Myelination Flashcards
What is the range for the speed of action potentials?
1 to 100m/s.
What is the speed of action potentials increased by?
A larger size (giant axons), increased body temperature due to faster diffusion and fatty insulation due to myelin sheaths.
What does a fast speed of action potential allow?
Faster responses of various things to the environment.
What is myelin?
A specialised membrane sheath surrounding most vertebrate axons.
What is the purpose of myelin?
It insulates axons from each other (electrical wiring) and speeds the conduction of nervous impulse - saltatory conduction between nodes.
What are the nodes of ranvier?
Points where there are gaps in the myelin where there is a movement of ions in and out of the axon.
What do nodes of ranvier allow?
Allows the action potential to jump between these points.
What cells myelinate the peripheral and central nervous system?
Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system.
How do Schwann cells and oligodendrocytes differ?
Schwann cells only insulate a single area of each axon, whereas oligodendrocytes can myelinate multiple axons- they send out branches to myelinate multiple areas.
What is the myelin sheath composed of?
Many loops of a glial process - they have many processes which produce a myelin sheath on different axons.
Where are Schwann cells derived from?
The neural crest - there is a small contribution from ventral neural tubes.
Where are oligodendrocytes formed?
The proliferating zone of the neural tube.
What are Schwann cells under the control of?
Peripheral axons - they migrate and differentiate under the control of these peripheral axons.
How does the neural tube form?
The neural plate forms a neural groove which then turns into a neural fold. This is then cleaved to form the neural tube.
What is the oligdendrocyte precursor cell?
It is a bipotential cell that arises after birth - the O2A cell.
What can O2A cells differentiate into?
Type-2 astrocytes and oligodendrocytes.
What does BMP4 cause O2A cells to differentiate into?
Type-2 astrocytes.
Where do oligodendrocytes arise from?
Region/stem cells of the spinal cord that previously gave rise to somatic motor neurons.
In the forebrain, where do oligodendrocytes arise from?
Ventral regions similar to the spinal cord.
What is a key factor in the migration of oligodendrocyte precursors?
Sonic hedgehog signalling.
What is the difference between oligodendrocyte precusors and mature oligodendrocytes, in terms of migration?
Oligodendrocyte precursors are migratory, whereas mature oligodendrocytes are not.
How do oligodendrocyte precursors know where to travel?
They follow radial glia outwards from the ventricular zone and follow developing axon pathways- dorso-ventral and longitudinal.
What do oligodendrocyte progenitors require for migration?
Cell adhesion molecules (integrins, PSA-NCAM).
What happens to excess oligodendrocytes that are produced?
They die by apoptosis.
How are progenitors and newly-differentiated oligodendrocytes competing against each other?
They are competing for limiting amounts of mitogens and survival factors - some don’t get enough and will die.
What involvement do neuregulin growth factors have with oligodendrocytes?
They bind receptors (ErbB) on undifferentiated neural crest cells that contact axons. This sets off a Ras-Raf MAPK signalling pathway.
What stimulates O2A proliferation?
Electrical activity - possibly by increasing production of mitogens by astrocytes.
What controls the negative regulation of oligodendrocyte differentiation?
Notch signalling.
What can cause thicker myelin?
Signals from neurons enhance myelin gene transcription - thicker axons get thicker myelin.
Where do neural crest-derived progenitors migrate?
Over the surface of the neural tube and then through the anterior somites.
