The Action Potential and its propagation

Question 1

How often are leak channels open?

Answer 1

All the time

Question 2

How often are gated channels open?

Answer 2

Open in response to particular events e.g. a voltage change

Question 3

What are some functional elements of voltage-gated ion channels?

Answer 3

Selectivity filter
Voltage sensor
Activation gate
Inactivation gate

Question 4

What is the selectivity filter for?

Answer 4

Na+ and K+

Question 5

What is the voltage sensor for ?

Answer 5

Na and K channels

Question 6

What is the activation gate for?

Answer 6

Na and K channels

Question 7

What is the inactivation gate for?

Answer 7

Na channel only

Question 8

What are the different states for a voltage-gated channels for Na+?

Answer 8

1. Closed
2. Open
3. Inactivated
4. Closed

Question 9

What are the different states for a voltage-gated channels for K+?

Answer 9

1. Closed
2. Open (1ms delay)
3. Closed

Question 10

When do the Na+ channels close?

Answer 10

When the K+ channels open

Question 11

What is patch clamp recording?

Answer 11

Electrode up against the cell measuring a patch of membrane without entering the cell

Question 12

How does the electrode measure the membrane without entering the cell?

Answer 12

By applying suction down the membrane to pull some of the membrane against the electrode

Question 13

What do you use the electrode to do?

Answer 13

Create a small depolarisation

Question 14

During a patch clamp recording what is the state of the cell at rest?

Answer 14

Na channels closed

K channels closed

Question 15

During a patch clamp recording what is the state of the cell at on depolarisation immediately?

Answer 15

Na channels open (activation)
pNA increases
Em is driven down towards ENa
K channels still closed

Question 16

During a patch clamp recording what is the state of the cell at depolarisation with a delay?

Answer 16

Na inactivation, gates close
Na channels are closed
K channels open 
pK increases
Em is driven back to Ek

Question 17

During a patch clamp recording what is the state of the cell has gone through the method?

Answer 17

Na activation gates closing
Na inactivation gates re-opening
K channels closing

Question 18

What does the Na+ K+ pump act in the background to do?

Answer 18

Move Na and K ions back to their original places

Question 19

Is the Na+/K+ essential?

Answer 19

No

A nerve can support about 10,000 action potentials even when the pump is poisoned

Question 20

What can be seen by recording 2 points along an axon?

Answer 20

Than an action potential (AP) moves away from the point of electrical stimulation (stimulus electrode)

Question 21

What is an electrotonic currrent/ local current?

Answer 21

Current flowing passively along an axon decays with distance beyond the block

Question 22

What do cable properties of the membrane dictate?

Answer 22

Decay of this current

Question 23

What is electrotonic spread?

Answer 23

Detriment and voltage faults

Tells time

Question 24

What is the stimulus applied to the axon hillock to create an action potential?

Answer 24

Graded potential

Question 25

What are graded potentials?

Answer 25

A flow of positive charge

Question 26

Once an action potential occurs in the axon hillock what happens?

Answer 26

A flow of local currents allowing each new part of the membrane to progress along the membrane until the full membrane has been propagated

Question 27

Why doesn't the local current flow backwards and create an action potential in the backward direction?

Answer 27

Flows backwards but doesn't trigger an action potential because the voltage gated channels in the previous segments are in a refractory period meaning they can't be activated again

Question 28

Why can't channels be re-activated once in the refractory period?

Answer 28

Channels closed by ball and chain mechanism | When ball is in the channel the channel is inactive and can't be triggered again

Question 29

What is the AP of the membrane at the peak?

Answer 29

Negative inside to the outside

Question 30

What occurs as a result after the peak of a AP?

Answer 30

A longitudinal electrical gradient inside the axon

Question 31

What does the longitudinal electrical gradient cause?

Answer 31

An electronic current to flow away from the point of the AP

Question 32

What happens as a result of the electronic current flowing away into the next segment of the membrane?

Answer 32

The membrane ahead of the AP becomes depolarised

Question 33

What then happens to this new depolarised part of the membrane?

Answer 33

Reaches threshold and a new AP exists

Question 34

What is repetition of this process known as?

Answer 34

Action Potential (AP) Propagation

Question 35

How fast and often do AP potentials occur during AP propagation?

Answer 35

Occur rapidly, so that the AP also conducts rapidly (rates in the range 1-10 m s-1 are typical)

Question 36

What does the electronic current release?

Answer 36

The potential energy that exists all along the nerve in the form of the electrical gradient and the concentration gradients for Na+ (and later K+)

Question 37

Which direction does electronic current flow if a point is electrically depolarised using an electrode?

Answer 37

Both directions resulting in an AP propagation along both directions of the axon

Question 38

What factors affect AP conduction velocity?

Answer 38

The effect of axon diameter | The effect of myelination

Question 39

What are possible effects of the axon diameter?

Answer 39

Conduction velocity is a function of axon diameter axons present less internal resistance to current flow (before it eventually leaks out) This is the basis for the high conduction velocities in giant axons

Question 40

What are the possible effects of myelination?

Answer 40

Conduction velocity also depends on the extent of electrical insulation around the axon. This increases the rate of spread of electrotonic current This is the basis for the higher conduction velocities in myelinated nerves Myelin is formed by Schwann cells, which wrap tightly around the axons (the internodes) and leave only small uninsulated patches (nodes of Ranvier)

Question 41

What does the salutatory mechanism rely on?

Answer 41

Voltage-gated channels at the nodes

Question 42

Where are the density channels highest?

Answer 42

Density channels at nodes is extremely high (3000 micrometers2) compared with an unmyelinated nerve (300micrometers2)

Question 43

What does high density also increase?

Answer 43

Conduction velocity since the membrane comes to threshold a greater rate

Question 44

Why is there no voltage-gated channels along the internode?

Answer 44

They are not involved in AP production

Question 45

What does the density of voltage-gated channels in a patch of membrane determine?

Answer 45

Electrical excitability

Question 46

What is the membrane of an unmyelinated axon?

Answer 46

Electrically excitable

Question 47

What are patches with even greater densities of channels?

Answer 47

Particularly sensitive to electrotonic currents

Question 48

What is the membrane of an internode?

Answer 48

Electrically inexcitable and cannot transmit an AP19

Question 49

Where are extremely sensitive patches with high densities also found?

Answer 49

Where an AP is initiated naturally in a neurone E.g. Axon hillock or spike initation zone (SIZ) Sensory ending of a mechano-sensory neurone

Question 50

What are the dendritic membrane and soma of nerve cells?

Answer 50

Electrically inexcitable | Site where decision is made whether or not to initiate an AP

Question 51

What type of events are APs?

Answer 51

Discrete

Question 52

What does an AP represent?

Answer 52

An AP represents one pulse in a frequency code, which is very effective mechanism for transmitting information, and is resistant to perturbation (noise)

Question 53

How is information contained in an AP?

Answer 53

Both frequency and the pattern of activity