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Neurology and Neuroscience (Bioregulatory Systems) > Lecture 2: Membrane potentials and action potentials > Flashcards

Lecture 2: Membrane potentials and action potentials Flashcards

1
Q

How do you measure membrane potential?

A

By placing a reference electrode outside the cell (zero-volt level) and another electrode is placed inside the cell - it measures a voltage difference that is negative compared with the outside.

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2
Q

What are ion channels?

A

permeable pores in the membrane that open and close depending on transmembrane voltage, presence of activating ligands or mechanical forces

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3
Q

How is a membrane potential generated?

A

due to diffusion through a selectively permeable membrane

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4
Q

When is electrochemical equilibrium achieved?

A

when electrical force prevents further diffusion across the membrane

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5
Q

What is the equilibrium potential?

A

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

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6
Q

How can the equilibrium potential be calculated?

A

Using the Nernst equation

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7
Q

Which ions contribute to the real membrane potential?

A

K+, Na+ and Cl-

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8
Q

What is the size of each ion’s contribution to the real membrane potential proportional to?

A

How permeable the membrane is to the ion

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9
Q

What does the GHK equation describe?

A

The resting membrane potential

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10
Q

What is mean by depolarisation?

A

membrane potential increases from negative towards 0 and becomes positive

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11
Q

What is meant by repolarisation?

A

membrane potential decreases towards resting potential

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12
Q

What is meant by overshoot?

A

membrane potential becomes more positive

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13
Q

What is meant by hyperpolarisation?

A

membrane potential decreases beyond resting potential

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14
Q

What are the most important ions for the resting potential of neurons?

A

Na+ and K+

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15
Q

What does P mean in the GHK equation?

A

Permeability or channel open probability

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16
Q

What does it mean if P=0?

A

channel is 100% closed

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17
Q

What does it mean if P=1?

A

channel is 100% open

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18
Q

What does it mean if P = 0.5?

A

channel is open 50% of the time

19
Q

What are graded potentials?

A

Change in membrane potential in response to stimulation

20
Q

What are the 2 key features of graded potentials?

A
  • the size depends on the strength of the stimulus

- they dissipate with distance from the stimulus as charge ‘leaks’ from the axon as the impulse propagates

21
Q

Where do graded potentials occur?

A

at synapses and in sensory receptors

22
Q

What do graded potentials contribute to?

A

initiating or preventing action potentials

23
Q

In what cells do action potentials occur?

A

in excitable cells (mainly neurons and muscle cells but also in some endocrine tissues)

24
Q

What roles do action potentials have?

A
  • cell-to-cell communication and can be used to activate intracellular processes
25
Q

What does permeability depend on?

A

conformational state of ion channels

26
Q

How is an ion channel opened?

A

by membrane depolarisation

27
Q

How is an ion channel inactivated?

A

by sustained depolarisation

28
Q

How is an ion channel closed?

A

by membrane hyperpolarisation/repolarisation

29
Q

What are the 5 phases of the action potential?

A
  1. resting membrane potential
  2. depolarising stimulus
  3. upstroke
  4. repolarisation
  5. after-hyperpolarisation
30
Q

What happens in phase 1 of an action potential?

A

RESTING MEMBRANE POTENTIAL:

  • P(K) > P(Na) therefore potassium moves out of the cell and very little sodium comes in,
  • nearer equilibrium potential for K+
31
Q

What happens in phase 2 of an action potential?

A

DEPOLARISING STIMULUS:

  • stimulus depolarises membrane potential,
  • moves it in positive direction towards threshold
32
Q

What happens in phase 3 of an action potential?

A

UPSTROKE:

  • starts at threshold potential
  • increased P(Na) because VGSCs open quickly
  • increased P(K) as VGKCs open slowly
  • less K+ leaving than Na+ entering
  • membrane potential moves toward Na+ equilibrium potential
33
Q

What happens in phase 4 of an action potential?

A

REPOLARISATION:

  • decreased P(Na) as VGSCs close –> Na+ entry stops
  • increased P(K) as more VGKCs open and remain open –> K+ leaves cell down electrochemical gradient
  • membrane potential moves towards K+ equilibrium potential
34
Q

What is the absolute refractory period?

A

Is the period of time during which a second action potential ABSOLUTELY cannot be initiated, no matter how large the applied stimulus is - the activation and inactivation gates are closed

35
Q

What happens in phase 5 of an action potential?

A

AFTER-HYPERPOLARISATION:

  • as membrane potential moves closer to K+ equilibrium some VGKCs close
  • returns to resting potential
36
Q

What is the relative refractory period?

A

stronger than normal stimulus required to trigger an action potential - inactivation gate is open

37
Q

When is an action potential triggered?

A

Once the threshold potential is reached

38
Q

What are action potentials described as?

A

‘All-or-nothing’ events

39
Q

When does the positive feedback behaviour of the depolarisation cycle stop?

A

When the voltage-gated Na+ channels inactivate (i.e. closed and voltage-insensitive)

40
Q

How is the electrochemical equilibrium restored following the action potential?

A

by K+ and Na+ ions moving through non voltage-gated ion channels and some ions are exchanged through pumps (like Na+K+ATPase)

41
Q

Where are the voltage-gated channels mostly located?

A

At the Nodes of Ranvier formed by gaps between the myelin sheath surrounding the axon

42
Q

What factors influence conduction velocity?

A
  • axon diameter (Larger diameter = faster AP)

- myelination (more myelin = faster AP)

43
Q

What does passive propagation result from?

A

Passive (graded) propagation results from a local change in ionic conductance (e.g. synaptic or sensory that produces a local current) that spreads along a stretch of membrane becoming exponentially smaller

44
Q

What is the process called when APs propagate along an axon using nodes of Ranvier?

A

saltatory conduction

Neurology and Neuroscience (Bioregulatory Systems) (10 decks)

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