Dahlin L 1996)). Between the Schwann cells, non-myelinated nerve fibers are located in a large number. Myelinated and non-myelinated nerve fibers are organized in bundles, called fascicles, and surrounded by a mechanical strong membrane consisting of laminas of flattened cells, the perineurial membrane. The bundles are usually organized in groups, held together by a loose connective tissue called the epineurium. In between the nerve fibers and their basal membrane is located an intrafascicular connective tissue—the endoneurium. The amount of the connective tissue components may vary between various nerves and also between various levels along the same nerve. For example, nerves located superficially in the limb or parts of the peripheral nerve that cross a joint contain an increased quantity of connective tissue, possibly as a response to repeated loading (Sunderland 1978).
The normal propagation of impulses in the nerve fibers as well as the communication and nutritional transport system in the neuron—axonal transport—require a sufficient energy supply. The peripheral nerve therefore contains a well developed microvascular system with vascular plexa in all connective tissue layers of the nerve (Lundborg 1970, 1975). The vessels approach the nerve trunk segmentally and these vessels have a coiled appearance so that the vascular supply is not impaired during the normal gliding or excursion of the nerve trunk. When the vessels reach the nerve trunk they divide into branches running longitudinally in various layers of the epineurium and also form numerous collaterals to vessels in the perineurial sheath. When the vessels pass through the perineurium into the endoneurium, which primarily contains capillaries, they often go through the perineurium obliquely thereby constituting a possible ''valve mechanism" (Lundborg 1970, 1975).
The perineurial layer and the endoneurial vessels play an important role in protecting the nerve fibers in the fascicles. The endoneurial milieu is preserved by a blood-nerve barrier, and the tissue pressure in the fascicle—endoneurial fluid pressure—is slightly positive (Myers et al. 1978). This is obvious when there is injury to the perineurium; following a transaction a "mushrooming" effect is observed. There are no lymphatic vessels in the epineurial space, therefore problems occur when an edema is formed in the endoneurial space. Following such an edema the pressure in the fascicle may increase and rapidly interfere with the endoneurial microcirculation (Lundborg and Dahlin 1996). The epineurial vessels are more vulnerable than the endoneurial vessels to trauma and even to surgical handling of the nerve.
The neuron itself is, as mentioned above, a unique cell with the cell body and the extending process (axon). The length of the axon may be 10 to 15,000 times the diameter of cell body. Therefore, there is a need for an intraneuronal transport system—axonal transport—where essential products are produced and constantly transported from the nerve cell body down the axon (anterograde transport), and disposal materials and trophic factors are also transported in the opposite direction (retrograde transport) (Grafstein and Forman 1980). The axonal transport consists of various components where fast axonal transport (up to around 410 mm per day) involves various enzymes, transmitter substance vesicles and glycoproteins and the various slow components (up to 30 mm per day) involve mainly cytoskeletal elements such as subunits of microtubules and neurofilaments. It should be noted that axonal transport is energy depend and disturbances in axonal transport may be involved not only in the development of diabetic neuropathy but also in nerve compression injuries (Dahlin et al. 1986).