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2.1 BRS Neurology > Membrane and Action Potentials > Flashcards

Membrane and Action Potentials Flashcards

1
Q

What is flux?

A

The number of particles that cross a unit area in a unit of time

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2
Q

What is voltage, current and resistance?

A

Voltage = potential difference —> ions produce a
charge gradient
- volts

Current = ion movement due to potential difference
- amps

Resistance = barrier preventing ion movement
- ohms

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3
Q

What equation links voltage, current and resistance?

A

Voltage = Current x Resistance

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4
Q

How are membrane potentials measured?

A
  • Reference electrode outside cell —> zero-volt level
  • Electrode inside cell —> negative voltage measured
    (compared to zero-volt level)
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5
Q

How do ions move across membranes and why?

A

Ion channels open/close depending on:
- Voltage
- Activating ligands
- Mechanical force

Because lipid membrane impermeable to ions

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6
Q

When is membrane potential 0mV?

A

No membrane ion channels —> no movement of ions —> no separation of charge

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7
Q

How is a membrane potential generated?

A

Ion channels open —> movement of ions across membrane —> separation of charge

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8
Q

What is equilibrium potential?

A

Potential at which electrochemical equilibrium is reached —> ions don’t move down conc grad

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9
Q

What is electrochemical equilibrium?

A

When electrical force balance prevents further diffusion of ions

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10
Q

What is the Nernst Equation?

A

Calculates equilibrium potential (E):
E = - ( RT / zF ) ln ( X2 / X1 )

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11
Q

What is the simplified Nernst Equation?

A

E = ( -61 / z ) log ( Xin / Xout )

Assume:
- R = 8.314
- T = 310 K (37°C)
- F = 96,485 C/mol

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12
Q

What is the Nernst Equation for K+?

A

Xin = 150 mM
Xout = 5 mM

E = ( -61 / 1 ) log ( 150 / 5 )
= -61 log30
= -90 mV

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13
Q

What is the Nernst Equation for Na+?

A

Xin = 10 mM
Xout = 150 mM

E = ( -61 / 1 ) log ( 10 / 150 )
= -61 log 1/15
= +72 mV

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14
Q

What is the GHK Equation?

A

Calculates membrane potential (Em):
E = -61 log ( PK[K]in + PNa[Na]in + PCl[Cl]out /
PK[K]out + PNa[Na]out + PCl[Cl]in )

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15
Q

What is used to calculate equilibrium potential?

A

Nernst Equation

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16
Q

What is used to calculate membrane potential?

A

GHK Equation (Goldman-Hodgkin-Katz)

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17
Q

Which figures are used for Na+ and K+ in the Nerst Equation?

A

Na+:
Xin = 10 mM
Xout = 150 mM

K+:
Xin = 150 mM
Xout = 5 mM

18
Q

Which figures are used for K+, Na+ and Cl- in the GHK Equation?

A

Na+:
Xin = 0.01 M
Xout = 0.15 M

K+:
Xin = 0.15 M
Xout = 0.005 M

Cl-:
Xin = 0.005 M —> bottom
Xout = 0.11 M —> top

19
Q

What is the GHK Equation for when all channels are open 100% of the time?

A

E = -61 log ( 1 x [0.15]in + 1 x [0.01] + 1 x [0.11] /
1 x [0.005] + 1 x [0.15] + 1 x [0.005] )

20
Q

What is membrane potential depolarisation?

A

Increases towards 0

21
Q

What is membrane potential repolarisation?

A

Decreases towards resting

22
Q

What is membrane potential overshoot?

A

Increases above 0

23
Q

What is membrane potential hyperpolarisation?

A

Decreases below resting

24
Q

What stimulates changes in membrane potentials? (2)

A
  1. External stimulus
  2. Neurotransmitters
25
Q

How are membrane potentials graded? (2)

A

Type and Strength

26
Q

What are graded potentials?

A

Initial change in membrane potential —> determines if action potential initiated or prevented (depending on if threshold crossed)

27
Q

Why may a potential decrease along an axon?

A

If charge leaks from axon —> cumulative decrease

28
Q

What stimulates voltage-gated ion channels to open, inactivate and close?

A

Open —> membrane depolarisation
Inactivate —> sustained depolarisation
Closed —> hyper/repolarisation

29
Q

What do ions follow when moving across a membrane?

A

Electrochemical gradient

30
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
31
Q

What occurs during phase 1 of an action potential and why?

A

Resting membrane potential:
- Membrane potential near EK because PK > PNa (flat)
- around -70 mV

32
Q

What occurs during phase 2 of an action potential and why?

A

Depolarising stimulus:
- Stimulus depolarises membrane potential —> starts
to increase (curve up)

33
Q

What occurs during phase 3 of an action potential and why?

A

Upstroke:
- VGSCs and VGKCs open —> Na+ influx and K+ efflux
—> Na+ influx faster —> membrane potential
increases towards ENa (steep rise)
- around +40 mV

34
Q

What occurs during phase 4 of an action potential and why?

A

Repolarisation:
- VGSCs inactivate —> PNa dec and PK inc —> Na+
influx stops and K+ efflux continues —> membrane
potential decreases towards EK (steep fall)

35
Q

What is the absolute vs relative refractory period?

A

Absolute Refractory:
- New AP cannot be triggered
- Na+ channel activation gate open and inactivation
gate closed —> Na+ channel activation inactivation
gate closed
- At repolarisation

Relative Refractory:
- New AP only triggered by stronger stimulus
- Some Na+ channels recovered —> some open
- At hyperpolarisation

36
Q

What occurs during phase 5 of an action potential and why?

A

After-hyperpolarisation:
- VGSCs closed and VGKCs open —> hyperpolarisation
—> some VGKCs close —> membrane potential rise
slightly back to resting (curve up)

37
Q

How does the “all-or-nothing” principle work?

A

All:
- Depolarisation reaches threshold —> AP generated
via positive feedback of depolarisation:
(depolarisation —> VGSCs open —> PNa inc —> Na+
influx inc —> more depolarisation etc. )

Nothing:
- Depolarisation doesn’t reach threshold —> no AP

38
Q

What is different about the membrane depolarisation and repolarisation mechanism?

A

De —> ion pumps not directly involved
Re —> ion pumps move ions against gradient
(eg. Na+/K+ ATPase)

39
Q

How does passive vs active propagation of an action potential work?

A

Passive:
- Sub-threshold depolarisations decay along axon
- Resting potential restored by more VGKCs opening
- Affected by internal resistance, membrane
resistance or axon diameter

Active:
- Local current flow depolarises adjacent region

40
Q

What does the action potential propagation graph look like and why?

A

Steep rise —> shallower fall
- Active propagation —> steep rise + steep fall
Passive propagation —> steep rise + shallower fall
- Dips between because VGCs conc at nodes of ranvier

41
Q

Which 6 factors affect conduction velocity?

A

Velocity decreases as:
1. Axon diameter dec
2. Myelination dec (linear) - MS, diptheria
3. Temp dec
4. Anoxia
5. Compression
6. Drugs - eg. some anaesthetics)

42
Q

What are 2 examples of diseases that slow the conduction velocity?

A

MS and Diptheria
- dec myelination

2.1 BRS Neurology (13 decks)

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