Membrane Potentials Flashcards
What is the membrane potential?
It is the voltage (difference in electrical charge) across the plasma membrane.
What is the resting potential?
It is the membrane potential of a cell not sending signals.
What is the resting potential of neuronal and cardiac cells?
NEURONAL: -70 mV
CARDIAC: -90 mV
How are Na+ and K+ gradients maintained?
They are maintained by the sodium-potassium pump.
The sodium-potassium takes 3 Na+ out for every 2 K+ brought in.
Why is there such a major voltage difference between the inside and outside of the cell, and how does it lead to the electrochemical gradient?
K+ channels are always open at the resting potential (“potassium leak”). There’s a net outflow of K+; this leads to a net negative charge inside the cell.
The excess negative charges inside the cell exert an attractive force that opposes the flow the additional positively charged potassium ions out of the cell. The electrical gradient counterbalances the chemical concentration of K+ - this is called the electrochemical gradient.
When can we measure the equilibrium potential (Eion)?
We can measure it at equilibrium, when both the electrical and chemical gradients are balanced.
How can we calculate the equilibrium potential?
It can be calculated using the Nernst Equation.
The Nernst Equation is:
Eion = 62 mV (log ([ion]outside/ [ion]inside))
Why is there a difference in the resting cell potential of neurones and cardiac cells?
Because, in the neurones, there are many more K+ channels than there are Na+ channels, so the resting potential is a slight compromise between EK and ENa.
In cardiac cells, the majority are K+ channels, so the resting potential is more towards EK.
How do we model the resting potential for membranes permeable to multiple ions?
We use the Goldman Equation (aka. the Goldman-Hodgkin-Katz equation).
(I fully cannot write this out, sorry, you’re gonna have to have your notes open)
Describe how hyperpolarisation comes about.
When gates K+ channels open, K+ diffuses out, making the inside of the cell more negative.
Describe how depolarisation comes about.
When gated Na+ channels open, Na+ diffuses in, making the inside of the cell more positive.
What brings about an action potential?
If a depolarisation shifts the membrane potential sufficiently, it results in a massive change in voltage called an action potential.
Describe the generation of an action potential in NEURONES.
1) THE RESTING STATE
At resting potential, most voltage-gated sodium and potassium channels are closed.
2) DEPOLARISATION
When an action potential is generated, voltage-gated sodium channel open first and sodium flows out of the cell.
3) RISING PHASE OF THE ACTION POTENTIAL
During the rising phase, the threshold is crossed and the membrane potential increases.
4) FALLING PHASE OF THE ACTION POTENTIAL
During the falling phase, voltage-gated sodium channels become inactivated; voltage-gated potassium channels open and potassium flows out of the cell.
5) UNDERSHOOT
During the undershoot, membrane permeability to potassium is, at first, higher than at rest, then voltage-gated potassium channels close and resting potential is restored.
What is the refractory period after an action potential, and what causes it?
During the refractory period after an action potential, a second action potential cannot be initiated. The refractory period is a result of a temporary inactivation of the sodium channels.
Describe how an action potential travels down the axon (ion-wise).
1) An action potential is generated as Na+ flows inwards across the membrane at one location on the axon.
2) The depolarisation of the action potential spreads to the neighbouring region of the membrane, reinitiating the action potential there. In the original position, the membrane is repolarising as K+ flows outwards.
3) The depolarisation process is repeated in the next region. Local currents of ions, therefore, propagate the action potential along the length of the axon.
