Membrane potential and action potential

Question 1

What is voltage or potential difference?

Answer 1

It is generated by ions to produce a charged gradient.

Question 2

What is current?

Answer 2

Movement of ions due to a potential difference.

Question 3

What is resistance?

Answer 3

• A barrier that prevents the movement of ions.
• Measured in ohms
• Barriers can change in biology generating different resistances.

Question 4

Why does a membrane potential arise?

Answer 4

• It arises due to ion flow across the membrane
• These ions can only move through selectively permeable ion channels
• The ion channels can only open in response to stimuli like a transmembrane voltage, presence of activating ligands or mechanical forces.

Question 5

What determines the passage of ions across a membrane?

Answer 5

• Potential difference (charge gradient)
• Concentration gradient

Question 6

What prevents the further influx of potassium into compartment 1 in this case?

Answer 6

• Even though potassium can still go down its concentration gradient into compartment one, the build-up of positive charge repels potassium ions achieving a state of electrochemical equilibrium.
• The Electrical gradient is balancing chemical gradient.

Question 7

What is the difference between case 2 and case 3?

Answer 7

• Difference in the sign of membrane potential due to the selectivity of the membrane.
• Both cases electrochemical equilibrium is achieved at which the concentration gradient exactly balances the electrical gradient.

Question 8

What is the equilibrium potential?

Answer 8

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

Question 9

What can the equilibrium constant be calculated using?

Answer 9

• The Nernst equation
• The way this is expressed [ln (in/out) instead of ln (out/in)], there should be a negative sign at the begining of the equation.
• Faradays constant is 96500 C mol^-1.

Question 10

How can you simplify the Nernst Equation

Answer 10

• By assuming the temperature is 37 degrees Celcius.
• Convert natural log into common log
• State E in mV
• Make compartment 2 the inside of the cell and compartment 1 the outside
• Use typical concentrations of K+ (150 mM inside and 5 mM outside) and Na+ (10 mM inside and 150 mM outside).

Question 11

What is the problem with calculating equilibrium constant such as E_Na or E_K?

Answer 11

• They are theoretical values
• 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+)
• Each ion’s contribution to the membrane potential is proportional to how permeable the membrane is to that ion at any time.
• Hence a typical resting potential (E_m) is -70 mV and not -90 mV which is E_K.

Question 12

What equation describes the membrane potential (E_m)?

Answer 12

• The Goldman-Hodgkin-Katz (GHK) equation.
• It describes the membrane potential more accurately where:
  
  • P= permeability or channel open probability (0= 100% closed, 1= 100% open).
  • Subscript on P indicates the ion [K⁺] etc and the I and o indicates whether it is inside or outside the cell.

Question 13

What is overshoot in relation to membrane potential?

Answer 13

It is when the membrane potential becomes positive (more than 0).

Question 14

What happens when a sensory body is stimulated?

Answer 14

• An external stimulus produces a change in membrane potential.

Question 15

Are all membrane potential changes the same?

Answer 15

• No, changes in membrane potentials are graded, meaning they differ according to the type and strength of the stimulation.
• Stimuli can either depolarise the membrane or hyperpolarise the membrane (such as inhibiting stimuli that prevent action potential).
• A strong stimulus might produce a larger depolarisation whereas a weaker stimulus might produce a smaller depolarsation.

Question 16

What happens to graded potentials as they move from the initial site of depolarisation?

Answer 16

• Graded potentials tend to decay away (become smaller) as they move away from the site of initial depolarisation.
• The decremental spread of graded potentials is due to charge leaking from the axon into the extracellular fluid, decreasing the size of the potential change along the axon.

Question 17

If graded potentials decay as they move down how can action potentials be generated and propagated?

Answer 17

• If the graded potential reaches a threshold for the activation of the VGNCs then an action potential is generated in an all or nothing event.
• Once an action potential starts, then nothing can stop it from being completed.
• Action potentials are not graded potentials and travel along the length of the axon.

Question 18

What cells can action potentials occur in?

Answer 18

All excitable cells like neurones and muscles cells along with some endocrine tissue.

Question 19

What are action potentials known as in neurones and why are they important?

Answer 19

• They are known as nerve impulses and allow transmission of information reliably and quickly over long distances.

Question 20

How can action potentials be useful in cells apart from neurones?

Answer 20

They play a central role in cell to cell communication and can be used to activate intracellular processes like contraction in muscle cells and in beta cells of the pancreas, insulin release.

Question 21

What does the permeability of ionic channels depend on?

Answer 21

• Permeability depends on the conformational state of the ion channels.
• Opened by depolarisation
• Inactivated by sustained depolarisation
• Closed by membrane hyperpolarisation/repolarisation

Question 22

What happens when membrane permeability of an ion is increased?

Answer 22

• The ion moves down its electrochemical gradient.
• This movement changes the membrane potential to the equilibrium potential of that ion.

Question 23

What are the 5 phases of the action potential?

Answer 23

Question 24

What happens during phase 1 of the action potential?

Answer 24

Question 25

What happens during phase 2 of the action potential?

Answer 25

Question 26

What happens during phase 3 of the action potential?

Answer 26

Question 27

What happens during phase 4 of the action potential?

Answer 27

Question 28

What happens at the start of repolarisation?

Answer 28

• Sodium channels have 2 gates
• The first gate opens following the activation event.
• The other gate closes very rapidly after the channel opens stopping or impeding the flow of Na+ leading to sodium channel inactivation
• At this point, you cannot trigger any new action potentials even with very strong stimulus as the nerve moves into the absolute refractory period.

Question 29

What happens later in repolarisation?

Answer 29

Question 30

What happens during phase 5 of the action potential?

Answer 30

Question 31

What follows the absolute refractory period and how is it different from the absolute refractory period?

Answer 31

• Relative refractory period

Question 32

Recall the time course of change in permeability during

Answer 32

Question 33

Describe the ion movement that happens during AP

Answer 33

Question 34

Describe passive propagation of the action potential

Answer 34

• A graded potential can decay from the site of depolarisation.
• The rate at which it decays is dependant on the size of the axon which is the internal diameter.
• The internal resistance
• Insulation of the neurone

Question 35

Describe active propagation of the action potential

Answer 35

Question 36

With reference to passive and active propagation, recall how saltatory conduction works

Answer 36

• Graded potential propagates between the insulated areas.
• There is more passive propagation and this allows for faster transmission of the action potential across the neurone.

Question 37

What determines conduction velocity?

Answer 37

Question 38

Session overview

Answer 38