Lecture 4- Actions potentials

Question 1

What is an absolute refractory period?

Answer 1

A period where no more action potentials are possible

This is when the sodium channel inactive gates are close as these cannot be reopened

Question 2

What is a relative refractory period?

Answer 2

Where action potentials are possible but difficult to initiate
This is when the Na+ inactivation gates are open (and so Depolarisation could occur) but potassium channels are also open meaning we are lot further away from threshold (hyperpolisation) and so are unlikely to reach where an all or none action potential would occur (-60mv)

Question 3

What two things does the conductance of an ion across the cell membrane depend on?

Answer 3

• Permeability (ion channels)

- Equilibrium (driving force)

Question 4

What do these symbols stand for in a diagram showing conductance and voltage per unit time..
Vm
GNa+
GK+

(note after the first letter should be subscript)

Answer 4

Vm= Membrane potential
GNa+= Sodium conductance
GK+= Potassium conductance

Question 5

What are the important components of a voltage gated sodium channel?

Answer 5

• Activation gate which senses voltage
• Pore that has a selectivity filter (only sodium can pass through)
• Four repeating motifs with 1-6 transmembrane helices
• Inactivation gate

Question 6

What is the pore in a voltage gated sodium channel made of?

Answer 6

• Two beta-subunits
• Four Alpha subunits
• There is also a TTX binding site

Question 7

What is the consequence of the pore containing a TTX binding site?

Answer 7

• A blocker called tetrodotoxin (from puffer fish) can bind and stop sodium from coming in by blocking the channels. This is irreversible.
• Saxitoxin from algal bloom (“red tide”) also has this effect
• As does Lidocaine with the exception of being short acting. This means dentists can use it as a way to numb patients.

NOTE: none of these damage the channel

Question 8

What is the function of the inactivation gate on a voltage gated sodium channel? (Describe at each stage of the channel activation what its role is)

Answer 8

This is the ball and chain like structure that exists as the bottom of the pore (however is not actually apart of the pore)

• When the channel is not active the pore is closed. This is not as a result of the inactivation gate but Instead what stops sodium coming through is a positive charge in the pore which sits in the way/ repels.
• When the channel is activated the pore is open with the positive charge nearby for selectivity but not blocking the way. Sodium can still pass through and the inactivation gate once again does not come into play.
• When the inactivation gate/ ball and chain does become useful is when the pore is still open but the channel needs to be deactivated. In this case the inactivation gate as a physical block stopping any more sodium coming through.

Question 9

What exists around the pore in a voltage gated potassium channel?

Answer 9

4 subunits

Question 10

How is a potassium channel similar and then different from a sodium channel?

Answer 10

• Both selective
• Both activated by Depolarisation
• But the potassium channel has slower kinetics meaning it takes longer for it to close and open
• Also the potassium channel doesn’t have both an activation gate and inactivation gate like the sodium channel does (only has one)

Question 11

What is a voltage gated potassium channel blocked by?

Answer 11

tetraethylammonium

Question 12

Describe the sequence of gates opening and closing throughout the course of an action potential….

Answer 12

At rest=

• For Na+ channel… activation gate is closed, inactivation gate is open.
• For K+ channel… gate is closed

During Depolarisation + AP (on the way to peak)=

• For Na+ channel… activation gate is open, inactivation gate is open meaning sodium can flow in (aligns with a peak in sodium conductance)
• For K+ channel…. gate is closed

Straight after peak and beginning of repolarization=

• For Na+ channel…activation gate is open, inactivation gate is open meaning sodium can still flow in
• For K+ channel… gate is open meaning potassium can flow out (aligns with a peak in potassium conductance)

As repolarization continues=

• For Na+ channel… first the inactivation gate closes then the activation gate closes blocking off the pore from sodium entering. There is a huge drop in sodium conductance here. This is called the absolute refractory period
• For K+ channel… gate remains open meaning potassium can flow out bringing voltage further and further down to reach hyperpolarization

Then there is the relative refractory period=

• For the Na+ channel the inactivation gate opens but the activation gate remains closed.
• For the K+ channel the activation gate remains open

And the cycle starts again when the potassium gate closes

Question 13

During the course of an action potential are a lot of ions moving across the membrane?

Answer 13

No, only small amounts. We know this because the concentrations of the ICF and ECF for both potassium and sodium do not drastically alter throughout the course of the AP.

Question 14

In what direction does an action potential spread along an axon?

Answer 14

In normal cases (not artificial) it can only go in one direction starting at the axon hillock. The channels behind are in absolute and relative refractory periods so will not generate more APs.

Question 15

How does an action potential spread along an axon?

Answer 15

With an action potential there is an influx of sodium at a specific point in the axon. The positive charges on the sodium’s then repel each other pushing the ions further along and resulting in localised changes in charge at neighbouring segments. This causes the threshold for AP generation to be met at these points down the line and thus AP propagation.

Question 16

When is speed of action potential propagation important and when is it not?

Answer 16

• Not important if you are a tiny animal like a fly as there is not much distance for the APs to travel to reach extremes of the body
• However, if you are something like a giraffe it becomes vital cause slow propagation would result in delays. Also have to work against gravity.

Question 17

How does axon diameter effect the speed of conduction?

Answer 17

The wider an axon is in a diameter the less internal resistance there is and therefore the faster AP propagation will be.

Question 18

How does myelination effect the speed of conduction?

Answer 18

Results in insulation decreasing rm. This means there is not as much leakage of ions. Also while there is a high amount of channels at nodes there is none between. This means that for myelinated axons action potentials can jump from node to node travelling down a lot faster.

Question 19

What is the term for conduction that occurs in myelinated axons?

Answer 19

Saltatory conduction

Question 20

How does a distance/ time graph look for the speed of conduction in a myelinated axon?

Answer 20

• Long distance in short time between nodes.

- Lots of time not much distance at nodes

Question 21

What are two adaptations of axons that effect conduction speed?

Answer 21

• Insulation/ myelin sheath

- Diameter of axon