L4: Action Potentials

Question 1

conductance = _________ + _________

Answer 1

Conductance = permeability (ion channels) and driving force (Nernst potential)
-> ion specific

Question 2

Describe Na+ conductance during an action potential

Answer 2

spike in Na+ conductance when ion channels open

Question 3

Describe K+ conductance during an action potential

Answer 3

• K+ conductance increases slowly because K+ channels have different mechanisms than Na+
  
  • they are activated by the AP itself
  • slow to open, slow to close

Question 4

What causes the after-hyperpolarisation?

Answer 4

K+ channels have different mechanisms than Na+, slow to close

Question 5

Describe the structure of a VG Na+ channel

Answer 5

• 4x a subunits and 2x b subunits
• activation gate - senses voltage around it
• selectivity filter - only lets Na+ through - very specific
• inactivation gate - ball and chain model

Question 6

What blocks VG Na+ channels?

Answer 6

• tetrodotoxin
• saxitoxin (alagal bloom)
• lidocaine - freezing for dentists

Question 7

Explain how lignocaine works as a temporary local anesthetic

Answer 7

short acting because it only block channel a little bit –> no APs, no signal from area, blocks signal from pain receptor, also sensory receptors

Question 8

How does activation of VG Na+ channels spread

Answer 8

• activation of one likely to activate those around it –> population based thing
• the further away, the less likely it will be activated

Question 9

Describe how the inactivation molecule works

Answer 9

inactivation molecule blocks the pore after a certain amount of time, causes sudden drop in Na+ conductance, lasts 1-3 milliseconds

Question 10

How are VG K+ channels activated?

Answer 10

VG K+ channels activated by depolarisation, slower to open and close

Question 11

What blocks VG K+ channels?

Answer 11

tetraethylammonium (TEA)

Question 12

Describe characteristics of an AP

Answer 12

• RMP must reach threshold for AP to happen
• all or nothing
• AP is always same size
• AP is regenerative

Question 13

What timeframe for absolute refractory period?

Answer 13

1-2ms

Question 14

What can/can’t happen during absolute refractory period? Why?

Answer 14

no AP possible because inactivation molecule blocking Na+ channel

Question 15

What can/can’t happen during relative refractory period? Why?

Answer 15

AP possible, but needs more stimuli because RMP further from threshold

Question 16

Why does AP only travel forwards?

Answer 16

Na+ channels behind depolarisation are inactivated, refractory period

Question 17

What increases speed of conduction? (2)

Answer 17

in unmyelinated axons → diameter
- larger pipe = faster flow because less internal resistance

myelination (insulation)
- much bigger impact
- reduced current leak/ion loss

Question 18

How does myelination increase speed of AP conduction?

Answer 18

High density Na+ channels at nodes give large Na+ ion influxes, only small, local currents in between → reduced current leak