Question 1

How does the resting membrane potential differ between neurons and skeletal&cardiac muscle?

Why is there this difference?

Answer 1

• 70mv in neurons
• 90mv in skeletal and cardiac muscle

The difference is caused by varying permeability of the plasma membrane to Na+. This differs between different cell types, the greater the Na+, the more positive the membrane potential is at rest due to inward flow of Na+

Question 2

How does the resting membrane potential differ between neurons and skeletal&cardiac muscle?

Answer 2

• 70mv in neurons

- 90mv in skeletal and cardiac muscle

Question 3

What happens to the resting membrane potential during depolarisation?

Answer 3

It becomes more positive

Question 4

What happens to the resting membrane potential during hyperpolarisation?

Answer 4

It becomes more negative

Question 5

How is the resting membrane potential generated?

Answer 5

The plasma membrane is naturally more permeable/leaky to specifically K+ ions.

This results in K+ ions moving out of the cell down its concentration gradient.

Intracellular proteins have a large net negative charge and are unable to leave the cell, hence the net negative charge remains inside the cell.

At equilibrium/rest the concentration gradient causing K+ ions to leave the cell becomes equal to the electrical force gradient causing K+ ions to move back into the cell.

Question 6

Define the equilibrium potential Ek

How is it calculated?

Answer 6

The potential for a particular ion when the concentration and electrical driving forces are equal.

By the Nernst equation: RT/zF x Ln (CONCout/CONCin)

T = 310k
R = 8.31
z = 1
F = 96485
Ln = 2.303 x log10

Question 7

What happens to the resting membrane potential during depolarisation?

Answer 7

It becomes more positive

Question 8

What happens to the resting membrane potential during hyperpolarisation?

Answer 8

It becomes more negative

Question 9

How is the resting membrane potential generated?

Answer 9

The plasma membrane is naturally more permeable/leaky to specifically K+ ions.

This results in K+ ions moving out of the cell down its concentration gradient.

Intracellular proteins have a large net negative charge and are unable to leave the cell, hence the net negative charge remains inside the cell.

At equilibrium/rest the concentration gradient causing K+ ions to leave the cell becomes equal to the electrical force gradient causing K+ ions to move back into the cell.

Question 10

What are the intracellular and extracellular concentrations of Na+ and K+ ions?

Answer 10

[Na+]i 10mM
[Na+]o 145mM

[K+]i 120mM
[K+]o 4mM

Question 11

Define the equilibrium potential Ek

Answer 11

The potential for a particular ion when the concentration and electrical driving forces are equal.

Question 12

What is the term used to describe high levels of K+ in the plasma?

Answer 12

Hyperkalaemia -> depolarisation

Question 13

What is the term used to describe low levels of K+ in the plasma?

Answer 13

Hypokalaemia ->hyperpolarisation

Question 14

True or False: Some membranes are permeable to other ions

Answer 14

True

Axon membranes have some permeability to Na+ for instance.

This is why the graphs predicted by the Nernst equation differ from actual results.

Question 15

True or False: Action of the Na+/K+ ATPase generates an outward current and so can be described at electrogenic.

Answer 15

True

The outward current caused by 3 Na+ out and 2 K+ in, results in a slight hyperpolarisation (MP becomes more negative)

This however only amounts to around 5mV maximum in tissues which have a high pump density, such as in cardiac muscle.

Question 16

True or False: At rest, the electrochemical gradient for K+ is close to zero

Answer 16

True

Question 17

How are the ionic gradients maintained at rest?

Answer 17

By action of the Na/K+ ATPase pump, that transports 3 Na+ out of the cell and transports 2 K+ into the cell.

Question 18

What is the equilibrium potential for potassium?

Answer 18

Ek = -90mv

Question 19

What is the equilibrium potential for sodium?

Answer 19

ENa = +71mv

Question 20

True or False: Action of the Na+/K+ ATPase generates an outward current and so can be described at electrogenic.

Answer 20

True

The outward current caused by 3 Na+ out and 2 K+ in, results in a slight hyperpolarisation (MP becomes more negative)

This however only amounts to around 5mV maximum in tissues which have a high pump density, such as in cardiac muscle.

Question 21

Inhibition of the Na+/K+ ATPase pump, by digitalis for example, has what effect on the membrane potential?

Answer 21

An immediate small (5mv) depolarisation, followed by a slow depolarisation as the concentration gradients run down.