Myelination

Question 1

Basics for intro

Answer 1

• various types of glia cells
• astrocytes play important role at synapse
• microglia clear debris
• Schwann cells and oligodendrocytes most relevant for current topic: form myelin sheath, a layer around the axon

Question 2

Glial cells form insulating sheaths

Answer 2

• oligodendrocytes and Schwann cells create an insulating material around axon –> rapid conduction of signals
• produce thin sheath that wraps around multiple times
• oligodendrocytes: CNS *one cell sheaths up to 30 axons
• Schwann cells: PNS *each myelin segment is a new cell

–> axons are not continuously wrapped in myelin –> facilitates propagation of APs

Question 3

What influences the thickness of myelin sheath?

Answer 3

• number of layers is proportionate to the diameter of the axon
• the larger the axon, the larger the sheath
• very thin axons are not myelinated

–> conduct AP much slower because of small diameter and lack of myelin

–> C fibers do not need to be fast e.g. temperature change, chronic aches

Question 4

Nodes of Ranvier

Answer 4

= unmyelinated gaps where axon is exposed to cellular space

• increases conduction speed
• signals jump from one node to the next
• nodes have low threshold, easily excitable
• 50x higher density of Na+ than myelinated regions

Question 5

Clinical significance of demyelination (can it be repaired?)

Answer 5

• hallmark of many neurodegenerative autoimmune diseases e.g. MS (CNS), Guillan-Barre (PNS)
• myelin plays important role in normal motor function, sensory function, and cognition –> loss has serious consequences
• immune system thought to contribute to demyelination e.g. by inflammation leading to overproduction of cytokines

Symptoms

• depends on affected region but e.g.
  blurred vision, weakness of limbs, cognitive disruption e.g. memory loss, speech impairment, coordination impairment

Can myelin be repaired?

• research still ongoing but implanting oligodendrocyte precursors has been successful in animal models