Membrane potentials and APs

Question 1

Diffusion

Answer 1

Spontaneous with no energy input

Question 2

Flux

Answer 2

Number of molecules that cross a unit area per unit time

Question 3

How is the membrane potential of a cell measured?

Answer 3

→ A reference electrode is placed outside the cell (zero-volt level)
→ Another electrode placed inside the cell
→ Measures the voltage that is negative compared with the outside (i.e. reference)

Question 4

What is the membrane potential?

Answer 4

Difference between voltage inside and outside of the membrane

Question 5

Key components of lipid cell membrane

Answer 5

• Hydrophobic
• Barrier to ion movement and separates ionic environments
• Can selectively change its permeability to specific ions
• Contains ion channels which open and close depending on transmembrane voltage, presence of activating ligands or mechanical forces

Question 6

When will there be movement of ions across ion channels?

Answer 6

• When there is an imbalance across the membrane
• Until an equilbrium is reached
  [Direction of flux is determined by direction of concentration grad]

Question 7

The Nerst Eqn

Answer 7

Used to calculate equilibrium potential E, using concentrations inside and outside membrane

Question 8

What does the Nerst Eqn assume?

Answer 8

Perfectly unique selectivity for ions

Question 9

Goldman-Hodgkin-Katz GHK equation

Answer 9

Describes membrane potential Em more accurately
→ Membranes have mixed and variable permeability to all ions (e.g. at rest permeability to K+ is greater than Na+)
→ Each ion’s contribution to membrane potential is proportional to how permeable the membrane is to the ion

Question 10

Depolarisation

Answer 10

Membrane potential becomes more positive than resting potential

Question 11

Repolarisation

Answer 11

Membrane potential goes back to resting potential following depolarisation

Question 12

Overshoot

Answer 12

Membrane potential becomes more positive thant 0mV

Question 13

Hyperpolarisation

Answer 13

Decreased beyond RMP

Question 14

What is the concept of a graded potential?

Answer 14

Changes in MP varying in response to type/strength/duration of stimualtions

Question 15

How do graded potentials spread?

Answer 15

Decrementally
→ Charge build up caused by initial depolarisation of graded potential can leak from axon
→ Size of potential charge decreases along the axon

Question 16

What kind of event is an action potential?

Answer 16

All or nothing
→ Graded potential has to reach a threshold for activation i.e. the opening of many Na+ channels
→ Same response regardless of size, intensity and duration of stimuli

Question 17

Where do APs occur?

Answer 17

In excitable cells e.g. neurons, muscle cells and some endocrine tissues

Question 18

What are APs called in neurones? What do they allow for?

Answer 18

Nerve impulses
- Reliable and quick transmission of information over long distances

Question 19

What intracellular processes can APs activate?

Answer 19

• Muscle cells → AP first in a series of events leading to contraction
• Beta cells of pancreas → AP stimulates insulin release

Question 20

What triggers each phase of an AP?

Answer 20

Change in membrane permeability for Na⁺ (Pₙ) and K⁺ (Pₖ)

Question 21

What does membrane permeability depend on?

Answer 21

Conformational state of ion channels

• Opened by membrane depolarisation
• Inactivated by sustained depolarisation
• Closed by membrane hyperpolarisation/repolarisation

Question 22

Phases of the AP

Answer 22

1. RMP
2. Depolarising stimulus
3. Depolarisation/Upstroke
4. Repolarisation
5. Hyperpolarisation

Question 23

Depolarising stimulus

Answer 23

→ Graded potential
→ Depolarisation below threshold potential of -55mV
→ All or nothing AP

Question 24

Depolarisation

Answer 24

→ Starts at threshold potential
→ VGSCs open v. quickly (Increase in Pₙ)
→ Influx of Na+ → depolarisation
→ VGKCs open slowly (Slight increase in Pₖ)
→ Efflux of K+
→ Less K+ leaving than Na+ entering
→ Membrane potential becomes more positive

Question 25

Repolarisation

Answer 25

→ Peak depolarisation → VGSCs close and become inactivated (absolute refractory period) → Increase in Pk as VGKCs are fully open → Efflux of K+ → Membrane potential becomes more negative again

Question 26

Hyperpolarisation

Answer 26

→ VGKCs still open i.e. continued K+ efflux → Eventually VGKCs close as MP reaches Ek → Both Na+ and K+ channels are at resting state → RMP restored by Na⁺-K⁺ ATPase VGSC is active but closed - **relative refractory period**

Question 27

What is the refractory period?

Answer 27

Time when neuronal membrane is unresponsive to further threshold depolarisations

Question 28

When is the absolute refractory period?

Answer 28

Happens in repolarisation 1. Early on → inactivation gate of VGSC closes 2. Later → activation gate of VGSCs closes

Question 29

What is the absolute refractory period?

Answer 29

Neuronal membrane is unable to respond to further stimulation, and **cannot** initiate another action potential

Question 30

When is the relative refractory period?

Answer 30

Happens in hyperpolarisation → Inactivation gate is open but activation gate remains closed

Question 31

What is the relative refractory period?

Answer 31

When another AP may be initiated by a stimualtion that is larger than normal

Question 32

How is unidirectional flow of the AP achieved?

Answer 32

→ Absolute refractory periods ensure that neurons that have already been fired recently can't be stimulated again so soon.

Question 33

What are the ion movements during AP mainly carried out by?

Answer 33

Voltage gated ion channels not ion pumps

Question 34

How big are the changes in ion concentration that cause changes in MP?

Answer 34

Very small

Question 35

What can affect the rate of decay of a depolarisation?

Answer 35

- Insulation → slower decay - Internal or axial membrane resistance - Diameter of axon → slower decay

Question 36

Where are VG channels mostly located?

Answer 36

At Nodes of Ranvier, to allow for saltatory conduction

Question 37

RMP is re-established by.. (in an AP)

Answer 37

Repolarisation with more K+ channels opening

Question 38

How does myelination affect conduction velocity?

Answer 38

Presence of myelin **increases** speed of conduction

Question 39

What could cause a reduction of myelination?

Answer 39

Diphtheria MS

Question 40

How does axon diameter affect conduction velocity?

Answer 40

Increased axon diameter increased conduction velocity

Question 41

How does temperature affect conduction velocity?

Answer 41

Increased temp increased speed of conduction

Question 42

Diseases or conditions that can affect conduction velocity

Answer 42

→ Regrowth of axon after injury is smaller and thinner → Reduced myelination with multiple sclerosis and diphtheria → Cold → Anoxia → Compression → Some drug inc. anaesthetics