Myelin

Question 1

What is myelin?

Answer 1

Lipid rich sheath that wraps around axons, originating from oligodendrocytes in the CNS and schwann cells in the PNS.

Question 2

Where are myelinated neurons found?

Answer 2

Found in white matter tracts of the brain and spinal cord and peripheral nerves.

Question 3

How much of myelin is composed of protein?

Answer 3

Myelin is also composed of 30% protein which maintains myelin structure and the integrity of axon-myelin interactions.

Question 4

Is myelin exclusive to mammals?

Answer 4

Myelinis not exclusivetomammals as it is present invertebratesandinvertebrates and therefore must have evolvedindifferent speciesof the animal kingdom atdifferent time points.

Question 5

What is the general principle for myelinating an axon?

Answer 5

Oligodendrocytes send out multiple projections, known as myelin lamellae, that wrap around multiple axons.

Question 6

How are myelin lamellae formed?

Answer 6

Myelin lamellae are formed byfusionof the opposedinner leafletsof theplasma membraneof oligodendrocyte projections and have a trapezoidshape, therefore some sections of myelin closer to the cell body are thicker than others.

Question 7

What are paranodal loops?

Answer 7

Paranodal Loops are areas of non-compact myelin on the edge of the myelin lamellae. So they sit adjacent to each other instead of on top.

Question 8

What are dense lines?

Answer 8

apposition of inner faces of the same lamella plasma membrane.

Question 9

What are interperoid lines?

Answer 9

apposition of outer faces of plasma membrane of subsequent wraps.

Question 10

Where are Na+ channels found on myelinated neurons?

Answer 10

Found at nodes

Question 11

Where are K+ channels found on myelinated neurons?

Answer 11

Juxtaparanodes

Question 12

How does conduction occur at a non-myelinated neurone?

Answer 12

At thesite of injectionofcurrentthere is adepolarisation(more positive) in voltageof the membrane. Thiscurrent dissipates alongtheaxonand therefore thevoltage decreasesalongtheaxon.

Question 13

What is the length constant (λ)?

Answer 13

determineshowfarthe depolarisation spreads

Question 14

What is the input resistance (rinput)?

Answer 14

determineshow muchdepolarisation is initiallyproduced at the injection of current

Question 15

What does rinput and λ both depend on?

Answer 15

themembrane resistance (rm)and theaxoplasmic resistance (ri)
Asrmincreases,λandrinputincreases
Asriincreases,λdecreasesandrinputincrease

Question 16

What is membrane resistance (rm)?

Answer 16

• due to presence ofleak channelsin the lipid bilayer
  
  • More channels=smaller resistance

Question 17

What is axoplasmic resistance (ri)?

Answer 17

• due to thecontentof theaxoplasm
  
  • Moreelectrically non-conductivemolecules=larger resistance

Question 18

How do you increase conduction velocity in a non-myelinated neurone?

Answer 18

Increase axon diameter

Question 19

How does myelin increase conduction velocity?

Answer 19

Wrapping myelin around an axonpreventstheloss of current acrosstheaxonmembrane therefore forcing current flow longitudinally down the axon

Question 20

What are the advantages of myelination? (5)

Answer 20

• Increases speedofconductioni.e.increases speed ofresponsee.g.escapereflex
• Enablesgreaternervous systemcompactness
• Allows for saltatory conduction so depolarisation only occurs at nodes to increase conduction velocity.
• Increases ability ofprocessingof complex information
• Increasessynchronyof responsee.g.muscle contraction

Question 21

What is the g ratio? And calculation.

Answer 21

optimum numberof myelinwraps,
gratio = axon diameter (di) / axon and myelin diameter (do)
gratio~0.75for allCNSaxons