0% Resting and Action Potentials

Question 1

Define equilibrium potential?

Answer 1

The potential at which electrochemical equilibrium has been reached. It is the potential that prevents diffusion of the ion down its concentration gradient.

Question 2

Explain why the resting membrane potential of most cells is around -70 mV.

Answer 2

The equilibrium potential of potassium is around -90 mV and the membrane is very permeable to potassium so the resting membrane potential is around the same as the equilibrium potential of potassium.

• Membranes have mixed K+ and Na+ permeability but at rest K+»»>Na+
• mainly due to K+ but a little bit of Na+ also leaks in, giving it a more positive.

Question 3

What does the Nernst equation show and what is it dependant on?

Question 4

What does the Goldman-Hodkin-Katz equation show?

Question 5

What is electrochemical equilibrium?

Answer 5

The point at which the concentration gradient balances the electrical gradient.

Question 6

What is the difference between graded potentials and action potentials?

Answer 6

Action potentials are an all-or-nothing event - they have the same amplitude every time
Graded potentials can vary in amplitude and the amplitude is affected by the strength of the stimulus. Graded potentials can be positive or negative and they decrease in altitude as they travel away from the point of origin.
– Changes depending on stimulus and it can be bi-directional (positive or negative)
- Weak stimulus = small potential
- Strong stimulus = large potential

Question 7

Where do graded potentials occur?

Answer 7

Synapses and sensory receptors

Question 8

What is the role of graded potentials?

Answer 8

Generate or prevent action potentials

Question 9

What do the graded responses allow?

Answer 9

Allow an integration of receptors e.g. the neurotransmitter from the pre synaptic neurone to the post synaptic neurone allows gradation at the post synaptic, and if it is strong enough then another action potential is created.
If 2 different neurotransmitter are delivered to the post synaptic neurone, a1 is inhibitory the other is excitatory the gradation cancels it out

Question 10

What are the two gates in voltage gated ion channels?

Answer 10

Activation gate - opened by conformational changes

Inactivation gate - ball and chain plugs the channel

Question 11

What happens during depolarisation?

Answer 11

Depolarisation makes the voltage gated sodium channels open so there is a large increase in the permeability to sodium. The membrane potential drives towards the equilibrium potential of sodium. The voltage gated potassium channels are much slower to activate.

Question 12

What happens during the start of repolarisation? What makes this the absolute refractory period?

Answer 12

The inactivation gate of the VGSCs close meaning that regardless of the stimulus strength, another action potential cannot be generated - absolute refractory period

Question 13

What is the relative refractory period?

Answer 13

The activation gate closes and the inactivation gate opens so an action potential can be generated but only with a stimulus of greater than normal strength.

Question 14

How long does a normal action potential last?

Answer 14

2 ms

Question 15

What factors affect conduction velocity?

Answer 15

Myelination

• prevents loss of charge by acting as an insulator - allows the charge to travel further than with cable transport
• Saltatory conduction

Diameter
(greater diameter = faster)

Question 16

Why do cells have an electrical potential?

Answer 16

• Transmits info reliably + quickly over large distances
• Controls entry of calcium into cell
• Calcium is important in releasing chemical signals e.g. neurotransmitter

Question 17

Define Flux

Answer 17

• Number of molecules that cross a unit area per unit time i.e. molecules.m−2.s−1
• When at diffusion equilibrium, there is no net flux

Question 18

What is the resting membrane potential of most excitable cells?

Answer 18

-70mV

Question 19

How is a membrane potential measured?

Answer 19

• A zero reference electrode is placed outside the cell
• Another electrode is placed inside the cell
• It measures a voltage difference that is negative compared to the outside

Question 20

How are ions movement across the membrane?

Answer 20

• Permeable pores in the membrane (ion channels) open and close depending on transmembrane voltage, presence of activating ligands or mechanical forces.
• Ion channels can be selective for different types of ion (K+, Na+, Cl-, Ca2+) and movement across the membrane will occur when the concentration of the permanent species is different on one side of the membrane

Question 21

How are membrane potentials generated

Answer 21

Due to diffusion through a selectively permeable membrane

Question 22

Define Depolarisation

Answer 22

Change in a positive direction

Question 23

Define overshoot

Answer 23

Change from 0 in a positive direction

Question 24

Define Repolarization

Answer 24

Change in a negative direction towards the resting potential

Question 25

Define hyperpolarization

Answer 25

Voltage drops below resting potential

Question 26

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

Answer 26

NO - it’s due to potassium moving out of cells (NOT DUE TO SODIUM/POTASSIUM PUMP)

Question 27

What is happening at the resting membrane potential?

Answer 27

• 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

Question 28

What happens during depolarisation

Answer 28

• Depolarisation causes opening of voltage-gated sodium channel allowing sodium into the cell as there is a large increase in the permeability to sodium
• The membrane potential drives towards the equilibrium potential of sodium.
• Voltage-Gates potassium channels open much slower than VGSCs

Question 29

What happens during the start of repolarisation? What makes this the absolute refractory period?

Answer 29

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, the inactivation gate of the VGSCs close meaning that regardless of the stimulus strength, another action potential cannot be generated
- absolute refractory period

Question 30

Describe the after-hyperpolarization phase

Answer 30

• 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

Question 31

How is the electrochemical equilibrium restored following the action potential?

Answer 31

• Electrochemical equilibrium is restored following the action potential by the ions moving through NON voltage-gated ion channels
• Some ions move through pumps but this is a relatively SLOW process

Question 32

what is the membrane potential dependant on

Answer 32

– The size of each ion’s contribution to membrane potentials is proportional to the permeability of the membrane to that ion