2 Action Potentials

Question 1

Describe an Action potential

Answer 1

An Action Potential as a neuronal signal

• allows parts of the body to communicate quickly
• it is generated by a rapid influx of Na+ ions, (stimulus, or synapse transmission)/slower efflux of K+ out of cell

Question 2

Describe the all or nothing law, with relation to an Action potential

Answer 2

An action potential is triggered only if there is a minimum level of depolarisation
(threshold potential)

So, it either happens or doesn’t

Question 3

Describe the different parts of an action potential

Answer 3

Resting potential - about -70mV

• Rising stroke: Upstroke; Depolarisation
• Half-width: The measure of the time taken to reach half the height of the peak of AP
• Peak
• Falling phase: downstroke, repolarisation, hyperpolarisation

Question 4

Describe the ionic basis of the generation of an action potential

Answer 4

The Action Potential is dependent on sodium ions

• A stimulus means lots of Na+ ions move into the neuron
• and this rapidly makes the inside of the cell more positive
• This will increase the membrane potential of the neuron - Depolarisation

If this level of depolarisation reaches a threshold potential

• Voltage-Gated Na+ channels open, and more Na+ moves into the neuron
• further depolarising the membrane

Question 5

Describe the roles of Voltage-Gated ion channels, in the production of an action potential

(and how this can lead to positive feedback, AP upstroke)

Answer 5

These are protein channels, which are closed at rest

• but once the threshold potential is reached, these channels open
• allowing both Na+ and K+ ions to move down their respective concentration gradients

This is known as positive feedback

• triggering event
• membrane depolarisation (more +ve)
• Increased Na+ permeability (VG channels)
• Increased Na+ influx
• More membrane depolarisation

This occurs fast

Question 6

Describe the ionic mechanisms for the changes in permeability to Na+ ions
(and or K+)

(compare their speeds)

Answer 6

• Permeability of Na+ ions changes quickly
• Permeability of K+ ions changes slowly

The rapid influx of Na+ is needed to generate the action potential

BUT the slow efflux of K+ ions out of the cell is needed to repolarise the membrane
- in preparation for the next action potential

THIS is known as negative feedback

Question 7

Describe the AP downstroke as an example of slow negative feedback

Answer 7

• Membrane depolarisation
• increased K+ permeability
• Increased K+ outflow
• Membrane hyperpolarisation

So, the inactivation of Na+ VG channels (to reduce Na+ influx) is not enough for AP repolarisation
- needs the opening of VG K+ channels

For AP to occur, VG Na+ must activate more rapidly than VG K+

Question 8

Describe the refractory period and VG Na+ inactivation

Answer 8

This is the period after the action potential

- in which another action potential cannot be stimulated

Question 9

Describe the relative refractory period

Answer 9

If the stimulus is the same size and is being triggered during the refractory period
- an AP cannot be produced

A larger stimulus can trigger an AP earlier, but there is a limit on how soon

Question 10

Describe the absolute refractory period

Answer 10

This is the time when no large amount of stimulus can generate an AP
- this is because, during this time, all the VG Na+ channels are inactivated (closed)

This is possible only if at least one of these channels are activated/open, in order to trigger another AP (earlier)

Question 11

Describe how another AP can be triggered (after a refractory period)

Answer 11

Triggering another AP depends all on Na+ permeability

• in order to move into the neuron and depolarise the cell membrane
• so when voltage-gated channels are activated/inactivated is important

How soon the next AP can be triggered depends on this fact

• as soon as voltage-gated Na+ channel opens
• a large enough stimulus can produce another AP

Question 12

Describe propagation and conduction of action potentials (passive conduction, and why this doesn’t work)

Answer 12

Passive conduction will ensure that the adjacent membrane depolarises, so the action potential ‘travels’ down the axon
- But transmission by continuous action potentials is relatively slow and energy-consuming (Na+/K+ pump)

A faster, more efficient mechanism has evolved, saltatory conduction

Question 13

Describe saltatory conduction

Answer 13

Myelination provides saltatory conduction
- Myelin is insulating, preventing the passage of ions over the membrane

Saltatory conduction
- Action potential moves through the axon
- As the charge moves through the axon, it can weaker
- So, the myelin sheath insulates the axon, maintaining the signal strength
- Gaps in Myelin Sheath are the Nodes of Ranvier
> which allow another AP to be triggered here and produce another full-strength Action potential

Question 14

What is nerve conduction velocity (NCV) affected by?

Answer 14

Nerve Conduction Velocity (NCV) is affected by:

• Axon diameter
• Myelination
• Temperature

Question 15

Describe the effects of Axon diameter on Nerve conduction velocity (NCV)

Answer 15

Increase in Axon diameter = Increase in NCV

• less internal resistance
• current travels further before being dissipated

Question 16

Describe the effects of Myelination on Nerve conduction velocity (NCV)

Answer 16

• Decrease in current leakage across membrane over the internodal surface
• Current travels further before being dissipated
• Voltage-gated channels concentrated at the Nodes of Ranvier
• Gating of channels only has to occur at nodes, not continuously as in unmyelinated tissue
• Saltatory conduction

increase in NCV