Action Potentials 2 - Myelin

Question 1

Which axons tend to be myelinated?

Answer 1

• Large diameter axons (e.g. motorneurones) - myelinated

- Small diameter axons (e.g. C-fibres - sensory neurones) - unmyelinated.

Question 2

What is myelin and what is it produced by?

Answer 2

• Compose of lipids and proteins.
• Formed by glial cells (specialised supporting cells):
  
  • Schwann cells - myelinated axons in peripheral nerves
  • Oligodendrocytes - myelinated axons in CNS
• Glial cells wrap layer upon layer of their own PM in a tight spiral around the axon.

Question 3

When does myelination occur?

Answer 3

• Begins early in 3rd trimester (although little myelin exists in brain at time of birth).
• Occurs quickly during infancy and continues through adolescence.

Question 4

What are Nodes of Ranvier?

Answer 4

• Regions with no myelin.

- Contain a high density (~10,000/node) of Na+ channels.

Question 5

Why does myelination increase conduction velocity? What is this process called?

Answer 5

Saltatory conduction:
- large increase in membrane resistance
- large decrease in membrane capacitance
This increases length constant - slight decrease in time constant.

Question 6

What is conduction velocity proportional to in myelinated and unmyelinated nerve fibres?

Answer 6

• Myelinated nerve fibres (0.2-1.5 um): velocity proportional to diameter.
• Unmyelinated nerve fibres (1-10 um): velocity proportional to square root of diameter.

Question 7

Give examples of disease states affecting conduction of the AP in the CNS?

Answer 7

• Multiple Sclerosis: myelin damage in all CNS nerves

- Devic’s disease: myelin damage in optic and spinal cord nerves

Question 8

Give examples of disease states affecting conduction of the AP in the PNS?

Answer 8

• Charcot-Marie-Tooth disease

Question 9

What is thought to be the cause of pathological demyelination?

Answer 9

Inflammation from overproduction of cytokines (via upregulation of TNF or interferon).

Question 10

Why does demyelination affect AP generation?

Answer 10

In regions of demyelination:

• density of action current is reduced because of resistive and capacitive shunting
• failure to reach threshold
• no AP.