Lecture 9: Conduction of AP along axons

Question 1

All-or-none nature of APs

Answer 1

AP close to the cell body is the same shape and magnitude as an AP at the axon terminal

Question 2

Dendrites decay where as axons are…

Answer 2

conducted without decrement, and thus travel long distances due to the presence of Na+ VG channels along its structure

Question 3

What happens if Na+ VG channels are removed from the axon?

Answer 3

The AP decays over a very short distance

Question 4

What happens if we put some Na+ VG channels back within the section they were removed?

Answer 4

Nothing: the electrotonic current decays below the threshold potential and therefore cannot propagate the new channel to open to create new APs
-the AP is NOT conducted

Question 5

BUT if the new Na+ channels are close enough…

Answer 5

that the AP is still above threshold potential it will cause the new channels to open and generate new APs

• therefore the AP is conducted further down the axon
• therefore we do not need continuous chain of Na+ VG channels, but just enough that the action potential remains about Threshold pot to activate the following channels

Question 6

Axon conduction of AP is due to 2 things:

Answer 6

1. Electrotonic current spreading and decaying along the axon
2. Generation of new, identical AP, which generates NEW electrotonic currents

Question 7

Why does the AP at one channel not activate channels both ways and create APs in both directions?

Answer 7

previous channel is inactivated, inact. gate is closed due to being in refractory period, and is unable to generate an AP

Question 8

AP propagation and conduction summary

Answer 8

• local change in ionic conductance (open/closing of ion channels; stim with electrodes) generates an AP
• AP generates an electrotonic current
• Electrotonic current spreads quickly & as it spreads away from site of change:
  - it decays over short distance
  - it changes Vm, which also decays
• if change in Vm >/= TP, channels open
• opening generates identical AP further away, generating a new electrotonic current, etc.

Question 9

Do AP travel?

Answer 9

No, they are self-propagating, as one AP generation creates and electrotonic current, generating a new AP, making a new current, etc. etc.

Question 10

Why is AP unidirectional

Answer 10

Immediately after depolarization/start of repolarization the inactivation gate closes, starting the absolute refractory period in which new AP cannot be generated

Question 11

AP summary

Answer 11

• all-or-none: occurs or doesn’t, identical without degradation
• self-propagating: an AP triggers the next AP in adjacent areas
• Cycle: VGNa+ channels open, Na+ entry depolarizes membrane, electrotonic current spreads, depolarization/current triggers new AP
• refractory period prevents “reverse” conduction

Question 12

Is there variation in AP conduction /velocity/?

Answer 12

Yes: between both species and neuron types