Nerve
A nerve is an organ of the PNS, conveying electrical information into and out of the CNS. It is basically an organized bundle of axons, also known as nerve fibers. An axon is the only part of a neuron that can generate and transmit nerve impulses, also known as action potentials. Axons are usually covered by a protective sheath called the myelin sheath.
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Endoneurium: The innermost layer of connective tissue in a nerve. It is a thin layer of loose connective tissue that encloses each individual axon, including myelinated axons.
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Perineurium: A sleeve of connective tissue that surrounds a fascicle. A fascicle is a single bundle of axons contained within a nerve. Nerves typically contain several fascicles. Adipose tissue and blood vessels are found outside the perineurium, in between neighboring fascicles.
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Epineurium: A strong, collagen-rich sleeve of connective tissue that forms the outer boundary of the nerve. It contains the fascicles, adipose tissue, and blood vessels.
Figure 1: General structure of a nerve
Nerves are categorized based on their function. While their base structure is the same, the information can flow in different directions, giving rise to three types of nerves: motor, sensory and mixed.