Topic 3 - Neuronal conduction and myelination

Question 1

What 2 factors determine the speed of an action potential?

Answer 1

1. Space constant (length constant)

2. Time constant

Question 2

What is the space constant, the equation and what affects the space constant?

Answer 2

It is how far the current spreads passively along the axon.

Space constant = square root (of rm/ri)

The size of the axon as wider axons have a larger space constant. Internal resistance = depends on area and membrane resistance depends on axon circumference.

Question 3

What is Rm membrane resistance, Ri internal resistance and membrane capacitance ?

Answer 3

Rm = membrane resistance = how un-leaky - current spreads further if the membrane is less leaky - bigger the SA the smaller the membrane resistance.

Ri = internal resistance = current spreads further if there is little resistance to it moving down the axon – larger cross sectional area = smaller internal resistance

membrane capacitance = how stretchy the hose wall is.

Question 4

What is the time constant and what is the equation?

Answer 4

How long it takes for the membrane to “charge up”.

T = RmCm

Question 5

What is the impact of myelin on membrane resistance and membrane capacitance?

Answer 5

Increases membrane resistance:

• longer space constant
• current spreads further down axons

Decreases membrane capacitance (as myelin moves 2 capacitator plates further apart):
- counteracts the effect of increased membrane resistance on the time constant

Question 6

If there is a loss of myelin, signals will not travel properly and can leads to two diseases. What are they?

Answer 6

MS & GBS

1. ) Multiple sclerosis:
   - autoimmune disorder - immune system attacks myelin
2. ) Guillian-Barre syndrome:
   - autoimmune disorder - affects PNS myelin
   – patients usually recover because PNS myelin can regenerate (unlike CNS myelin)