Myelination

Question 1

What drives myelination in PNS / CNS?

Answer 1

Schwann cells / oligodendrocytes

Question 2

Schwann cells - myelination

Answer 2

Can myelinate only one segment of a single axon

Question 3

Oligodendrocytes - myelination

Answer 3

Can form myelin sheath around several axons

Question 4

What does myelin sheath consist of?

Answer 4

Spiral of concentric layers of a glial cell process containing no cytoplasm and purely lipid membrane (fat)

Thick, fatty myelin sheath

Question 5

Function of myelin sheath?

Answer 5

Increases speed at which action potential is conducted along axon

Insulation

Question 6

What are periodic gaps in myelin sheath called?

Answer 6

Nodes of Ranvier

Question 7

What are Nodes of Ranvier?

Answer 7

Where segments of myelin produced by neighbouring Schwann/oligodendrocytes meet

Question 8

What is function of Nodes of Ranvier?

Answer 8

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)

Question 9

What do non-myelinated nerves have?

Answer 9

Supporting Schwann cell

A single Schwann cell supports several axons

Question 10

For non-myelinated nerves, what is axon embedded in?

Answer 10

Channel called the mesaxon - where the Schwann cell is right next to the axon

Question 11

What is a demyelinating disease?

Answer 11

Condition that results in damage to myelin sheath

Question 12

What are consequences of demyelination?

Answer 12

1. Nerve impulses slow/stop - neurological problems

2. Deficiency in sensation/movement/cognition (functions specific to nerves involved)

Question 13

What is extensive myelin loss usually followed by?

Answer 13

Axonal degeneration and often cell body degeneration

Question 14

How are demyelinating diseases classified?

Answer 14

By basis of cause

Question 15

What is primary demyelinating disease called?

Answer 15

Demyelinating leukodystrophic (dysmelinating) disease

Question 16

What occurs during demyelinating leukodystrophic diseases? Cause?

Answer 16

Myelin is abnormal and degenerates

Caused by genetics

Question 17

What is secondary demyelinating disease called?

Answer 17

Demyelinating myelinoclastic disease

Question 18

What are demyelinating myelinoclastic diseases caused by?

Answer 18

Healthy myelin destroyed by a toxic (alcohol), infectious agents, chemical or autoimmune substances

Question 19

What is MS?

Answer 19

Most common demyelinating disease of CNS

Autoimmune in nature

Question 20

What occurs during MS?

Answer 20

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)

Question 21

Role of T cells in MS?

Answer 21

1. BBB disruption - allows T cell entry to brain
2. T cells recognise myelin as foreign - attack it
3. T cells release cytokines - promote degradation of myelin and BBB - B cells and macrophages enter brain

Question 22

Role of B cells and macrophages in MS?

Answer 22

B cells enter brain and make antibodies to myelin which target it for further degradation

Macrophages degrade myelin

Question 23

Result of MS?

Answer 23

Physical, mental, psychiatric problems

Question 24

What is a salutatory conduction?

Answer 24

Action potential jumps along axon (due to Nodes of Ranvier)