1.08 - Membrane & Action Potentials Flashcards
Define: Potential Difference
A charge separation across a membrane of a cell. whenever separation of charge occurs, a potential difference exists.
What is potential difference measured in?
Volts
How is the charge separation formed?
it is a combination of unequal intracellular and extracellular ion concentrations and a membrane that is ‘leaky’ to ions
What ions are more highly concentrated extracellularly and intracellularly respectively?
Intra: K+
Extra: Na+, Cl-, Ca2+
What is the conductance?
The leakiness of the membrane to an ion. It is the measure of the ease with which an ion can cross a unit area of 1cm2 of membrane
What factors influence the conductance?
p= the probability a channel is open y = the number of ions per sec a channel can conduct n = the number of channels
What is the value of the resting membrane potential?
~-60 to -70mV
What structure maintains the membrane potential?
Na/K ATPase.
It uses ATP to pump 3Na+ out of the cell and 2K+ in to the cell –> against their concentration gradient
Define: Action Potential
A sudden change in resting membrane potential from -70mV to ~+20mV.
What is the duration of an Action Potential?
~1-2ms
What are the three stages of an action potential?
Depolarisation (upstroke)
Repolarisation (downstroke)
Hyperpolarisation (undershoot below RMP)
What is a voltage gated channel?
A channel whose probability of opening varies with membrane potential
What happens when the resting membrane potential reaches -55mV?
voltage gated Na+ channels open
voltage gated K+ channels open but do so more slowly
What are the stages of depolarisation?
Initial depolarisaion to -55mV
Opens v-g Na+ channels
Influx of Na+
Vm becomes more positive, approaches Ena (~+60mV)
What are the stages of repolarisation?
Initial depolarisaion to -55mV
Opens v-g K+ channels (slow opening –> delayed effect, longer lasting)
Efflux of K+
Vm becomes more -ve, approaching Ek
Describe depolarisation and hyperpolarisation
As membrane potential approaches Ena, the electrochemical force pushing Na+ into the cell decreased
Voltage gated Na+ channels inactivate
Continued efflux of K+ repolarises the membrane.
Voltage gated Na+ inactivation is removed by depolarisation.
This hyperpolarisation must occur before another action potential can take place
What is the purpose of the leaky channels in the action potential
Brings about the small change in resting membrane potential to -55mV to open the V-G Na+ channels
What is the absolute refractory period?
is the period when a
second action potential cannot be initiated
Due to closed Na+ inactivation gates that must be
reset (opened) by membrane repolarisation
What is the relative refractory period?
is the period where a
second action potential requires a stronger than normal stimulus to be initiated
Due to leaky K+ channels that keep Vm hyperpolarised
Summarise the Action Potential
When RMP reaches -55mV voltage gated Na+ channels open
Na conductance increases
Vm shifts towards ENa
Slowly, voltage gated K+ channels open
Increased K+ conductance
Na+ channels begin to inactivate and
electrochemical force on Na+ diminishes
Combination of K+ channels opening (efflux of K+
ions) and inactivation of Na+ channels (influx of Na+ stopping) leads to repolarisation of the membrane
The after hyperpolarisation occurs because voltage-gated K+ channels are still open
Vm returns to rest when they close (only leak channels open at rest)
