Action Potential Mechanisms Flashcards
what exists across the membrane of all cells
A potential difference exists across the membrane of all cells
resting membrane potential
in the range 20-90mV
is the ICF negative or positive with respect to the ECF
negative
what are the charges like in the ICF and the ECF
equal numbers of positive and negative charges in the ECF and ICF
ion / charge distribution is polarised
At the very membrane side - there are only negative charges around the inside and only positive charges around the outside
This is created (it is not natural)
= resting membrane potential
Just because it is called resting does not mean it happens without the involvement of energy
what is the composition of Na+, K+ and Cl- ions in the ECF and ICF
Ion ECF (mM) ICF (mM)
Na+ 145 15
K+ 4 150
Cl- 110 10
what are the diffusion gradients
Na wants into the cell
K wants out of the cell
= driving force
what is the diffusion potential
resting membrane is impermeable to Na+
resting membrane is very permeable to K+ (there is a channel that allows K to pass through - K helps to create the arrangement of the membrane potential)
diffusion of K+ leaves excess negative charge inside the cell
this potential gradient arising from diffusion is the resting membrane potential
what does the RMP arise from
The RMP arises from the separation of charges on either side of the membrane
the RMP is due mainly to the diffusion of K+ from cell interior through K+ channels
the small amount of Na+ that leaks into the cell is expelled from the Na+/K+ pump
what else causes the RMP apart from the diffusion of K+ from cell interior
- the Na+/K+ pump contributes by exchanging unequal numbers of Na+ and K+
the pump moves 3 Na+ outwards and 2 K+ inwards
the Na+/K+ pump is electrogenic
Na enters the cell through diffusion but leaves the cell by active transport (the pump)
K leaves the cell through diffusion but returns to the cell by active transport (the pump)
ATP is converted to ADP
what is action potential
the process of bringing from the RMP to an inverted arrangement and back again
rising phase = Na influx - voltage gated Na channels
falling phase = K efflux - voltage gated K channels
what are ion channels
- membrane proteins (also termed transmembrane protein), usually 4 domains
- aqueous channel through membrane
- gated opening
> ligand
> voltage (needs to reach a certain voltage before they will open or close) - ion selective / specific
what type of gated opening does Na+, K+ and Ca++ have?
- Na+ = voltage gated
> has more than 1 gate
> m-gate and h-gate - K+ = voltage gated
- Ca++ = ligand gated
> something connects to the channel and causes it to open this way
explain voltage-gated sodium channel
- when the channel is closed initially
> m-gate is closed
> h-gate open - channel opens
> m-gate opens
> h-gate open
> there is an influx of Na - channel closed (refractory)
> m-gate open
> h-gate closes
> happens once the action potential has over shot and the reverse now happens
explain voltage-gated potassium channel
just open gate opening and closing
- channel close
- channel open
> potassium leaves the cell
how does action potential begin
a stimulus is applied and causes depolarisation
the membrane potential moves towards the threshold (-55mV)
> m gate closed
> h gate open
(assume RMP is -70mV)
what happens when the membrane potential reaches the threshold
threshold = -55mV > Na+ channels start opening (m gate opens) > Na+ influx (enters the cell) > more depolarisation and more channels are recruited to cause a greater level of depolarisation > K+ channels remain closed
what happens when all the Na+ channels are open
there is maximum Na+ influx and the membrane protein over shoots 0mV
Na+ channel still open
K+ channel still closed
what happens when the membrane potential reaches +35mV
> Na+ channels shut - inactivation / h-gate closes
K+ channels open = efflux of K+ begins
the reverse of the process that has been happening occurs
- stops movement of Na
- starts movement of K
- start repolarising the cell
what happens during K+ efflux of AP
AP down stroke = recovery phase
Na channels shut = refractory period
K channels open and K efflux continues
what happens when the membrane potential returns to the resting level
kinda like everything resets
> ion channels return to resting state
> excitability restored
what is a big factor in the refractory period
h-gate
give a summary of action potential
• AP is all or none; amplitude is independent of stimulus
• At 'threshold' ○ As it reaches -55mV - triggered ○ Voltage-gated Na+ channels open ○ Na+ diffuse in; = further depolarisation ○ Positive feedback involved here
• ‘peak’
○ Na+ channels close; voltage gated K+ channels open;
○ K+ diffuse out = repolarisation
• Return to resting membrane potential
where are the gates of the Na channels found
inside the cell
This is important because if we are going to block the sodium channel then it needs to be blocked from inside the channel
Important with LA
what is the refractory period
After an AP is initiated, the neuron cannot generate another AP until the first one has ended
The period of inexcitability is called the refractory period
It is due to the inactivation of voltage-gated sodium channels
The inactivation (‘h’) gates are shut, and so Na+ cannot diffuse into the neuron
Action potentials cannot add together; they are all or none events
If we didn’t have the refractory period then the AP could move in one direction and then in another direction
what are the consequences of the refractory period
- Limits maximum firing frequency of action potentials in axons
- Ensures unidirectional propagation of action potentials
- Prevents summation of APs
• Prevents summation of contraction in cardiac muscle
The cardiac AP lasts as long as the ventricular contraction
