NERVE INJURIES

SCHWANN CELL n Surrounds individual myelinated axons (produces myelin), or surrounds several unmyelinated axons NODE OF RANVIER Gap at junction between Schwann cells n Allows rapid nerve conduction: Action potential depolarization jumps from one node of Ranvier to the next (3-150 m/sec versus 2-2.5 m/sec for unmyelinated axons) FASCICLES Grouped arrangement of axons with endoneurium surrounding each axon and perineurium surrounding each fascicle n Invested by internal (inner) epineurium n Logistically the smallest unit that can be treated surgically n Interconnect and branch proximally2 • Makes matching fascicles more difficult proximally, but fibers can be adjacent in a fascicular group PERINEURIUM Connective tissue encircling each fascicle n Selective permeability (tight junctions that create a blood-nerve barrier, similar to the blood- brain barrier) n Thin but dense; high tensile strength n Encircled by internal (inner) epineurium • Sutured in fascicular repair OUTER EPINEURIUM Outer sheath of peripheral nerve n Sutured in epineurial repair n Contains collagen and elastin fibers n Surrounded by loose areolar tissue (mesoneurium) MESONEURIUM Outer adventitial layer of nerve n Incorporated in epineurial repair n Critical for nerve excursion and gliding BLOOD SUPPLY n Arteriae nervosum (vaso nervosus) • Enter nerve segmentally • Divide into longitudinal superficial branches in epineurium • Help guide proper orientation of nerve ends during repair n Extrinsic vessels supply intrinsic longitudinal vessels in epineurium, which communicate with capillary plexus in perineurium n Capillary plexus • Enters the endoneurium obliquely • Becomes occluded if endoneurial pressure rises because of injury (crush, edema) MARTIN-GRUBER ANASTOMOSIS n Motor connections between median and ulnar nerves in forearm, or distally between anterior interosseous nerve and ulnar branches n Present in up to 17% of the population n May mask the actual site of nerve injury (e.g., a median nerve injury at elbow level with loss of all intrinsic muscle function, even though the ulnar nerve is intact) RICHE-CANNIEU ANASTOMOSIS n Motor connections between median and ulnar nerves in palm n Present in up to 70% of the population3 n Can mask injury pattern FROMENT-RAUBER ANASTOMOSIS n From radial nerve (either posterior interosseous nerve or superficial radial nerve) to ulnar motor branches innervating the first, second, or third dorsal interosseous muscles n Rare n Can mask injury pattern BERRETTINI CONNECTION n Sensory connections between the ulnar and median nerves in the palm n Usually travels deep to the superficial volar arch or immediately distal to the carpal tunnel n Present in approximately 80% of all cadaver specimens n Can produce confusing findings in physical examination for nerve injury WALLERIAN DEGENERATION n Nerve stump undergoes degeneration distal to axonotomy. n Cytoplasmic calcium increases. n Macrophages proliferate; myelin phagocytosis. n Schwann cells proliferate and myelin breaks down. n Schwann cells and macrophages replace neural tube and organize into bands of Bunger, providing a scaffold for regenerating axons.