Nervus ischiadicus histology (100X)

The outermost layer around the entire nerve is a layer of connective tissue called the epineurium. Inside the nerve, the nerve fibers are often grouped into distinct fascicles that are separated by eachother by the epineurium. Peripheral nerves contain a lot of connective tissue that protects and supports the slender nerve fibers.

Around the fascicles themselves, there is a layer of connective tissue called the perineurium, which is an important characteristic of nerve bundles. The perineurium acts as a diffusion barrier to maintain a stable environment that is optimal for axonal conduction. The perineurium can be damaged causing symptoms in the extremety distal to the damage itself.