Membrane and Action Potentials Flashcards
What is flux?
The number of particles that cross a unit area in a unit of time
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
What equation links voltage, current and resistance?
Voltage = Current x Resistance
How are membrane potentials measured?
- Reference electrode outside cell —> zero-volt level
- Electrode inside cell —> negative voltage measured
(compared to zero-volt level)
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
When is membrane potential 0mV?
No membrane ion channels —> no movement of ions —> no separation of charge
How is a membrane potential generated?
Ion channels open —> movement of ions across membrane —> separation of charge
What is equilibrium potential?
Potential at which electrochemical equilibrium is reached —> ions don’t move down conc grad
What is electrochemical equilibrium?
When electrical force balance prevents further diffusion of ions
What is the Nernst Equation?
Calculates equilibrium potential (E):
E = - ( RT / zF ) ln ( X2 / X1 )
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
What is the Nernst Equation for K+?
Xin = 150 mM
Xout = 5 mM
E = ( -61 / 1 ) log ( 150 / 5 )
= -61 log30
= -90 mV
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
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 )
What is used to calculate equilibrium potential?
Nernst Equation
What is used to calculate membrane potential?
GHK Equation (Goldman-Hodgkin-Katz)
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?
- Resting membrane potential
- Depolarising stimulus
- Upstroke
- Repolarisation
- 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
