Resting Potential and Action Potential Flashcards
Why do cells have an electrical potential?
Transmits information reliably + quickly over large distances
- controls Ca2+ entry into cells.
- Ca2+ is involved in gene regulation, growth and death
What is flux?
The number of molecules that cross a unit area per unit time.
At diffusion equilibrium there is no net flux.
Define Ohm’s Law
Voltage = Current x Resistance
Voltage = Potential:
- Generated by ions to produce a charge gradient
Current:
- Movement of ions due to potential
Resistance:
- Barrier that prevents the movement of ions - cell membrane
- Permeability of membrane is key to the resting membrane potential
The resting membrane potential of excitable cells?
-70mV
- The Zero reference is placed outside the cell.
The inside of the cell is negative compared to the outside - Resting potential is established by diffusion of ions through a selectively permeable membrane.
What are ion channels types?
Selective for different ions (K+,Na+,Cl-,Ca2+)
- voltage-dependent = open by change in membrane potential
- voltage-independent = open all the time, responsible for producing resting potential
What is Electrochemical Equilibrium?
This is when electrical forces balance concentration gradient, a stable membrane potential is established.
What is the Equilibrium Potential?
Potential that prevents diffusion down the ion’s concentration gradient
What is the Nersnt Equation
A method of predicting the equilibrium potential for a given ion
Ex = (RT/ZF).ln(Co/Ci)
- Ex+ = equilibrium potential of ion X
- R = gas constant
- T = absolute temperature
- Z = charge on ion
- F = Faraday’s number 96,500 coulombs of charge/mol of a singly charged ion
- Co = (X+) outside cell
- Ci = (X+) inside cell
- Substituting values of constants at 37oC
Using the Nernst Equation what is the Equilibrium Potentials of Potassium and Sodium?
- -92mV (K+) and +73mV (Na+)
- however this is not the real membrane potential.
- Also to consider is the permeability of the membrane to all ions
- POTASSIUM is the main ion which controls resting membrane potential
What is the Goldman-Hodgkin-Katz equation?
- derivation of the Nernst equation which includes membrane permeability.
- can manipulate the permeability of the membrane to various ions you can predict the resting potential.
Define the different changes in membrane potential?
Depolarisation - change in a positive direction
Overshoot - change from 0 in a positive direction
Repolarisation - change in a negative direction towards the resting potential
Hyperpolarisation - voltage drops below the resting potential.
What is a graded potential?
Graded Potentials:
- change in amplitude
- changes depending on stimulus
- bidirectional
- decrease in amplitde with increasing time and distance from origin
- They only occur at synapses + sensory receptors
- Function = generate/prevent action potential from forming.
Action Potential:
- uniform amplitude (all or nothing)
What are the 5 main phases of an action potential?
1 - resting membrane potential:
- membrane more permeable to K+ (RMP closer to equil of K+)
- Voltage-gated ion channels closed.
2- Depolarising stimulus:
- opens VGSCs
- membrane potential increases
- stimulus has to be above threshold to generate AP.
3- Upstroke/Depolarising phase
- starts at threshold potential
- increased membrane permeability to Na+
- Sodium moves into cell and mp increases to equilibrium potential
- VGKCs open much slower than VGSCs
4- Repolarisation
- VGCSs inactivated
- more VGKCs open - K+ leaves cell and membrane potential decreases back towards equil. potential to K+
- sodium inactivation gate closed
- k+ channel open
- no new AP generated
5- After-Hyperpolarisation
- VGKCs remain open for a few miliseconds (hyperpolarisation)
- inactivation gate open
- stronger than normal stimulus is required to trigger an AP = Relative Refractory Period
Describe Ion movements during the action potential?
- Very small changes in concentration during an AP
- Ion pumps not directly involved in ion movements during AP
- Electrochemical equil restored after AP by ions moving through NON VGICs
What affects the propagation of an Action Potential?
- diameter of neuron and myelination
- myelination = insulator so prevents loss of charge
- Absolute Refractory Period = blocks VGSCs by inactivation gate, hyperpolarised cant be depolarised.
- AP speed 120m/s
