Astrocytes - Location, Structure, and Function
Schwann Cells and Unmyelinated Axons
- In the peripheral nervous system, Schwann cells also surround and give support to small-diameter axons (C fibers; 2 um or less).
- A series of Schwann cells covers the length of each axon. Abutting Schwann cells are tightly joined and nodes of Ranvier do not form.
- Each Schwann cell typically contains several axons (up to 20), which are often brought into the cell by invaginations of the plasma membrane.
- The internalized axons are not myelinated by the Schwann cell.
- Instead, they are held in longitudinal channels created by the invaginated plasma membranes.
- Indentations of the Schwann cell plasma membrane called mesaxons connect the channels to the cell surface.
- Narrow openings between the paired mesaxon membranes allow extracellular fluids and ions to freely move in and out of the channels and circulate around the embedded axons.
- Thus, even though small axons reside inside Schwann cells, they are still fully exposed to the extracellular environment.
- As a result, action potentials are generated along the entire length of small axons and saltatory (jumping) conduction doesn’t occur.
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