Membrane potentials and action potentials

Question 1

What is an ion flux?

Answer 1

Number of molecules that cross a unit area per unit of time

Question 2

What are the properties of ions?

Answer 2

Charged molecules
Opposite charges attract
Like charges repel

Question 3

Define voltage

Answer 3

Voltage = p.d.

Generated by ions to produce a charge gradient

Question 4

Define current

Answer 4

Movement of ions due to a p.d.

Question 5

Define resistance

Answer 5

Barrier that prevents the movement of ions

Question 6

How do you measure membrane potential?

Answer 6

Reference electrode is placed outside cels (zero-volt level)

Another electrode is placed inside cell, measure a voltage difference that is negative compare with the outside

Question 7

How do ions cross the cell membrane?

Answer 7

Through selectively permeable pores called ion channels in, which open and close in response transmembrane voltage, presence of activating ligands or mechanical forces

Question 8

What is an electrochemical equilibrium?

Answer 8

Electrical gradient is balancing the concentration gradients. Stable transmembrane potential is achieved

Question 9

Define equilibrium potential

Answer 9

potential at which electrochemical equilibrium has been reached. The potential that prevents diffusion of the ion down its concentration gradient

Question 10

What equation can be used to calculate the equilibrium potential?

Answer 10

Nernst Equation

Question 11

What are the most important ions for the resting potential for neurones?

Answer 11

Na+ and K+

Question 12

Why does the Goldman-Hodgkin-Katz (GHK) equation describe membrane potential more accurately?

Answer 12

Because it takes into account the permeability of the membrane to the particular ions

Question 13

Define depolarisation

Answer 13

Membrane potential becomes more positive towards zero

Question 14

Define repolarisation

Answer 14

Membrane potential decreases towards resting potential

Question 15

Define overshoot

Answer 15

Membrane potential becomes positive

Question 16

Define hyperpolarisation

Answer 16

Membrane potential decreases beyond resting potential

Question 17

What can cause a change in membrane potential?

Answer 17

External stimulation or neurotransmitters

Question 18

How is change in membrane potential graded?

Answer 18

Type or strength of stimulation; i.e. weak or strong

Question 19

What happens to graded potentials after they’ve moved a short distance (i.e 1mm)

Answer 19

Decay over the length

Question 20

Why do graded potentials decay down the length of the axon?

Answer 20

Charge leaks from the axon and the size of the potential change decreases along the axon

Question 21

What happens when a graded potential reaches a threshold for the activating of Na+ channels?

Answer 21

Action potential is generated

Question 22

What are the 5 phases of an action potential?

Answer 22

1. RMP
2. Depolarising stimulus
3. Upstroke
4. Repolarisation
5. After-hyperpolarisation

Question 23

What is the Nernst Equation?

Answer 23

E = (RT/zF).ln.(X2/X1)

E = Eqm potential 
R = gas constant
T = Temperature in Kelvin
z = charge on ion (-1 for Cl-, +2 for Ca2+)
F = Faraday’s number - charge per mol of ion
ln = natural logarithm (log to base e)
X2 = intracellular ion concentration
X1 = extracellular ion concentration

Question 24

What can the Nernst Equation be simplified to?

Answer 24

E = (-61/z).log (Xinside/Xoutside) (mV)

Question 25

What are the typical concentrations of K+?

Answer 25

150 mM inside and 5 mM outside

Question 26

What are the typical concentrations of Na+?

Answer 26

10 mM inside and 150 mM outside

Question 27

Why does the Goldman-Hodgkin-Katz equation describe membrane potential (Em) more accurately than the Nernst equation?

Answer 27

In reality biological membranes are not uniquely selective for an ion. Membranes have mixed and variable permeability to all ions (but, for neurones at rest K+ >> Na+). GHK takes into account permeability.

Question 28

What produces the initial change in membrane potential that determines what happens next i.e. initiate or prevent AP?

Answer 28

Graded potentials

Question 29

In what type of cells do APs occurs?

Answer 29

Neurones and muscle cells but also some endocrine tissues.

Question 30

What are APs also known as in neurones?

Answer 30

Nerve impulses and allow the transmission of information reliably and quickly over long distances

Question 31

What does permeability of a membrane depend on?

Answer 31

Conformational state of ion channels Opened by membrane depolarisation Inactivated by sustained depolarisation Closed by membrane hyperpolarisation/repolarisation

Question 32

Outline phase 3 - upstroke of an AP.

Answer 32

Starts at threshold potential ^Na+ because VGSCs open quickly [Na+ enters the cell down electrochemical gradient] ^PK as the VGKCs start to open slowly [K+ leaves the cell down electrochemical gradient]. Less than Na+ entering. Membrane potential moves toward the Na+ equilibrium potential

Question 33

Outline phase 4 - repolarisation of an AP.

Answer 33

^Na+ because the VGSCs close - Na+ entry stops ^K+ as more VGKCs open & remain open. K+ leaves the cell down its electrochemical gradient. Membrane potential moves toward the K+ equilibrium potential.

Question 34

What happens after hyperpolarisation?

Answer 34

At rest voltage-gated K+ channels are still open. K+ continues to leave the cell down the electrochemical gradient. Membrane potential moves closer to the K+ equilibrium - some voltage-gated K+ channels then close. Membrane potential returns to the resting potential