Action potential propagation

Question 1

Why do neurons need action potentials?

Answer 1

• because electrical signals decay over distance
• with AP generation, the electrical signal gets replenished all along the axon, ensuring that the signal can be transmitted a long distance

Question 2

What is the “safety factor”?

Answer 2

• there are 5-10 times the minimum Na+ channels necessary
• axons branch, so the safety factor insures that action potentials will spread down both branches of the axon
• sodium channel inactivation - allows absolute refractory period to end sooner (less time until enough inactivation gates have reopened)

Question 3

How does refractoriness prevent action potentials from reversing direction?

Answer 3

the axon segment more proximal to the soma than the active locus was, a moment ago, an active locus. So now it is in refractory period and the axoplasmic current cannot make it spike

Question 4

What does conduction velocity depend on?

Answer 4

• the time it takes for an active locus to depolarize the membrane ahead of it
• the length of the axon membrane (length constant proportional to [(resistance of cell membrane)/(resistance of axoplasm against current flow)] As the axon becomes fatter, the axoplasm gets better at conducting current (increasing the length over which signals can propagate). Increasing the membrane resistance (e.g. insulating myelin) also increases the distance over which signals propagate

Question 5

How does myelin sheathing support AP conduction?

Answer 5

• acts as an electrical insulator
• because it increases the resistance capacity of the membrane between nodes, little current is lost between them and the next node depolarizes to threshold very quickly (active locus) - salutatory conduction

Question 6

What determines how far electrical signals can propagate along the axon without being regenerated?

Answer 6

The length constants (usually 1mm for most neurons, so several mm blocked axon is enough to stop us from feeling pain at the dentist)

Question 7

Why do we have small neurons if they’re slower?

Answer 7

more information can be obtained by sacrificing several msec in conduction time

Question 8

Why are larger neurons easier to excite?

Answer 8

when the axon diameter increases, the membrane resistance decreases (more surface area), so more current enters the axon when you apply the voltage