Lecture 7: Myelin Flashcards
Why does white matter in the brain appear white?
because it is composed of myelinated axons which are lipid rich
What are the myelinating cells of the peripheral nervous system?
Schwann cells
myelinate in a 1:1 ratio
What are the myelinating cells of the central nervous system?
oligodendrocytes
myelinate in a 50:1 ratio
What are the two different types of Schwann cell?
myelinating and non-myelinating
What is the role of non-myelinating Schwann cells?
bundle together unmyelinated axons
Schwann cells myelinate multiple axons within the central nervous system and all nerve fibres are myelinated by Schwann cells.
TRUE/FALSE
false
What is the oligodendroglial cell lineage?
oligodendrocyte precursor cells (OPC) -> immature oligodendrocyte -> myelinating oligodendrocyte
Is myelin single-layered or multi-layered?
myelin is a multi-layered, compacted membranous sheath
How is myelin organised?
into internodes
What are the regions of unmyelinated axon called? What are these regions characterised by?
nodes of Ranvier
here there is clustering of voltage gated sodium channels
there are very few voltage gated sodium channels on the myelinated part of the axon
What does myelination induce in the axon?
induces architectural rearrangementinduces architectural rearrangement
What are the three distinct molecular domains myelin divides the axon into?
the node of Ranvier, the paranode and the juxtaparanode
What are characteristics of the node of Ranvier?
short, unmyelinated segments of axon which contains clusters of voltage-gated sodium channels
What is the role of ankyrin proteins in the node of Ranvier?
anchors contents of the node to the underlying actin-spectrin cytoskeleton
What are characteristics of the paranodes?
flank the nodes and acts as a junction between noncompacted paranodal myelin loops and the underlying axolemma
What is the role of NF155-Caspr-Contactin complexes in the paranodes?
anchor paranodes to the underlying actin-spectrin cytoskeleton
What are characteristics of the juxtaparanodes?
between paranodes
contain voltage-gated potassium channels
the periaxonal space
What is found in the node of Ranvier, the paranode and the juxtaparanode respectively?
K+ channels, Caspr and βIV spectrin
What are different ways of increasing conduction speed?
myelination or increasing axon diameter
What is the advantage of having larger axons?
less resistance to current flow
What is the advantage of having myelinated axons?
pushes voltage gated sodium channels into clusters which results in the clustering of sodium ions as they are attracted to the exposed negatively charged membrane of the axon
this leads to increased capacitance at the node of Ranvier
What does increased capacitance of the nodes of Ranvier lead to?
very rapid activation of multiple voltage gated sodium channels
action potential jumps from node to node which results in very fast conduction
What is capacitance at the node of Ranvier and what is capacitance at the internode?
capacitance is higher at the node of Ranvier and lower at the internode
How does lower capacitance at the internode affect propagation of the action potential?
lower capacitance at the internode means that a lower change in ion concentration is required to propagate the action potential -> results in greater speed and metabolic efficiency
