Myelination

Question 1

Why do signals decay along the axon?

Importance of myelin

Answer 1

Membrane resistance 
-thicker membrane => less decay
Axial resistance
-larger axon diameter => less decay
Membrane capacitance 
-thicker membrane => smaller capacitance => less decay

Myelin decreases capacitance

• oligodendrocytes have many branches that wrap around the axons to form many layers
• 1 Schwann cell only myelinates 1 area of the axon

Increases speed of AP when space is limited
Fewer ion channels and pumps needed, making it more efficient at charge transfer

Question 2

Saltatory conduction

Answer 2

Cluster of VGNC/pumps at Nodes or Ranvier where AP is regenerated

Question 3

Myelinogenesis

Answer 3

Oligodendrocyte progenitors develop => oligodendrocytes
-develop when environment permits

Starts in 3rd trimester
Rapid increase from birth, corresponds with motor and cognitive skills
Continues throughout life

Dependent on neuronal activity

• via neuroglial signalling molecules
• can be directly or indirectly via astrocytes
• growth factors and NTs

Question 4

Myelin pathologies

• developmental
• myelin damage
• neurodegenerative/AI

Answer 4

Developmental - failure to myelinate/dysmyelination
Leukodystrophy
Cerebral Palsy

Myelin damage

• brain injury
• infection
• toxin
• ischemia/stroke

Neurodegenerative/AI

• MS
• GBS
• CPM

But demyelinated lesions increase with age

Question 5

Symptoms of myelin loss

Answer 5

Failure of conduction of axons
-often slower, can stop

Impaired vision, hearing
Tingling
Weakness
Cognitive disruption
Memory loss
Heat sensitivity
Loss of dexterity, coordination, balance
Difficulty controlling bowel movements,

Question 6

MS

Answer 6

1 in 1000 in the UK
More common in women than men in reproductive years but men often get it worse

Unknown cause

• 30% incidence explained genetically
• VitaminD/sun exposure before 15 is protective
• Infection (EBV)
• inflammation

Prognosis
-5-10 years less than unaffected

Question 7

Pathology

Answer 7

AI attacks myeline

Demyelinating episodes occur in flare (relapses) but areas often remyelinate

• can be complete but often results in thinner myelin
• after time, remyelination fails due to failure of differentiation of progenitors