Peripheral Nerve Basics

• Consist of processes of cell bodies found in the DRG, anterior horn, and autonomic ganglia • Organized by several distinct connective tissue layers – the epineurium, perineurium, and endoneurium • Vascular supply provided by the vasa nervorum

Peripheral Nerve Basics

• Neuronal processes bound into fascicles by perineurium • Fascicles bound into nerves by epineurium • Endoneurium is a division of the perineurium which form thin layers of connective tissue surrounding neuronal fibers in a fascicle

Sural nerve in cross-section Epineurium

• Loose areolar tissue with sparse, longitudinally-oriented collagen fibers • Some elastic fibers where epineurium abuts perineurium • Able to accommodate a significant amount of nerve stretching and movement • Increases in thickness where nerves cross joints • Constitutes an increasing proportion of nerves as they increase in size • Epineurial fat helps cushion nerves from compressive injury • Decreased epineurial fat found in patients with diabetes Perineurium

• Cellular component composed of laminated fibroblasts of up to 15 layers in thickness which are bounded by a basal lamina • Semi-permeable: inner lamellae have tight junctions, providing a barrier to intercellular transport of macromolecules – Tight junctions can be loosened with topical anaesthetics and with osmotic change

Perineurium

• Exhibits a slightly positive internal pressure – Fascicular contents herniate upon perineurial injury • Under tension longitudinally – Nerve segment shortens upon transection – may complicate surgical repair as nerve can be stretched only approximately 10% before being inhibited by collagen

Endoneurium

• Intrafascicular connective tissue consisting of a collagenous matrix in the interstitial space • Develops into partitions of dense connective tissue between diverging fascicles and eventually becomes perineurium when the fascicles separate • Collagen fibers are longitudinally-oriented and run along nerve fibers and capillaries