The Basis of Cell Excitability

Question 1

What is the timescale for an action potential?

Answer 1

1-2msec

Question 2

What causes depolarisation?

Answer 2

Na+ influx

Question 3

What is depolarisation?

Answer 3

The cell becoming less negative

Question 4

What does depolarisation do to PNa

Answer 4

It causes the opening of Na+ channels and PNa increases

Question 5

What causes repolarisation?

Answer 5

K+ efflux

Question 6

What is repolarisation?

Answer 6

The return of Em to resting potential

Question 7

What does repolarisation do to Na+ gated channels?

Answer 7

Closes them

Question 8

What is conductance of ions?

Answer 8

It is equivalent to permeability
Measured instead of permeability, membrane acts as an electrical resistor (R)
Conductance, g=1/R
Each ion has its own conductance

Question 9

Conductance is proportional to what?

Answer 9

To the number of open ion channels

Question 10

Change in gion will change what?

Answer 10

Em

Question 11

What causes cells to further depolarise?

Answer 11

opening of voltage gated Na+ ion channels

Question 12

What causes the initial depolarisation?

Answer 12

Synaptic activity
Generator potential (sensory neurone)

Question 13

What does Em approach in the depolarisation phase?

Answer 13

ENa

Question 14

What happens to Na+ channels in repolarisation?

Answer 14

They inactivate

Question 15

When do K+ channels open?

Answer 15

As the neurone is repolarising, they open at positive values of Em

Question 16

What is hyperpolarisation?

Answer 16

Em has returned to its initial value but K+ channels are still open
Em approaches Ek

Question 17

How is hyperpolarisation dealt with?

Answer 17

The K+ channels finally close

The leak channels restore Em to resting value

Question 18

What is the all-or-nothing principle?

Answer 18

An action potential either happens or it doesn’t. They have no difference in size or power

Question 19

What causes the all-or-nothing principle?

Answer 19

Existance of a threshold value

Question 20

What is the threshold?

Answer 20

The point at which an action potential will fire

Question 21

What is happening in the neurone at the threshold value?

Answer 21

Na+ influx is greater than K+ efflux

Question 22

What is the absolute refractory period?

Answer 22

No further action potential by any stimulus regardless of size

Question 23

What is the relative refractory period?

Answer 23

A stronger stimulus will open sufficient Na+ channels and overcome increased gK that makes the threshold greater

Question 24

Explain absolute refractory period in terms of voltage-gated channels?

Answer 24

Na+ are mostly inactivated

K+ are mostly open

Question 25

Explain relative refractory period in terms of voltage-gated channels?

Answer 25

Na+ are recovering from inactivation | K+ some are still open

Question 26

Why do action potentials not travel backwards in axons (unidirectional)?

Answer 26

The membrane behind the action potential is in the refractory period

Question 27

What is electrotonic spread?

Answer 27

The propagation of an action potential in an unmyelinated axon

Question 28

How much faster is the speed of an impulse in a myelinated neurone?

Answer 28

1000x

Question 29

What happens to the impulse in a myelinated neurone

Answer 29

Jumps between nodes of Ranvier

Question 30

What does the myelin sheath do?

Answer 30

Increases the speed of action potential conduction

Question 31

What is saltatory conduction?

Answer 31

The jumping of the impulse between gaps in the myelin sheath

Question 32

Explain how an action potential is generated?

Answer 32

- Initial stimulus causes generator potential and starts initial depolarisation - Opening of voltage gated na ion channels increases gNa (sodium conductance) - Na influx into cells leads to further depolarisation and even greater gNa - Na influx is greater than K efflux - Em surpasses threshold value (-55mV) and approaches ENa - Depolarisation peaks at +40mV where Na channels inactivate and Na influx stops despite stimulus - Decreased gNa coincides with the opening of Voltage gated K channels. - This opening is delayed to prevent Em from remailing equal - K efflux leads to repolarisation and increased gK returns to rest. - Em approaches Ek at hyperpolarisation - Voltage gated K channels close - Em returns to resting value (-65mV) via leak channels.

Question 33

Why is there a delay between the peak of depolarisation and the opening of Voltage gated K channels?

Answer 33

To prevent the Em from having equal charge in/out of the cell at the peak of depolarisation.

Question 34

What is caused when voltage gated ion channels are open?

Answer 34

Depolarisation

Question 35

What is caused when voltage gated ion channels are closed?

Answer 35

Repolarisation

Question 36

What affects the speed of conduction in an axon?

Answer 36

Diameter of the axon | smaller = more constricted and therefore the conduction speed is slower.