In nerves, fascicles are covered by the Perineurium while the entire nerve is covered by the Epineurium.
The endoneurium consists of an inner sleeve of material called the glycocalyx and a mesh of collagen. Nerves are bundled along with blood vessels, which provide essential nutrients and energy to the enclosed, and metabolically demanding, neurons. Within the endoneurium, individual nerve fibers are surrounded by a liquid called the endoneuria fluid.
The endoneurium has properties analogous to the blood–brain barrier. It prevents certain molecules from crossing from the blood into the endoneuria fluid. In this respect, endoneuria fluid is similar to cerebrospinal fluid in the central nervous system. During nerve irritation or injury, the amount of endoneuria fluid may increase at the site of damage. This increase in fluid can be visualized using magnetic resonance neurography to diagnose nerve damage.
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