Origin Of Membrane Potenials / Action Potential

Question 1

Basic components of the plasma membrane

Answer 1

Extracellular water:
- Plasma water (3L)
- Interstitial water (13L)
Intracellular water:
- 25L

Lipid bilayer - prevents diffusion of hydrophilic molecules

Question 2

Ways in which molecules can cross the plasma membrane:

Answer 2

Passive diffusion

Aqueous diffusion:
- Channel protein
- Required concentration gradient

Facilitated diffusion

Active transport

Endocytosis

Question 3

How do concentration and electrical gradients allow for the movement of ions across the plasma membrane?

Answer 3

Conc. gradient: Ions flow from high conc to low conc
Ion gradient: Ions flow to areas of the opposite charge

Membrane resting potential:
- -70mV

Question 4

Equilibrium potential and how it can be calculated by the Nernst equation:

Answer 4

Potential at which the electrical force on the ion will exactly balance the opposing force of the concentration gradient

Membrane potential where there is no net flux of ions across a membrane

Nernst equation:
Eion = 61.5log10[(iono)/(ioni)]

Question 5

Distribution of Na+ and K+ across the cell membrane and the direction that they flow:

Answer 5

Outside cell:
- Na+ 145mM
- K+ 4.5mM

Inside cell:
- Na+ 15mM
- K+ 120mM

3Na+ out for every 2K+ in - more positive on outside

Question 6

Depolarisation and hyperpolarisation:

Answer 6

Depolarisation (upstroke):
- Influx of Na+ into cell
- Membrane potential becomes less negative
- Mediated by opening of voltage-activated Na+ channels
- Opening of channels causes more channels to open causing further depolarisation
- Closure of Na+ and opening of K+ causes REpolarisation (downstroke), membrane returns back to resting value

Hyperpolarisation (undershoot):
- Membrane potential becomes more negative
- Voltage gated K+ channels remain open after resting potential has been reached

Question 7

What is a driving force?

Answer 7

For Na+ influx = Vm - ENa
For K+ influx = Vm - EK+

Question 8

Neuronal action potential:

Answer 8

Brief electrical signals in which the polarity of the nerve cell membrane is momentarily reversed

Constant magnitude and velocity along axon, allowing signalling over long distances

AP only generated if the threshold potential is reached

Question 9

Refractory period

Answer 9

Closed state → depolarisation → open state → maintained depolarisation → inactivated state → repolarisation → back to closed state

Absolute:
- No stimulus can elicit a second action potential
- All Na+ channels activated

Relative:
- Stronger than normal stimulus, may elect a second action potential
- Mix of inactivated and closed channels, membrane in hyperpolarised

Question 10

Impulse propagation in un-myelinated and myelinated nerve fibres

Answer 10

Myelination increases conduction velocity