Neuroscience 3 - Resting and Action Potentials Flashcards
What is “Flux”?
- The number of molecules that cross a unit area per time.
2. When diffusion reaches equilibrium, there is no net flux.
How is a voltage generated in cells?
It is generated by ions that produce a charge gradient.
How is a current generated in cells?
It is due to the movement of ions due to potential.
How is there resistance in cells? And what is key to the resting membrane potential?
- This is due to the barrier that prevents the movement of ions - cell membrane.
- The permeability of the membrane is key to the resting membrane potential.
What is the membrane potential of most excitable cells? Where is the zero reference placed?
- -70mV.
2. The zero reference is placed outside the cell and the inside of the cell is negative.
What are ion channels? And how can they be opened?
- They permeable pores in the membrane that are selective for different ions and allow the ions to move in and out of cells.
- They can be opened as a results of a change in voltage, because of ligand binding to it or because of stress (change in size of cell).
What is the electrochemical equilibrium?
- Is when the concentration gradient is balanced by the electrical gradient across the membrane.
- A stable membrane potential is established.
What is the equilibrium potential?
The potential that prevents diffusion down the ion’s concentration gradient.
Which ion is mainly responsible for the control of the resting potential?
Potassium.
What governs the overall membrane potential?
The permeability of the membrane to all the ions.
What is the Goldman-Hodgkin-Katz (GHK) voltage equation?
- It is a derivative of the Nernst equation.
2. It generates a value for the resting membrane potential based on the ions, concentrations and membrane permeability.
What is depolarisation?
Change in a positive direction.
What is overshoot?
Change from 0 in a positive direction.
What is repolarisation?
Change in the negative direction towards the resting potential.
What is hyperpolarisation?
Voltage drops below resting potential.
What are graded potentials?
- It is a change in amplitude due to a stimulus and can be bi-directional.
- It occurs at synapses and sensory receptors and may help generate or inhibit an action potential.
- They also decrease in amplitude over time and distance.
How many phases are there of an action potential?
5.
What is phase 1 of the action potential?
- Resting membrane potential - the VGSC and VGKC are both closed.
- Membrane more permeable to K+ than Na+.
What is phase 2 of the action potential?
- Depolarising stimulus - VGSC open and allow Na+ into the cell.
- Changes the direction of the equilibrium potential of sodium.
- Stimulus needs to be above threshold to generate an action potential.
What is phase 3 of the action potential?
- Starts at threshold potential.
- Upstroke depolarising - Increase in permeability of membrane to Na+.
- Na+ moves down electrochemical gradient and membrane potential moves towards equilibrium potential of Na+.
- VGKC open much slower than VGSC.
What is phase 4 of the action potential?
- Repolarisation - VGSC become inactivated = permeability to Na+ decreases.
- More VGKC open = permeability to K+ increases and more K+ leaves the cell and the membrane potential moves in the direction of the equilibrium of K+.
- This is the absolute refractory period = another action potential cannot be generated yet as activation and inactivation gates are closed.
What is phase 5 of the action potential?
- After-hyperpolarisation - VGKC remain open and hence there is undershoot.
- Membrane potential moves towards the equilibrium potential of K+.
- VGKC close eventually, Inactivation gate is open and hence another action potential can be generated - however a greater stimulus is needed due to hyperpolarisation = relative refractory period.
The membrane remains in a refractory (unresponsive) state until the ……… recover from ……..
The membrane remains in a refractory (unresponsive) state until the VGSC recover from inactivation.
How is the electrochemical equilibrium restored after an action potential?
- Ions move through non-voltage gated ion channels.
2. Some ions move through pumps (Sodium/potassium ATPase) - this is a slow process.
Does the sodium/potassium pump produce membrane potential changes?
No.
What determines the velocity and distance travelled by an action potential?
The diameter of the axon and how myelinated it is.
How is the action potential prevented by growing the wrong direction?
The blocking of the inactivation gate of the VGSC ‘s means that the section of the membrane becomes hyperpolarised and hence cannot be depolarised again.
What is saltatory conduction?
Propagation of an action potential down nodes of ranvier.
How is the conduction velocity decreased? (3)
- Reduced axon diameter.
- Reduced myelination.
- Cold, anoxia, compression and drugs.
What are voltage-dependent and voltage-independent ion channels?
- Voltage-depndent = open by a change in membrane potential.
- Voltage-independent = open all the time, are responsible for producing a resting membrane potential.
What is the Nernst equation?
A way of predicting the equilibrium potential for a given ion.
E = -61/Z * log( [X] outside/ [X] inside )
Z = charge of ion
How are graded potentials and action potentials different?
- Graded = Change in amplitude.
2. Action = Uniform amplitude, all or nothing event.
Are the changes in membrane potential during an action potential due to ion pumps?
Nah
Where are axon potential generated?
At the axon hillock.
Do Potassium channels open and close faster than sodium channels?
Nope.
