4 & 5. Resting and Action potentials

Question 1

Flux

Answer 1

the rate of transfer of molecules

= number of molecules that cross a unit area per unit time e.g. m-2. s-2

Question 2

Is there net flux in dynamic equilibrium?

Answer 2

No

Question 3

Equilibrium potential

Answer 3

The potential at which electrochemical equilibrium has been reached.
Concentration gradient balanced by electrical gradient

Question 4

When is electrochemical equilibrium achieved?

Answer 4

when electrical force prevents further diffusion across the membrane

Question 5

What does the Nernst equation show?

Answer 5

It allows you to calculate the equilibrium potential of a cell (dependent on temperature and charge)

Question 6

What are the intracellular and extracellular concentrations of the 4 main ions involved in action potentials?

Answer 6

Na+: inside: 10 outside: 150
K+: inside: 150 outside: 5
Cl-: inside: 5 outside: 120
Ca2+: inside: 0.1 outside: 2

Question 7

Explain why the resting membrane potential of most cells is around -70 mV

Answer 7

Equilibrium potential of K+ is -90 mV and Na+ is +72
Membranes have mixed K+ and Na+ permeability
But more permeable to K+ so resting membrane potential is closer to equilibrium potential of K+.

Question 8

What does the Goldman-Hodkin-Katz equation show?

Answer 8

Accounts for relative permeability of the membrane at any one time to different ions so you can figure out the resting membrane potential.

Question 9

Resting potential

Answer 9

-70mV

Question 10

Depolarising

Answer 10

-70 to 0 (becoming more +)

Question 11

Hyperpolarising

Answer 11

-70 to -90 (becoming more -)

Question 12

Overshoot

Answer 12

0 to +

Question 13

Repolarising

Answer 13

+ to -

Question 14

What is the difference between graded potentials and action potentials?

Answer 14

APs are an all-or-nothing event (have the same amplitude every time)
GPs can vary in amplitude and the amplitude is affected by the strength of the stimulus. GPs can be positive or negative and they decrease in altitude as they travel away from the point of origin.

Question 15

How can nerve cells use graded depolarisations and hyperpolarisations for signalling?

Answer 15

Contribute to initiating or preventing action potentials

May summate or cancel each other out, create a more integrated signal

Question 16

Where do graded potentials occur?

Answer 16

Synapses

Sensory receptors

Question 17

Where do action potentials occur?

Answer 17

In excitable cells
Mainly neurones and muscle cells e.g. in muscle cells lead to contraction
Also in some endocrine tissues e.g. In B cells of pancreas provoke release of insulin

Question 18

What does permeability to ions depend on?

Answer 18

Conformational state of ion channel

Most channels in neurology are voltage operated

Question 19

What are the 5 phases of an action potential?

Answer 19

1. Resting membrane potential
2. Depolarising stimulus
3. Upstroke
4. Repolarisation
5. After hyperpolarisation

Question 20

Describe phase 1 of an action potential

Answer 20

Caused by K+ moving out of cells
Finite permeability to Na+
Membrane potential nearer equilibrium potential for K+ than for Na+

Question 21

Describe phase 2 of an action potential

Answer 21

Stimulus depolarises membrane potential
(Graded response- small and slow depolarisation)
Moves it in +ve direction towards threshold

Question 22

Describe phase 3 of an action potential

Answer 22

Starts at threshold potential
At threshold potential lots of Na+ channels respond
PNa increases because voltage-gated Na+ channels open quickly
Na+ ions enter the cell down their electrochemical gradient
Depolarisation
Membrane potential moves toward Na+ equilibrium potential
PK increases as the voltage-gated K+ channels start to open slowly
K+ ions leave the cell down their electrochemical gradient
(Slowly- Less than Na+ entering)

Question 23

Describe phase 4 of an action potential

Answer 23

PNa decreases because voltage-gated Na+ channels inactivate (Channels blocked)
Na+ entry stops
PK increases as more voltage-gated K+ channels open and remain open
K+ leaves the cell down its electrochemical gradient
Membrane potential moves toward the K+ equilibrium potential

Question 24

Describe phase 5 of an action potential

Answer 24

(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
Additional K+ channels are still open, so slight hyperpolarisation (Close slowly)
Some voltage-gated K+ channels then close

Membrane potential returns to the resting potential

Question 25

What occurs at the start of repolarisation?

Answer 25

Part of channel protein reacts to change in voltage
and blocks the channel so Na+ cant get through
Na+ channel gate is open, but blocked by inactivation protein
Na+ entry to cell stops
Depolarisation recognised by K+ channels and more open

Question 26

What is the underlying mechanism for the absolute refractory period? What is this?

Answer 26

Na+ channel inactivation is the underlying mechanism for
the Absolute refractory period
Inactivation gate is closed
New AP cannot be triggered even with very strong stimulus

Question 27

What happens later in repolarisation?

Answer 27

Absolute refractory period continues
Na+ Channel activation gate closes
Inactivation protein still present

Question 28

What occurs during post-hyperpolarisation?

Answer 28

Relative refractory period
Inactivation gate is open
Stronger than normal stimulus required to trigger an action potential

Question 29

How long does a normal action potential last?

Answer 29

2 ms

Question 30

Threshold potential

Answer 30

Once this potential is reached an AP is triggered

Question 31

Refractory state

Answer 31

Unresponsive to threshold depolarization

Question 32

What is the regenerative relationship between PNa+ and membrane potential?

Answer 32

Once threshold is reached the cycle continues
Positive feedback behaviour
Cycle continues until the voltage-gated Na+ channels inactivate- closed and voltage-insensitive
Membrane remains in a refractory (unresponsive) state until the voltage-gated Na+ channels recover from inactivation

Question 33

Ion channels vs Ion pumps in an action potential

Answer 33

Ion channels: Main pathway for ion movement, rapid

Ion pumps: NOT directly involved in ion movement during an AP, slow, need ATP

Question 34

Why can the AP only move in 1 direction?

Answer 34

Originating area is refractory

Question 35

Nodes of ranvier

Answer 35

Unmyelinated gaps along the axon
Allow rapid impulse propagation as impulse can jump to areas of high concentration of Na+ channels
Allows movement via saltatory conduct

Question 36

What influences conduction velocity?

Answer 36

Axon diameter

Axon myelination

Question 37

Large diameter myelinated axons speed

Answer 37

120 m/s

Low internal resistance

Question 38

Small diameter non-myelinated axons speed

Answer 38

1 m/s
Can’t travel by saltatory conduct
High internal resistance

Question 39

How does conduction velocity change with increased axon diameter?

Answer 39

Increases

Less resistance to current flow inside large diameter axons

Question 40

How does conduction velocity change with myelination?

Answer 40

Increases

AP’s only occur at nodes of Ranvier

Question 41

What slows conduction velocity?

Answer 41

Reduced axon diameter (i.e. re-growth after injury)
Reduced myelination (i.e. MS and diphtheria)
Cold, anoxia, compression and drugs (some anaesthetics)