Factors that affect the speed of conduction 5.3.7

Question 1

What three factors determine the speed of conduction?

Answer 1

• Myelination
• Axon diameter
• Temperature

Question 2

Why is speed of conduction low in unmyelinated neurones?

Answer 2

Because depolarisation must occur along the whole membrane of the axon

Question 3

What is the myelin sheath formed by?

Answer 3

Schwann cells

Question 4

Where can depolarisation not occur in the neurone?

Answer 4

In areas surrounded by myelin sheath, as depolarisation cannot occur due to the myelin sheath stopping the diffusion of sodium and potassium ions

Question 5

Where can action potentials only occur (myelinated)?

Answer 5

Nodes of Ranvier (uninsulated areas)

Question 6

What is saltatory conduction?

Answer 6

Where action potentials ‘jump’ from one node to the next (local circuits of current) in the presence of myelin

Question 7

What is the impact of saltatory conduction?

Answer 7

Allows the impulse to travel much faster (up to 50 times faster) than in an unmyelinated axon of the same diameter

Question 8

Describe the node at refractory period

Answer 8

• Membrane becoming repolarised
• Na channel proteins closed
• K channel proteins open

Question 9

Describe the node at action potential

Answer 9

• Membrane fully depolarised (+40mv)
• All Na channel proteins open
• K channel proteins closed

Question 10

Describe the node becoming depolarised

Answer 10

• Membrane potential moving towards threshold level
• Na channels starting to open but many still closed
• K channel proteins closed

Question 11

Describe the node at resting potential

Answer 11

• Membrane potential around -70mv
• Na channel proteins closed
• K channel proteins closed

Question 12

What does a thicker axon result in?

Answer 12

Faster speed of conduction compared to those with thinner axons

Question 13

Why is there a faster speed of conduction in thicker axons? (2 reasons)

Answer 13

• Thicker axons have an axon membrane with a greater surface area over which the diffusion of ions can occur - increasing the rate of diffusion of sodium and potassium ions through protein channels
• Thicker axons also possess more cytoplasm which reduces their electrical resistance so that an action potential can push into the next section faster

Question 14

What effect does temperature have on speed of conduction?

Answer 14

Colder conditions can slow down the conduction of nerve impulses - as there is less kinetic energy for the facilitated diffusion of potassium and sodium ions during an action potential