Neuroscience 3 - Resting and Action Potentials

Question 1

What is “Flux”?

Answer 1

1. The number of molecules that cross a unit area per time.

2. When diffusion reaches equilibrium, there is no net flux.

Question 2

How is a voltage generated in cells?

Answer 2

It is generated by ions that produce a charge gradient.

Question 3

How is a current generated in cells?

Answer 3

It is due to the movement of ions due to potential.

Question 4

How is there resistance in cells? And what is key to the resting membrane potential?

Answer 4

1. This is due to the barrier that prevents the movement of ions - cell membrane.
2. The permeability of the membrane is key to the resting membrane potential.

Question 5

What is the membrane potential of most excitable cells? Where is the zero reference placed?

Answer 5

1. -70mV.

2. The zero reference is placed outside the cell and the inside of the cell is negative.

Question 6

What are ion channels? And how can they be opened?

Answer 6

1. They permeable pores in the membrane that are selective for different ions and allow the ions to move in and out of cells.
2. 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).

Question 7

What is the electrochemical equilibrium?

Answer 7

1. Is when the concentration gradient is balanced by the electrical gradient across the membrane.
2. A stable membrane potential is established.

Question 8

What is the equilibrium potential?

Answer 8

The potential that prevents diffusion down the ion’s concentration gradient.

Question 9

Which ion is mainly responsible for the control of the resting potential?

Answer 9

Potassium.

Question 10

What governs the overall membrane potential?

Answer 10

The permeability of the membrane to all the ions.

Question 11

What is the Goldman-Hodgkin-Katz (GHK) voltage equation?

Answer 11

1. 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.

Question 12

What is depolarisation?

Answer 12

Change in a positive direction.

Question 13

What is overshoot?

Answer 13

Change from 0 in a positive direction.

Question 14

What is repolarisation?

Answer 14

Change in the negative direction towards the resting potential.

Question 15

What is hyperpolarisation?

Answer 15

Voltage drops below resting potential.

Question 16

What are graded potentials?

Answer 16

1. It is a change in amplitude due to a stimulus and can be bi-directional.
2. It occurs at synapses and sensory receptors and may help generate or inhibit an action potential.
3. They also decrease in amplitude over time and distance.

Question 17

How many phases are there of an action potential?

Answer 17

5.

Question 18

What is phase 1 of the action potential?

Answer 18

1. Resting membrane potential - the VGSC and VGKC are both closed.
2. Membrane more permeable to K+ than Na+.

Question 19

What is phase 2 of the action potential?

Answer 19

1. Depolarising stimulus - VGSC open and allow Na+ into the cell.
2. Changes the direction of the equilibrium potential of sodium.
3. Stimulus needs to be above threshold to generate an action potential.

Question 20

What is phase 3 of the action potential?

Answer 20

1. Starts at threshold potential.
2. Upstroke depolarising - Increase in permeability of membrane to Na+.
3. Na+ moves down electrochemical gradient and membrane potential moves towards equilibrium potential of Na+.
4. VGKC open much slower than VGSC.

Question 21

What is phase 4 of the action potential?

Answer 21

1. Repolarisation - VGSC become inactivated = permeability to Na+ decreases.
2. 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+.
3. This is the absolute refractory period = another action potential cannot be generated yet as activation and inactivation gates are closed.

Question 22

What is phase 5 of the action potential?

Answer 22

1. After-hyperpolarisation - VGKC remain open and hence there is undershoot.
2. Membrane potential moves towards the equilibrium potential of K+.
3. 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.

Question 23

The membrane remains in a refractory (unresponsive) state until the ……… recover from ……..

Answer 23

The membrane remains in a refractory (unresponsive) state until the VGSC recover from inactivation.

Question 24

How is the electrochemical equilibrium restored after an action potential?

Answer 24

1. Ions move through non-voltage gated ion channels.

2. Some ions move through pumps (Sodium/potassium ATPase) - this is a slow process.

Question 25

Does the sodium/potassium pump produce membrane potential changes?

Answer 25

No.

Question 26

What determines the velocity and distance travelled by an action potential?

Answer 26

The diameter of the axon and how myelinated it is.

Question 27

How is the action potential prevented by growing the wrong direction?

Answer 27

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.

Question 28

What is saltatory conduction?

Answer 28

Propagation of an action potential down nodes of ranvier.

Question 29

How is the conduction velocity decreased? (3)

Answer 29

1. Reduced axon diameter.
2. Reduced myelination.
3. Cold, anoxia, compression and drugs.

Question 30

What are voltage-dependent and voltage-independent ion channels?

Answer 30

1. Voltage-depndent = open by a change in membrane potential.
2. Voltage-independent = open all the time, are responsible for producing a resting membrane potential.

Question 31

What is the Nernst equation?

Answer 31

A way of predicting the equilibrium potential for a given ion.

E = -61/Z * log( [X] outside/ [X] inside )

Z = charge of ion

Question 32

How are graded potentials and action potentials different?

Answer 32

1. Graded = Change in amplitude.

2. Action = Uniform amplitude, all or nothing event.

Question 33

Are the changes in membrane potential during an action potential due to ion pumps?

Answer 33

Nah

Question 34

Where are axon potential generated?

Answer 34

At the axon hillock.

Question 35

Do Potassium channels open and close faster than sodium channels?

Answer 35

Nope.