Myelin

2022-02-04 links: reference:

Myelin #

• Ranvier channels are rich in Na+ channels, and this is where the action potentials become regenerated. This type of conduction is known as saltatory conduction.

• Myelin in the CNS is created by Oligodendrocytes; in the PNS: Schwann Cells.

• ~40% water. The remaining dry mass is 60-75% lipid (Mostly galactoCerebroside, then sphingomyelin and 25% Cholesterol.) And 15-25% protein (many specified types).

  • Cholesterol is not imported into the brain from the circulation. R

• There are 3 classes of nerve fiber:

  • A: 1-20μm; 5-120m/s (or more?)
  • B: 1-3μm; 3-15 m/s.
  • C: <2μm; 0.6-2 m/s. (Unmyelinated).
    • For an axon without myelin, the speed of impulse conduction is proportional to 1/2 the diameter. The sheath can be made up of up to 300 cocentric layers! Each sheath is ~0.2 - 1 mm in length.

• It’s at the nodes of Ranvier ($l$ = ~1-2μm) where current is (re)generated, which have a very high density (relative to what? None?) of voltage-gated Na+ Ion Channel. Myelin is not excitable and has no channels.