Membrane Physiology

Question 1

Describe the relative compositions of K+, Na+, Ca2+ and Cl- outside and inside the axon phospholipid bilayer.

Answer 1

• High K+ inside.
• High Na+ outside.
• High Ca2+ outside.
• High Cl- outside.

Question 2

What maintains the ionic gradients over an axon’s phospholipid bilayer?

Answer 2

Ion pumps.

Question 3

What is the membrane potential (Em) of most neurones?

What does this mean?

Answer 3

~-65mV.

• This means there is a relative excess of negative charge inside.

Question 4

What is the function of leak channels on neuronal membranes?

Answer 4

To set the membrane potential by allowing passive movement of ions across the membrane.

Question 5

What is the function of voltage gated channels on neuronal membranes?

Answer 5

To generate action potentials.

Question 6

What is the function of ligand gated channels on neuronal membranes?

Answer 6

To generate membrane potential changes.

Question 7

What determines a membrane’s permeability to particular ions?

Answer 7

The number of channels for that ion present on the membrane.

Question 8

What two factors influence the movement of ions across a membrane?

How do these factors influence movement of ions across neuronal membrane specifically?

Answer 8

• Electrical and chemical.
• In a neurone, [K+] is higher inside so drive for K+ efflux.
• In a neurone, [Na+] is higher outside so drive for Na+ influx.
• At a negative membrane potential, there is a drive for both K+ and Na+ to move in.

Question 9

What is the equilibrium potential for K+?

Describe how this would be set up in the hypothetical situation where the membrane is only permeable to K+.

Answer 9

• EK = -80mV.
• Initially, Em is 0 and [K+] is greater inside, so K+ efflux occurs.
• This causes Em to become more negative.
• Negative Em creates electrical force causing K+ influx.
• When Em is negative enough (-80mV), influx = efflux.

Question 10

What is the equilibrium potential for Na+?

Describe how this would be set up in the hypothetical situation where the membrane is only permeable to Na+.

Answer 10

• ENa = +62mV
• Initially, Em is 0 and [Na+] is greater outside, so Na+ influx occurs.
• This causes Em to become more positive.
• Positive Em creates electrical force causing Na+ efflux.
• When Em is positive enough (+62mV), influx = efflux.

Question 11

How do the chemical and electrical influences of Na+ and K+ across a membrane differ at resting membrane potential of -65mV?

What is the implication for the driving force of both ions?

Answer 11

• K+ chemical influence is efflux as [K+] is higher inside, whereas its electrical influence at -65mV is influx.
• Na+ chemical influence is influx as [Na+] is higher outside, and Na+ electrical influence at -65mV is also influx.
• Implication is Na+ has a greater driving force for influx than K+ does for efflux.

Question 12

Define equilibrium potential.

Answer 12

The voltage at which an ion is in equilibrium (when chemical gradient and electrical force are in balance).

Question 13

What does the value of membrane potential depend on?

Answer 13

The relative permeabilities of K+ and Na+.

Question 14

Why is membrane potential closer to the equilibrium potential of K+ than Na+?

Answer 14

Because the permeability of potassium is 40x greater than that of sodium.