Nerve (cross section) (English)

A nerve is formed by a bundle of axons (or nerve fibers) outside the CNS. These nerve fibers can be myelinated (surrounded by a myelin sheath) or nonmyelinated. Nerves are also composed of connective tissue coverings, as well as nourishing blood vessels (vasa nervorum). The three layers of connective tissue are the epineurium, surrounding the outer surface of a nerve, the perineurium, surrounding each nerve fascicle (which is a group of axons), and the endoneurium, which surrounds each individual axon. Spinal nerves arise from the spinal cord as anterior and posterior roots whereas cranial nerves arise from the brain.