The basis of 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

Depolarisation opens which voltage-gated channels?

Answer 11

Na+ 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 value of Ek?

Answer 18

-80mV

Question 19

What is the value of ENa?

Answer 19

+62mV

Question 20

What is the all-or-nothing principle?

Answer 20

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

Question 21

What causes the all-or-nothing principle?

Answer 21

The existence of a threshold

Question 22

What is the threshold?

Answer 22

The point at which an action potential will fire

Question 23

What is happening in the neurone at the threshold value?

Answer 23

Na+ influx is greater than K+ efflux

Question 24

What is the absolute refractory period?

Answer 24

No further action potential by any stimulus regardless of size

Question 25

What is the relative refractory period?

Answer 25

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

Question 26

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

Answer 26

Na+ are mostly inactivated

K+ are mostly open

Question 27

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

Answer 27

Na+ are recovering from inactivation

K+ some are still open

Question 28

Why do action potentials not travel backwards in axons?

Answer 28

The membrane behind the action potential is in the refractory period

Question 29

What is electrotonic spread?

Answer 29

The propagation of an action potential in an unmyelinated axon

Question 30

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

Answer 30

1000x

Question 31

What happens to the impulse in a myelinated neurone?

Answer 31

Jumps between nodes of Ranvier

Question 32

What does the myelin sheath do?

Answer 32

Increases the speed of action potential conduction

Question 33

What is saltatory conduction?

Answer 33

The jumping of the impulse between gaps in the myelin sheath