Membrane Potentials Flashcards
What are the different ion concentrations in the cell and how are these concentration achieved?
[K+] is high inside relative to outside
[Na+] is low outside relative to inside
This happens through a Na+/K+ pump, a sodium-potassium pump
This is an active transport process, so uses ATP to pump sodium and potassium against their concentration gradient
The pump takes in 3 molecules of sodium and exports them outside of the cell and then takes in 2 molecules of potassium and releases them inside of the cell
What by “potassium ‘leak’”?
Because of this conc. gradient, potassium moves out of the cell down its concentration gradient through K+ channels in the cell membrane
This leaves behind negatively charged anions
This creates a separation of charge across the membrane where the inside of the cell becomes much more negative than the outside- an electrical gradient
Because we have a separation of positively and negatively charged ions this leads to a potential difference
How come there’s a membrane potential?
There is an electrochemical gradient across the membrane that results in a membrane potential
Membrane potential means potential energy is kind of stored within the membrane
It is similar to voltage (difference in electrical charge) across the plasma membrane
The resting potential is the membrane potential of a cell not sending signals usually around -70mV
What is the Nernst Equation?
E∗ion=62mV ((log[ion]∗outside)/([ion]∗inside)). ∗=subscript
Used to calculate the equilibrium potential of a specific ion in a cell
The equilibrium potential is the membrane potential at which the electrical and chemical gradients of a specific ion are balanced
Ek= Potassium Equilibrium Potential
Ena= Sodium Equilibrium Potential
What is the Goldman Equation used for?
Used to calculate the resting membrane potential of a cell taking into account multiple ion permeabilities
How does the membrane potential being closer to E*k mean?
The membrane potential is closer to E*k as the membrane is more permeable to K+
If the membrane suddenly became more permeable to Na+ the membrane potential would rise closer to that of Na+
What is depolarisation?
Depolarisation is what occurs when the membrane potential is more positive than the resting potential
What is repolarisation?
Repolarisation is what occurs when the membrane potential returns to resting potential after depolarisation
What is hyperpolarisation?
Hyperpolarisation is what occurs when the membrane potential is more negative than the resting potential
What is an action potential?
If a depolarisation shifts the membrane potential sufficiently, it results in an action potential
The two key types of cells that this arises in are in our neurones and our cardiac myocyte cells in our heart
How does a neuronal action potential happen?
The cell bodies, they’re going to be receiving inputs, and when we have sufficient depolarisation, an action potential is going to spread all the way along the axon to the nerve terminal
The nerve terminal could synapse with another nerve or with a neuromuscular junction or skeletal muscle etc. and cause an effect there
So the inputs cause depolarisation but it needs to be sufficiently high of a depolarisation to fire an action potential, if it doesn’t we get a failed initiation
The depolarisation needs to pass a threshold level of about -55 mV
Once a successful stimulus occurs , our Na+ channels are opening allowing the sodium ion into the cell causing depolarisation.
We then reach a membrane potential of around +40 where our sodium channels inactivate and start to close
As they start to close, potassium channels start to open and potassium leaves the cell taking positive charge with it which aids the repolarisation of the cell membrane
The channels exceed the resting membrane potential, where we end up with this hyperpolarisation
In the hyperpolarisation phase, its going to be much harder for another action potential to be generated
The activity of the sodium-potassium pump returns the membrane potential to the resting state of around -70mV
How does the cardiac action potential occur?
A signal comes in which results in depolarisation when our sodium channels open
What happen in the cardiac action potential is that when repolarisation begins, when sodium channels inactivate and potassium channels open, we get a plateau phase.
Here potassium channels are still open but some calcium channels are activated too which results in this plateau phase we don’t see in neurons
This is a key feature because we need that calcium to come into the cell, as calcium is what initiates a contraction of our cardiac myocytes.
Then, in the next phase, repolarisation occurs where calcium channels close and potassium channels open
Finally most sodium and potassium channels are closed
What do action potentials have t do with the heart beat?
Action potentials underlie the electrical conduction system in the heart
This controls the rhythm and synchronicity of the contractions of the heart
