Neuro - Membrane and Action Potentials Flashcards
What is ions flux?
Number of molecules that move across a particular area in a given time
When does movement of ions occur across a membrane?
When there is a concentration gradient of ions and the channel is open.
Explain how an electrochemical gradient is formed using an example
There are two compartment with sodium on one side and potassium/chloride on the other side, but the membrane is only permeable to sodium. Sodium diffuses down its concentration gradient from A to B however will eventually make B so positive and A negative that it is pulled back into A, reaching the electrochemical equilibrium.
What is the equilibrium potential?
When the diffusion forces of an ion match the electrical gradient.
How can we work out the equilibrium potential?
Nernst Equation
What ions are most important when considering equilibrium potential?
Sodium
Potassium
What is the main difference between the Nernst equation and the Goldman-Hodgkin-Katz equation?
GHK equation accounts for the variation in membrane permeability. Each ions contribution to membrane potential is proportional to its permeability.
What is the membrane potential when all channels are open all the time?
-14mV
What is the membrane potential when potassium channels open but sodium/chloride channels are closed?
-90mv
What is the membrane potential when we increase permeability to sodium by 5%?
-66mV
What does a resting membrane potential of -66mV tell us?
Even at resting potential, there is some degree of sodium permeability (PNa), allowing sodium to enter into the cell.
What are the 4 main stages of membrane potential change?
Depolarisation - Em tends towards 0mv
Repolarisation- MP decreases towards RMP
Overshoot - MP becomes positive
Hyperpolarisation - MP becomes more negative than RMP
What is meant by a graded change?
The change in membrane potential is proportional to the stimulation intensity of the sensory body
What happens if we measure potential 1mm away from stimulus site?
The potential will be lower than at the stimulus site due to decay
What are stimuli?
Can cause depolarisation or hyperpolarisation
What do graded potentials do?
Induce and action potential if sufficient.
Where is an action potential found?
Can be found in all excitable cells
What is meant by an all or nothing response?
AP is not graded - when the threshold is reach wall there is a mass activation of VGSCs
What does permeability depend on?
The conformational state of channels
What depolarisation lead to?
Opening of channels
What does sustained depolarisation lead to?
Inactivation of channels
What does hyper polarisation or repolarisation lead to?
Closing of channels
What does the ion move down when permeability increases?
Electrochemical gradient
What happens to the membrane potential when permeability increases?
The Mp changes towards the equilibrium potential for that particular ion.
What types of proteins are responsible for ion movement during the action potential?
Ion channels, not ion pumps
What are the 5 phases of the action potential?
- RMP
- Depolarising stimulus
- Upstroke
- Repolarisation
- After hyperpolarisation
What happens in the resting membrane potential phase?
Pk > PNa
MP much closer to equilibrium potential for potassium than for sodium
What happens in the depolarising stimulus phase?
Simulus depolarises MP
Moves in positive direction towards the threshold
What happens in the upstroke phase?
Starts at the threshold potential. There’s an increase in PNa as VGSCs open so Na enters the cell. Pk also increases therefore there is a slow efflux of potassium.
What happens in the repolarisation phase?
PNa decreases as VGSCs close, however Pk doesn’t decrease but increases, so potassium still continues to leave from the cell. The Mp moves back towards the potassium equilibrium potential.
What is the sodium equilibrium potential?
+72mV
What is the potassium equilibrium potential?
-90mV
Before depolarisation, how do VGSCs close?
Rapid entry of Na leads to the inactivation of another gated channel whereby a portion of the protein physically blocks the channel to prevent sodium from entering
What happens as a result of VGSCs closing?
This alongside the opening of potassium channels allows for repolarisation
How does an absolute refractory period allow for repolarisation?
This means that a new action potential depolarising stimulus cannot be triggered as sodium channels are physically blocked.
How do we get hyperpolarisation from repolarisation?
Sodium channels are closed however potassium channels are still open, therefore potassium will continue to leave the cell towards its equilibrium potential of -90mV, causing hyperpolarisation
In the after hyperpolarisation phase, what happens?
Potassium still leaves the cell down its electrochemical gradient.
However, some VGKCs tend to close as MPs tend towards K equilibrium potential, meaning K cannot reach -90mV, and we eventually get a return to the RMP.
How do we get the return from a hyerpolarised membrane potential back to the RMP?
Some VGSCs open, however this is a finite amount, and some VGKCs close, therefore some sodium enters and less potassium leaves. This overall increases the membrane potential to a more positive -70mV.
What happens in the relative refractory period?
Some VGSCs open therefore it is possible to initiate depolarisation again, however the stimulus intensity must be a lot greater due to there still being a lot of VGSCs closed.
Can we still get depolarisation during the relative refractory period?
Yes the stimulus however must be much larger than normal
Explain the positive feedback loops of the action potential after its initiation
Depolarisation leads to opening of VGSCs. This causes a sodium influx which therefore increases depolarisation, creating a positive feedback loop.
How much ion movement is there during the action potential?
Not much during the AP however small amounts of ion movements induce substantial changes
Is the Na/K ATPase involved in the action potential?
No - however it still maintains the concentration gradient of the ions upon which the action potential depends.
What is passive propagation?
Only resting K+ channels are open
Internal (axial) resistance and membrane resistance along with the diameter of the axon alters the propagation distance and velocity.
What types of neurones have the most rapid decay from depolarisation?
Small diameter , unmyelinated neurones (1m/s)
What types of neurones have the slowest decay from depolarisation?
Large diameter, myelinated neurones (120m/s)
What is active propagation?
Local current flow depolarises the adjacent region towards the threshold potential. Once the TP is reached, adjacent area VGSCs open and initiate upstroke, whilst the old active region gradually returns to RMP.
How does diphtheria or MS affect myelination?
Reduce myelination
How do colds/anaesthetics and drugs affect conduction velocity?
Reduce conduction velocity.
