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How do individual neurons relate to the nerves that can be seen grossly? The structures that are visible in the body are not axons from single neurons, but rather groups of axons from many neurons bundled together. Individual axons, called nerve fibers, are grouped into fascicles, which are wrapped in perineurium. Fascicles are then bundled into groups and wrapped in epineurium. It is actually a bundle of fascicles surrounded by epineurium that is observed grossly and called a nerve. Realizing that what we call nerves are actually collections of many nerve fibers makes it easier to understand several very important characteristics of the nervous system. Firstly, it is easier to understand how some nerves can be much larger than other nerves. For example, the sciatic nerve, a very large nerve found in the posterior hip, has many more nerve fibers and many more fascicles than an intercostal nerve, which is much smaller. Secondly, it is easier to understand how a nerve can be a "mixed" nerve, carrying motor information to the periphery and sensory information to the spinal cord. Mixed nerves have both motor and sensory nerve fibers bundled in fascicles, allowing one nerve to have information traveling in both directions. It may be helpful to think of a nerve as a telephone cord with an outer coating around a collection of wires. Within the outside coating, some wires carry information to the earpiece and some wires carry information from the mouthpiece.
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