Transport of CO2 (Carbon Dioxide) in the Blood
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Unmyelinated and Myelinated Axons
- Larger diameter axons tend to be
myelinated (covered by a myelin sheath), which allows them to conduct action potentials at greater velocities than smaller diameter,
unmyelinated axons.
- Myelin sheaths are formed by
Schwann cells in the peripheral nervous system and
oligodendrocytes in the central nervous system.
- The sheaths consist of a series of myelinated segments, each extending about 1 millimeter along the outer surface of the axon.
- The segments are separated by small gaps (about 1 micrometer) called
nodes of Ranvier (enlarged for this illustration).
- Schwann cells and oligodendrocytes form
coats of myelin by wrapping the axon with concentric layers of their cell membranes, which are mostly lipid.
Learn more about the anatomy of the nervous system with this interactive quiz and labeling exercises guide.
- The layer of insulation created by the myelin coat inhibits current loss from the axon by blocking the movement of ions through the axon membrane.
- Ion channels in myelinated neurons are therefore mostly
concentrated in the nodes of Ranvier.
- In comparison, ion channels in unmyelinated neurons occur along the the
entire length of the axon.