Resting and Action Potential

Question 1

Flux

Units

Answer 1

the rate of transfer of molecules
molecules.m−2.s−1 (or similar)
Dynamic equilibrium reached – no net flux

Question 2

How membrane potential of a cell is measured

Answer 2

Reference electrode is placed outside the cell. This is the zero volt level.
Another electrode is placed inside the cell (in axon). Measures a voltage difference that is negative compared with the outside (i.e. reference).

Question 3

Importance of ion channels

Answer 3

Permeable pores in the membrane (ion channels) open and close depending on:
trans-membrane voltage
presence of activating ligands
mechanical forces.

Question 4

electrochemical equilibrium achieved when?

Answer 4

Electrochemical equilibrium is achieved when electrical force prevents further diffusion across the membrane

A stable trans-membrane potential is achieved

Question 5

Equilibrium potential

Answer 5

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

Question 6

Calculating equlibrium potential using Nernst equation

Answer 6

Normal equation:
E= (RT/zF)ln([X2]/[X1])
R = gas constant
T = Temp. Kelvin, assume 310 (37C)
Z = charge on ion, -1 for Cl-, +2 for Ca2+
F = Faraday’s number, charge per mol of ion
ln = log to base e

Simplified version
E(units=mV)= (-61/z) x log ([Xinside]/[Xoutside])

Question 7

Real membrane potentials (Em) do not rest at EK (–90 mV) or ENa (+72 mV)
Typical Em is -70 mV
Why?

Answer 7

Membranes have mixed K+ and Na+ permeability (but at rest K+&raquo_space; Na+)

Question 8

Goldman-Hodgkin-Katz (GHK) equation

slide 21, lecture 4

Answer 8

-

Question 9

Depolarisation
Overshoot
Repolarisation
Hyperpolarisation

Answer 9

Depolarisation = change in a positive direction.
Overshoot = change from 0 in a positive direction.
Repolarisation = change in a negative direction towards the resting potential.
Hyperpolarisation = voltage drops below the resting potential.

Question 10

Graded Potential
Meaning of graded potential?
Action potential meaning?
Features of a graded potential?
Function of a graded potential?

Answer 10

Graded Potential = change in amplitude.
Action Potential = Uniform amplitude (all-or-nothing event).

A Graded Potential:
▪ Is bi-directional – positive or negative depending on stimulus.
▪ Can have a weak stimulus = small potential, strong = large.
▪ Decreases in amplitude over time and distance from origin. Due to leakage of charge along the axon.
▪ Only occurs at SYNAPSES and SENSORY RECEPTORS.

Function = Generate or prevent an action potential forming.
NOTE: Graded Potentials have a DECREMENTAL SPREAD.

Question 11

Where is the action potential generated?

Answer 11

Axon hillock

Question 12

Phases of action potential

Answer 12

1) Resting Membrane Potential
2) Depolarising Stimulus
3) Upstroke/ Depolarising Phase
4) Repolarisation Stage
5) Post-hyperpolarisation Phase

Question 13

Phase 1 of action potential

slide 32, lecture 4

Answer 13

Resting Membrane Potential
PK&raquo_space; PNa therefore membrane potential nearer
equilibrium potential for K+ than for Na+

Voltage-gated Na+ channels (VGSC) and Voltage-gated K+ channels (VGKC):
▪ Sodium channel activation gate CLOSED.
▪ Sodium channel inactivation gate OPEN.
▪ Potassium channel CLOSED.

NOTE: only the sodium channel has an activation and
inactivation gate.

Question 14

Phase 2 of action potential

slide 33, lecture 4

Answer 14

Depolarising Stimulus
▪ Causes an opening of VGSC allowing sodium to flow into the cell.
▪ This means that the membrane potential changes in
the direction of the equilibrium potential of sodium towards the threshold
▪ Stimulus needs to be above the THRESHOLD to
generate an AP

Question 15

Phase 3 of action potential

slide 34, lecture 4

Answer 15

Upstroke/ Depolarising Phase
PNa increases because the VGSCs open quickly. The
upstroke then starts when the membrane potential
reaches the threshold potential:
▪ Na+ ions enter the cell down the electrochemical gradient.
▪ PK also increases as the VGKCs start to open (but
this is slower).
▪ K+ ions leave the cell down their electrochemical
gradient, but fewer than the Na+ ions entering

Net effect is the membrane potential moving
towards the Na+ equilibrium potential.
Voltage-gated Na+ channels (VGSC) and Voltagegated K+ channels (VGKC):
▪ Sodium channel activation gate OPEN.
▪ Sodium channel inactivation gate OPEN.
▪ Potassium channel CLOSED.

Question 16

Phase 4 of the action potential
Absolute Refractory Period meaning
(slide 35, lecture 4)

Answer 16

Repolarisation Stage
▪ PNa decreases because the VGSCs inactivate so
sodium entry into the cell stops.
▪ PK increases as the VGKCs remain open so K ions
leave the cell down their electrochemical
gradient.

The net effect is that membrane potential moves
towards the equilibrium potential for K+

During Repolarisation (early):
Voltage-gated Na+ channels (VGSC) and Voltagegated K+ channels (VGKC):
▪ Sodium channel activation gate OPEN.
▪ Sodium channel inactivation gate CLOSED.
▪ Potassium channel OPEN.

Absolute Refractory Period: The term used to
denote the sodium channel inactivation gate being
CLOSED. This means a new AP cannot be generated
even with a very strong stimulus.

Repolarisation (late):
Voltage-gated Na+ channels (VGSC) and Voltagegated K+ channels (VGKC):
▪ Sodium channel activation gate CLOSED.
▪ Sodium channel inactivation gate CLOSED.
▪ Potassium channel OPEN.

Question 17

Phase 5 of action potential

slide 38, lecture 4

Answer 17

Post-hyperpolarisation Phase
▪ The undershoot takes place as the VGKCs remain
open for a few milliseconds after repolarisation.
▪ The membrane potential moves closer to the K+
equilibrium potential until the VGKCs close. Then
the membrane potential returns to the resting
potential.

Voltage-gated Na+ channels (VGSC) and Voltagegated K+ channels (VGKC):
▪ Sodium channel activation gate CLOSED.
▪ Sodium channel inactivation gate OPEN.
▪ Potassium channel OPEN.
▪ The membrane enters a relative refractory
period – A stronger stimulus is required to open
the VGSCs as the membrane potential is already
more negative than normal (hyperpolarisation).

Question 18

How long does an action potential take?

Answer 18

2 milliseconds

Question 19

Time- course of changes in permeability of
PNa
PK

Answer 19

-

Question 20

Threshold meaning

Answer 20

Change in membrane potential required to open VGSCs.

Question 21

All or nothing nature meaning

Answer 21

Once the threshold has been reached, a full size AP can be produced.

Question 22

Refractory State

Answer 22

Unresponsiveness to stimulus
Allows to distinguish between impulses
Prevents AP from travelling wrong way down axon

Question 23

Regenerative relationship between PNa and membrane potential

Diagram (slide 43+ 45, lecture 4)

Answer 23

Initially, depolarisation is caused by an event outside
the cell and if the potential is less than the
threshold, graded potential returns to resting
potential.
1. Once the threshold is reached, the cycle can
continue – Positive feedback behaviour.
2. The cycle stops when the VGSCs are inactivated
– closed and voltage insensitive (inactivation
gate).
3. The membrane remains in an unresponsive state
until the VGSCs recover from inactivation.

Question 24

Ion Movements during the Action Potential
Extent of change
How is electrochemical equilibrium restored?

Answer 24

There are very small changes in concentration during an AP (<0.1%).
▪ Ion pumps are not directly involved in ion movements during the AP – This is a spontaneous event.
▪ Electrochemical equilibrium is restored following the AP by the ions moving through NON voltage-gated ion channels.
▪ Some ions move through pumps but this is a relatively slow process.

Question 25

Propogation of action potential
What affects speed of AP?
What prevents loss of charge?
What prevents AP going in wrong direction?
Speed of AP can be up to?

Answer 25

▪ Diameter of the neuron and myelination affects speed of AP.
▪ Myelination prevents loss of charge by acting as an insulator – allows the charge to travel further than with cable transport.
▪ The ABSOLUTE REFRACTORY PERIOD – Blocking the VGSCs by the inactivation gate means that the section of membrane which is hyperpolarised cannot be depolarised again and the AP cannot travel in the wrong direction.
▪ Speed of an AP can be up to 120m/s (and as low as 1m/s in small, un-myelinated axons).
▪ In myelinated, action Potentials only occur at the Nodes of Ranvier.

Question 26

Passive propogation vs active propogation

Graph of membrane potential as distance from depolarisation changes) (slide 48, lecture 4

Answer 26

Passive:
Only resting K+ channels open
Potential change declines down the axon during graded potential

Active:
Local current flow depolarises adjacent region toward threshold: if adjacent area suffienciently depolarised it travels down the axon
Can’t go back down because of refractory period

Question 27

List two structural features that affect the conduction velocity along normal axons and briefly explain why they affect velocity as they do.

Answer 27

▪ Diameter of neuron – Larger diameter = Lower resistance = Faster conduction speed due to
more elctronically charged ions.
▪ Degree of myelination – Myelination = Faster propagation, Non-myelination = Slower
propagation (of same diameter axon).

Question 28

Conduction velocity can be reduced by:

Answer 28

o Reduced axon diameter – i.e. re-growth after injury.
o Reduced myelination – i.e. Multiple Sclerosis.
o Cold, anoxia, compression and drugs – i.e. some anaesthetics.