Myelination Flashcards
What drives myelination in PNS / CNS?
Schwann cells / oligodendrocytes
Schwann cells - myelination
Can myelinate only one segment of a single axon
Oligodendrocytes - myelination
Can form myelin sheath around several axons
What does myelin sheath consist of?
Spiral of concentric layers of a glial cell process containing no cytoplasm and purely lipid membrane (fat)
Thick, fatty myelin sheath
Function of myelin sheath?
Increases speed at which action potential is conducted along axon
Insulation
What are periodic gaps in myelin sheath called?
Nodes of Ranvier
What are Nodes of Ranvier?
Where segments of myelin produced by neighbouring Schwann/oligodendrocytes meet
What is function of Nodes of Ranvier?
Axon not insulated by myelin sheath at these points - ions can move freely in and out of axon
Results in action potential jumping along axon (salutatory conduction)
What do non-myelinated nerves have?
Supporting Schwann cell
A single Schwann cell supports several axons
For non-myelinated nerves, what is axon embedded in?
Channel called the mesaxon - where the Schwann cell is right next to the axon
What is a demyelinating disease?
Condition that results in damage to myelin sheath
What are consequences of demyelination?
- Nerve impulses slow/stop - neurological problems
2. Deficiency in sensation/movement/cognition (functions specific to nerves involved)
What is extensive myelin loss usually followed by?
Axonal degeneration and often cell body degeneration
How are demyelinating diseases classified?
By basis of cause
What is primary demyelinating disease called?
Demyelinating leukodystrophic (dysmelinating) disease
What occurs during demyelinating leukodystrophic diseases? Cause?
Myelin is abnormal and degenerates
Caused by genetics
What is secondary demyelinating disease called?
Demyelinating myelinoclastic disease
What are demyelinating myelinoclastic diseases caused by?
Healthy myelin destroyed by a toxic (alcohol), infectious agents, chemical or autoimmune substances
What is MS?
Most common demyelinating disease of CNS
Autoimmune in nature
What occurs during MS?
Environmental/genetic factors leads to loss of tolerance to self-proteins
Inflammation and injury to myelin sheath and nerve fibres cause multiple areas of scarring (lesions, plaques)
Role of T cells in MS?
- BBB disruption - allows T cell entry to brain
- T cells recognise myelin as foreign - attack it
- T cells release cytokines - promote degradation of myelin and BBB - B cells and macrophages enter brain
Role of B cells and macrophages in MS?
B cells enter brain and make antibodies to myelin which target it for further degradation
Macrophages degrade myelin
Result of MS?
Physical, mental, psychiatric problems
What is a salutatory conduction?
Action potential jumps along axon (due to Nodes of Ranvier)
