<P> Each nerve is covered externally by a dense sheath of connective tissue, the epineurium . Underlying this is a layer of flat cells, the perineurium, which forms a complete sleeve around a bundle of axons . Perineurial septae extend into the nerve and subdivide it into several bundles of fibres . Surrounding each such fibre is the endoneurium . This forms an unbroken tube from the surface of the spinal cord to the level where the axon synapses with its muscle fibres, or ends in sensory receptors . The endoneurium consists of an inner sleeve of material called the glycocalyx and an outer, delicate, meshwork of collagen fibres . Nerves are bundled along with blood vessels, since the neurons of a nerve have fairly high energy requirements . </P> <P> Within the endoneurium, the individual nerve fibres are surrounded by a low - protein liquid called endoneurial fluid . This acts in a similar way to the cerebrospinal fluid in the central nervous system and constitutes a blood - nerve barrier similar to the blood - brain barrier . Molecules are thereby prevented from crossing the blood into the endoneurial fluid . During the development of nerve edema from nerve irritation or (injury), the amount of endoneurial fluid may increase at the site of irritation . This increase in fluid can be visualized using magnetic resonance neurography, and thus MR neurography can identify nerve irritation and / or injury . </P> <P> Herophilos 335--280 BCE, described the optic nerve and the oculomotor nerve for sight and eye movement . Analysis of the nerves in the cranium allowed him to differentiate between blood vessels and nerves i.e. Ancient Greek: νεῦρον (neûron), "string (plant fiber), nerve". </P> <P> Nerves are categorized into three groups based on the direction that signals are conducted: </P>

Based on the location of sciatic nerve what is afferent nerve