The Action Potential Flashcards
5 stages of an action potential?
Resting potential Stimulus Upstroke/depolarisation Repolarisation Hyperpolarisation
Resting potential value, permeability of K+/Na+ and important channels?
-70mV
Permeability of membrane for K+ > Na+
Ungated channels responsible for resting potential
What type of channels are the Na+/K+ voltage-gated channels and where are their gate(s) located?
Both are TRANSMEMBRANE channels
K+ channel has gate within hydrophobic core of the membrane (closed during resting potential)
Na+ channel has ACTIVATION gate in hydrophobic core of membrane (closed during resting potential) and INACTIVATION gate on cytosolic face of the membrane (open during resting potential)
Na+ voltage-gated channel activation/inactivation gate changes throughout action potential?
Resting potential - Activation gate closed, no Na+ movement into cell
Depolarisation - activation gate opens, Na+ moves into cell
Repolarisation - inactivation gate closes, no Na+ movement into the cell
What is the role of the stimulus in generating the action potential?
It depolarises the membrane potential.
If it depolarises the membrane potential beyond the threshold potential, an action potential is generated
What part of the action potential curve is the stimulus or foot?
Curve from resting potential to threshold potential
Process of depolarisation phase?
Starts at threshold potential
Na+ channel activation gate opens
Na+ moves into cell down electrochemical gradient
K+ voltage-gated channels open slowly, Na+ flow in still > K+ flow out
Membrane potential becomes more positive as it moves towards Na+ equilibrium potential
Process of repolarisation phase?
K+ channels fully open, Na+ activation gate closes
Permeability to Na+ decreases and Na+ stops entering the cell
Permeability to K+ increases and K+ leaves the cell down their electrochemical gradient
Membrane potential becomes more negative as it moves towards the K+ equilibrium potential
Late in repolarisation phase, Na+ inactivation gate opens and activation gate closes
What is the absolute refractory period and how does it work?
The period in the action potential where a new action potential cannot be triggered regardless of the strength of the stimulus. Works as inactivation gate is closed on Na+ voltage-gated channels. This gate does not respond to stimuli so Na+ influx cannot be triggered for start of an action potential
What is the threshold potential?
Membrane potential that, once reached, triggers an action potential
What does the “all-or-nothing” nature of an action potential mean?
Either an action potential does not occur at all or if it does, it always occurs at its maximum size
Difference between absolute and relative refractory periods?
Absolute = No possible generation of a new action potential, regardless of size of stimulus Relative = Greater difference between membrane potential and threshold potential so new action potential can be triggered but only by a larger-than-normal stimulus/depolarisation
What are the boundaries of the absolute refractory period?
Start of depolarisation (above threshold potential) and bottom of hyperpolarisation
What are the boundaries of the relative refractory period?
Bottom of hyperpolarisation and point of resting potential establishment
Why is depolarisation a positive feedback mechanism?
Initial depolarisation leads to opening of voltage-gated Na+ channels.
This leads to increased Na+ permeability, increased Na+ movement into cell and eventually, increased depolarisation
Ceases when inactivation gate of Na VG channel closes
