Axon cross-section. Transmission electron micrograph (TEM) of a cross-section through a nerve process called an axon, with various structures labelled. Axons are extensions of nerve cells that join them to other nerve cells. The cross-section here is circular, showing the myelin sheath (dark) that insulates the axon. The labels indicate an internal and external mesaxon, as well as a Schwann cell. The mesaxons are membrane junctions between the Schwann cells and the myelin sheath. The Schwann cells are part of the support structure for the nervous system. Here, they produce the axon's myelin sheath.
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