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Neurons feature many long, slender projections termed axons, along which electrochemical nerve impulses are transmitted. In the central nervous system (CNS) bundles of these axons are called tracts, whereas in the peripheral nervous system (PNS) they are called nerves.
Each nerve 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 fibers. Within the endoneurium, the individual nerve axons are 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.
Axon length and diameter can vary greatly from between 1 m to 1 mm in length and 1
µm to 20
µm in diameter. The longest axons in the human body are those of the sciatic nerve, which run from the base of the spinal cord to the big toe of each foot. Axons in the central nervous system typically show complex trees with many branch points allowing for the simultaneous transmission of messages to a large number of target neurons.
Axons are described as either un-myelinated or myelinated. Myelin is a layer of a fatty insulating substance, which is formed by two types of glial cells: Schwann cells en-sheathing peripheral neurons and oligodendrocytes insulating those of the central nervous system. Myelination enables an especially rapid mode of electrical impulse propagation called saltatory conduction. De-myelination of axons causes the multitude of neurological symptoms found in the disease multiple sclerosis.
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.