Membrane and Action Potentials Flashcards by Ananya Agarwal

What is flux?

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

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What is voltage, current and resistance?

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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What equation links voltage, current and resistance?

Voltage = Current x Resistance

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How are membrane potentials measured?

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

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How do ions move across membranes and why?

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

Because lipid membrane impermeable to ions

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When is membrane potential 0mV?

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

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How is a membrane potential generated?

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

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What is equilibrium potential?

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

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What is electrochemical equilibrium?

When electrical force balance prevents further diffusion of ions

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What is the Nernst Equation?

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

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What is the simplified Nernst Equation?

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

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

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What is the Nernst Equation for K+?

Xin = 150 mM
Xout = 5 mM

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

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What is the Nernst Equation for Na+?

Xin = 10 mM
Xout = 150 mM

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

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What is the GHK Equation?

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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What is used to calculate equilibrium potential?

Nernst Equation

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What is used to calculate membrane potential?

GHK Equation (Goldman-Hodgkin-Katz)

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

Na+:
Xin = 10 mM
Xout = 150 mM

K+:
Xin = 150 mM
Xout = 5 mM

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

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

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

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] )

What is membrane potential depolarisation?

Increases towards 0

What is membrane potential repolarisation?

Decreases towards resting

What is membrane potential overshoot?

Increases above 0

What is membrane potential hyperpolarisation?

Decreases below resting

What stimulates changes in membrane potentials? (2)

External stimulus
Neurotransmitters

How are membrane potentials graded? (2)

Type and Strength

What are graded potentials?

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

Why may a potential decrease along an axon?

If charge leaks from axon ---> cumulative decrease

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

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

What do ions follow when moving across a membrane?

Electrochemical gradient

What are the 5 phases of the action potential?

1. Resting membrane potential 2. Depolarising stimulus 3. Upstroke 4. Repolarisation 5. After-hyperpolarisation

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

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

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

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

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

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

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

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

What is the absolute vs relative refractory period?

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

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

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

How does the "all-or-nothing" principle work?

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

What is different about the membrane depolarisation and repolarisation mechanism?

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

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

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

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

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

Which 6 factors affect conduction velocity?

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)

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

MS and Diptheria - dec myelination