Arm like processes extend from the cell body of all neurons. Bundles of neurons processes are called tracts in the CNS and nerves in the PNS. Some examples of CNS tracts are the large pyramidal (corticospinal) tracts descending from the motor cortex to the medulla while continuing into the spinal cord.
Nerves in the PNS are typically divided into cranial and spinal nerves. There are twelve pairs of cranial nerves and thirty one pair of spinal nerves. Cranial nerves innervate parts of the head and connect directly to the brain (especially to the brainstem). They are typically assigned Roman numerals from 1 to 12, although cranial nerve zero is sometimes included. In addition, cranial nerves have descriptive names. Spinal nerves innervate much of the body, and connect through the spinal column to the spinal cord. They are given letter-number designations according to the vertebra through which they connect to the spinal column .
Each nerve consisting of many fascicles is covered externally by a dense sheath of connective tissue, the epineurium. Underlying this layer of flat cells, the perineurium, forms a complete sleeve around a bundle of axons called fascicles. Surrounding each axon is the endoneurium. The endoneurium consists of an inner sleeve of material called the glycocalyx and an outer delicate meshwork of collagen fibres. Within the endoneurium, the individual nerve axonsare surrounded by a protein liquid called endoneurial fluid. The endoneurium has properties analogous to the blood–brain barrier, in that it prevents certain molecules from crossing from the blood into the endoneurial fluid.