Excitable cells 2: action potential propagation

Question 1

What effect does (the factors determining) the length constant have on AP propagation? Time constant?

Answer 1

• Factors that increase the length constant of the axon (low Ra, high Rm) enhance AP conduction velocity by increasing longitudinal flow of current
• Factors that shorten time constant of axon (low Cm, low Rm) enhance rate of depolarisation (speed of AP upstroke)
  • > increases electrical gradients between active and resting zones, increases current spreading depolarisation along axon, and speeds up AP propagation

Question 2

What is local circuit theory? Evidence?

Answer 2

Model for neuronal conduction
- depolarisation of a small region of a neuronal plasma membrane produces transmembrane currents in the neighbouring regions, tending to depolarise them

Evidence:
1) Small voltage transients recorded even when AP is blocked by cooling section of a nerve (Hodgkin, 1937)

2) Changing axial and extracellular resistance alters conduction velocity
3) Since Ra decreases with increasing axon diameter – conduction velocity is greater in large than small diameter axons

Question 3

Why does myleination increase rate of AP propagation and conduction velocity? Mention time constant, Rm, Cm

Answer 3

Increases axon diameter -> conduction velocity greater in large diameter axons (increases membrane resistance & decreases capacitance)

Depolarisation at nodes of ranvier, current flow only occurs at nodes -> AP ‘skipping’

Decreases time constant at nodes -> decreased Rm, Cm