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Pharmacology > Action Potentials > Flashcards

Action Potentials Flashcards

1
Q

Depolarisation definition?

A

Membrane potential becomes LESS negative (even positive)

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2
Q

Hyperpolarisation definition?

A

Membrane potential becomes MORE negative

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3
Q

What does increasing membrane potential do?

A

Make it more negative

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4
Q

Factors causing change in Vm?

A

Depolarisation/hyperpolarisation depends on:

Direction of ion movement - influx (into cell) or efflux (out of cell)

Charge carried by ions (+ve or -ve)

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5
Q

Methods of ion movement across membranes?

A

Via ion channels/transporters

Passive movement of an ion through an ion channel is driven by its ELECTROCHEMICAL gradient

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6
Q

How does Na+ move through the membrane?

A

Response to opening of cell membrane sodium-selective channels Na+ flows INWARDLY as conc. gradient and electrical gradient are inward

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7
Q

Equilibrium potential of Na+?

A

ENa = +60 mV

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8
Q

What is the membrane potential and how does it compare to ENa?

A

Vm = -80 mV

This is negative compared to ENa (+60 mV)

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9
Q

Driving force of a sodium ion?

A

Driving force for Na+ influx: Vm - ENa

When negative, inward movement of Na+ occurs

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10
Q

How is the current carried by an ion calculated?

A

INa = gNa (driving force or Vm - ENa) or IK = gK (driving force or Vm - EK)

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11
Q

How does K+ move through the membrane?

A

Response to opening of cell membrane potassium-selective channels

K+ flows OUTWARDLY as conc. gradient is outward and electrical gradient is inward; conc. gradient energy exceeds energy of electrical gradient

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12
Q

Equilibrium potential of potassium compared to membrane potential?

A

EK = -90 mV; more negative than the membrane potential (-80 mV)

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13
Q

Driving force of a potassium ion?

A

Driving force for K+ efflux: Vm - EK, when positive, outward movement of K+ occurs

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14
Q

Effect of Na+/ K+ channel opening on Vm?

A

Na+ channel opening - Vm driven towards ENa

K+ channel opening - Vm driven towards EK

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15
Q

Describe ion channels and structure

A

Protein complexes spanning lipid bilayer to form a central pathway, allowing rapid flow of SELECTED ions

Have 4 modules

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16
Q

Regulation of ions channels?

A

Most are regulated and exist in:

Open (o)

Closed (c)

…and additional conformations cycling between these

17
Q

Opening of ion channels?

A

Opened (“gated”) by:

Membrane voltage - voltage-gated ion channels

Chemical substances - ligand-gated ion channels

Physical stimuli - like mechanical/thermal

18
Q

Ion channels responsible for depolarisation and hyperpolarisation?

A

Voltage-activated Na+ channels - depolarising

Voltage-activated K+ channels - hyperpolarising

19
Q

Steps of a neurone action potential?

A

Resting potential (for neurones, 60-70 mV)

Threshold - depolarisation to threshold which opens channels

Upstroke - rapid depolarisation moves Vm close to ENa, due to opening of voltage-activated Na+ channels

Downstroke - rapid hyperpolarisation moves Vm close to EK, due to opening of voltage-activated K+ channels and inactivation of voltage-activated Na+ channels

Undershoot - after action potential and is more negative than resting membrane potential, due to delayed closure of voltage-activated K+ channels

20
Q

Action potential properties?

A

Brief electrical signals in which polarity of nerve cell membrane is MOMENTARILY ( approx 2ms) REVERSED

“All or none” - action potentials generated when threshold is reached

21
Q

Describe “All of None” nature of action potentials

A

Cannot have half an action potential

Even if size of depolarising stimulus is large, action potential has a fixed amplitude

22
Q

What occurs at rest?

A

K+ channels are leaky

23
Q

Describe similarities between voltage-activated Na+ and K+ channels

A

Highly selective for respective cations

Activated by membrane depolarisation - Na+ channels rapidly and K+ channels with a slight DELAY

24
Q

Difference between voltage-activated Na+ and K+ channels?

A

Activation of Na+ channels is self-reinforcing - opening of a few channels causes more channels to open, causing more depolarisation, etc (POSITIVE FEEDBACK)

Activation of K+ channels is self-limiting - outward movement of K+ causes repolarisation, turning off the stimulus for opening (NEGATIVE FEEDBACK)

25
Q

Na+ channels during maintained depolarisation? Reversal?

A

Enter a non-conducting, inactivated state, even though they initially open due to depolarisation

Repolarisation causes the channel to enter a closed state in readiness for opening, to generate another action potential

26
Q

Why is maintained depolarisation of Na+ channels important?

A

Na+ channels will be inactivated, which contributes to the repolarising phase of action potential and is responsible for REFRACTORY PERIOD

27
Q

Repolarisation definition?

A

Returning the membrane potential to normal resting value

28
Q

Absolute refractory period definition?

A

No stimulus, however strong, can elicit a second action potential immediately after generation of an action potential, as all Na+ channels are inactivated

29
Q

Relative refractory period definition?

A

A stronger than normal stimulus may elicit a second action potential (mixed population of inactivated and closed channels)

30
Q

When do absolute and relative refractory periods occur?

A

During downstroke and undershoot respectively

31
Q

Briefly, how is impulse propagation achieved with unmyelinated and myelinated axons?

A

Unmyelinated axons - passive spread of current

Myelinated axons - saltatory conduction

32
Q

Why do passive signals not spread far from origin?

A

Nerve cell membrane is “leaky”, not a perfect insulator, so passive signals do not spread far (current is lost from the membrane along the axon)

33
Q

How can passive electrical signals be conducted over large distances without decaying?

A

Via action potentials

34
Q

Passive current spread increase causes?

A

Decrease ri - axial resistance (increase axon diamater)

Increase rm - membrane resistance (by adding insulating material like myelin)

35
Q

Describe myelin and creation

A

Insulating material created by SCHWANN cell in PNS and by OLIGODENDROCYTES in CNS

Conduction in myelinated axons is much faster than in non-myelinated axons of similar diameter

36
Q

Node of Ranvier definition?

A

Points of neurones at which myelin sheath disappears

37
Q

What is saltatory conduction?

A

Action potential jumps from one node of ranvier to next

Pharmacology (17 decks)

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