W4 Membrane and Action Potentials - Jason Potas Flashcards
What is a membrane potential?
A potential difference of charge across a membrane. Maintained by Na+/K+ ATPase.
This potential difference is measured in Volts, V.
Resting Membrane Potential (RMP) is also known as Vm
What is an action potential (AP)?
The ability of a voltage-gated ion channel to depolarise and repolarise.
What is conductance, G of an ion?
The measure of ease with which an ion can cross a unit are of 1cm^2 of membrane.
Given by the equation:
G(ion) = Po.n.gamma
where:
Po = the probability that channel is open (i.e. a Po of 1 means that the channels are open all of the time.
n = the number of channels
gamma = the number of ions per sec that a channel can conduct.
What is the Nernst potential?
The net effect of an ion wanting to move down its chemical gradient as well as its electrical gradient resulting in an equilibrium between the two forces as an electro-chemical gradient.
What is the Nernst Equation?
A calculation of the Equilibrium potential, Ex, for the potential difference that would arise if the membrane was leaky to just one ion.
What is the Goldman Equation?
A calculation for what the resting membrane potential should be.
Takes into account the relative conductance of the membrane of all ions as well as the electrochemical forces on each ion.
Why is Na+ important in creating an action potential?
The opening of voltage-gated Na+ channels results in Na+ influx that leads to depolarisation of the membrane (i.e. RMP becomes more positive)
What is the “Hodgkin-Huxley Cycle”?
The explosive upstroke of action potential and is associated with an increase in G(Na). (the increased conductance of Na+ into the cell)
Why are K+ channels central for repolarisation?
K+ channels are slower to open than Na+ channels, but are longer lasting. They lead to an efflux of K+ out of the cell resulting in repolarisation (downstroke) of the AP.
Define and state the mechanism behind absolute refractory period?
Absolute refractory period is the period when a second AP cannot be initiated. Due to the closed voltage-gated Na+ channels that must be “reset” by repolarisation.
Define and state the mechanism behind relative refractory period?
Relative refractory period is the period where a second AP requires a stronger than normal impulse to be initiated. It is due to the leaky K+ channels that keep the membrane hyperpolarised.
Why do different ion channels influence the shape of the action potential?
The channel properties and Nernst potential for each ion influences the AP shape. For example, cardiomyocytes also have Ca2+ channels, that affect the shape of the AP.
