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Neurology (BRS) > Membrane potentials and APs > Flashcards

Membrane potentials and APs Flashcards

1
Q

Diffusion

A

Spontaneous with no energy input

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2
Q

Flux

A

Number of molecules that cross a unit area per unit time

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3
Q

How is the membrane potential of a cell measured?

A

→ 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)

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4
Q

What is the membrane potential?

A

Difference between voltage inside and outside of the membrane

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5
Q

Key components of lipid cell membrane

A
  • 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
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6
Q

When will there be movement of ions across ion channels?

A
  • When there is an imbalance across the membrane
  • Until an equilbrium is reached
    [Direction of flux is determined by direction of concentration grad]
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7
Q

The Nerst Eqn

A

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

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8
Q

What does the Nerst Eqn assume?

A

Perfectly unique selectivity for ions

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9
Q

Goldman-Hodgkin-Katz GHK equation

A

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

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10
Q

Depolarisation

A

Membrane potential becomes more positive than resting potential

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11
Q

Repolarisation

A

Membrane potential goes back to resting potential following depolarisation

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12
Q

Overshoot

A

Membrane potential becomes more positive thant 0mV

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13
Q

Hyperpolarisation

A

Decreased beyond RMP

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14
Q

What is the concept of a graded potential?

A

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

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15
Q

How do graded potentials spread?

A

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

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16
Q

What kind of event is an action potential?

A

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

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17
Q

Where do APs occur?

A

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

18
Q

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

A

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

19
Q

What intracellular processes can APs activate?

A
  • Muscle cells → AP first in a series of events leading to contraction
  • Beta cells of pancreas → AP stimulates insulin release
20
Q

What triggers each phase of an AP?

A

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

21
Q

What does membrane permeability depend on?

A

Conformational state of ion channels

  • Opened by membrane depolarisation
  • Inactivated by sustained depolarisation
  • Closed by membrane hyperpolarisation/repolarisation
22
Q

Phases of the AP

A
  1. RMP
  2. Depolarising stimulus
  3. Depolarisation/Upstroke
  4. Repolarisation
  5. Hyperpolarisation
23
Q

Depolarising stimulus

A

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

24
Q

Depolarisation

A

→ 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

25
Q

Repolarisation

A

→ 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

26
Q

Hyperpolarisation

A

→ 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

27
Q

What is the refractory period?

A

Time when neuronal membrane is unresponsive to further threshold depolarisations

28
Q

When is the absolute refractory period?

A

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

29
Q

What is the absolute refractory period?

A

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

30
Q

When is the relative refractory period?

A

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

31
Q

What is the relative refractory period?

A

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

32
Q

How is unidirectional flow of the AP achieved?

A

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

33
Q

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

A

Voltage gated ion channels not ion pumps

34
Q

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

A

Very small

35
Q

What can affect the rate of decay of a depolarisation?

A
  • Insulation → slower decay
  • Internal or axial membrane resistance
  • Diameter of axon → slower decay
36
Q

Where are VG channels mostly located?

A

At Nodes of Ranvier, to allow for saltatory conduction

37
Q

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

A

Repolarisation with more K+ channels opening

38
Q

How does myelination affect conduction velocity?

A

Presence of myelin increases speed of conduction

39
Q

What could cause a reduction of myelination?

A

Diphtheria
MS

40
Q

How does axon diameter affect conduction velocity?

A

Increased axon diameter increased conduction velocity

41
Q

How does temperature affect conduction velocity?

A

Increased temp increased speed of conduction

42
Q

Diseases or conditions that can affect conduction velocity

A

→ Regrowth of axon after injury is smaller and thinner

→ Reduced myelination with multiple sclerosis and diphtheria

→ Cold

→ Anoxia

→ Compression

→ Some drug inc. anaesthetics

Neurology (BRS) (12 decks)

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