3- Resting Potential and action potential Flashcards
(13 cards)
What is electrochemical equilibrium
reached when concentration gradient is
balanced by the electrical gradient across the membrane
What is the equilibrium potential
potential that prevents diffusion down the ion’s
concentration gradient
Describe the Nernst equation
• The Nernst equation is Ex = (RT/ZF).ln(Co/Ci). You should know that:
Ex+ is the equilibrium potential of ion X R is the gas constant
T is absolute temperature
Z is the charge on the ion
F is Faraday’s number 96,500 coulombs of charge/mol of a singly charged ion
Substituting the values of the constants @ 37oC, Ex = (27/Z).ln(Co/Ci)
What is the equilibrium potential of potassium and sodium
Sodium (+73mV) or Potassium (-92mV)
What determines the overall membrane potential
It is the PERMEABILITY OF THE MEMBRANE to all the ions that governs the overall membrane potential
• POTASSIUM is the main ion which controls resting membrane potential
• The size of each ion’s contribution to membrane potentials is proportional to
the permeability of the membrane to that ion
What is the GHK voltage equation
GHK is a derivation of the Nerst equation which takes into account membrane permeability
• It considers all the ions involved to generate a value for resting membrane potential based on the ions, concentrations and permeability of the membrane to each ion
What is the difference between graded potentials and action potentials
Graded Potentials - change in amplitude
• Action Potentials - uniform amplitude - all-or-nothing event
• Graded potential changes depending on the stimulus and it can be bi-directional (positive or negative)
• Weak stimulus = small potential; Strong stimulus = large potential
• Graded potentials decrease in amplitude over time and with distance from its point of origin
• They get smaller because the movement of charge across the axon will slowly leak over time resulting in a decrease in the size of the graded potential
• Graded potentials only occur at:
Synapses
Sensory Receptors
• Function = GENERATE OR PREVENT AN ACTION POTENTIAL FROM FORMING
What is the absolute refractory period
ABSOLUTE REFRACTORY PERIOD - blocking of VGSCs by the inactivation gate means that the section of membrane which is hyperpolarised cannot be depolarised again and the action potential cannot travel in the wrong direction
Describe phase 1: resting membrane potential
Membrane is far more permeable to potassium than sodium
This is why the resting potential is closer to the equilibrium potential of potassium
At rest the VOLTAGE-GATED ion channels (Na+ and K+) are CLOSED
Activation Gate - conformational change closes the pore Inactivation Gate - ball and chain plugs the channel
The movement of these two gates is based on voltage At resting potential these gates are CLOSED
Describe phase 2: depolarising stimulus
Depolarisation causes opening of voltage-gated sodium channel allowing sodium into the cell
This means that the membrane potential changes in the direction of the equilibrium potential of sodium
Stimulus needs to be above THRESHOLD to generate an action potential
Describe phase 3: depolarising phase
Starts at threshold potential
Increase in the permeability of the membrane to sodium
Sodium moves down the electrochemical gradient and membrane potential moves towards the equilibrium potential of sodium Voltage-Gates potassium channels open much slower than VGSCs
At the start of the upstroke, VGSCs open first
As upstroke progresses, more an more VGKCs open
Describe phase 4: repolarisation
VGSCs begin to become inactivated so the permeability to sodium decreases
More VGKCs open - permeability to potassium increases
More K+ leaves the cell and the membrane potential moves in the direction of the equilibrium potential of K+
At the start of repolarisation - sodium inactivation gate is closed Potassium channel is open
This is the ABSOLUTE REFRACTORY PERIOD
As the inactivation gate is closed, you can’t generate another action potential in this state
As it returns towards resting potential the channels begin to close VGKCs remain open for longer allowing hyperpolarisation
5. PHASE 5: After-hyperpolarisation
Describe phase 5: after hyper-polarisation
The undershoot takes place because VGKCs remain open for a few milliseconds after repolarisation
Membrane potential moves closer towards the equilibrium potential of K+
VGKCs eventually close
During after-hyperpolarisation - sodium channel is closed and the inactivation gate is open
This ion channel CAN be opened again by a change in membrane potential but because the membrane potential is already more negative than normal (hyperpolarisation) - more stimulus is needed to reach threshold
This is the RELATIVE REFRACTORY PERIOD Relative Refractory Period:
• INACTIVATION gate is open
• Stronger than normal stimulus is required to trigger an action
potential
