PNS Last updated: September 5, 2017 Blood vessels Three levels of connective tissue (within which fibers and vessels lie) Epineurium external epineurium tough, surrounds periphery of nerve. – internal epineurium loose, occupies space between fascicles. – represents up to 50% of cross-sectional area of nerve trunk. thicker where nerve crosses joint. well-developed runs within epineurium. vascular plexus surrounds each fascicle. Perineurium – thin, dense, multilayered connective tissue. blood-nerve barrier: tight basement membranes within perineurium protect endoneurial space. tensile strength of perineurium maintains intrafascicular pressures. traverse perineurium obliquely to enter endoneurial space. vascular structures Endoneurium surrounds individual myelinated nerve fiber or group of unmyelinated – nerve fibers. delicate collagenous matrix with fibroblasts, mast cells, and capillary network Nerve fibers all axons are bundled together into fascicles: fascicles are located within internal epineurium. bounded by perineurium. fascicles are often grouped together into grouped fascicles. can be easily divided along internal epineurial planes. major peripheral nerves contain many grouped fascicles. there is constant redistribution of fascicular organization along peripheral plexuses allow for interconnections). nerve (interfascicular fascicles are more numerous and smaller where nerve crosses joint. smaller fascicles and more internal epineurium between them allows for increased protection of nerve fibers. up to 500 Schwann cells may myelinate single axon. peripheral nerves contain both and fibers (in average 4:1) traveling myelinated unmyelinated within each fascicle. Myelination process: Unmyelinated fibers: - multiple transport mechanisms: Anterograde transport: fast slow speed is high but frequent prolonged stops (average speed is slow) – all cellular and are produced in cell body; cell body may be at proteins neurotransmitters significant distance from terminal axon. (removes from distal axon back to cell body). Retrograde fast transport breakdown products Bibliography for ch. “PNS” → follow this
PNS Last updated: September 5, 2017 Blood vessels Three levels of connective tissue (within which fibers and vessels lie) Epineurium external epineurium – tough, surrounds periphery of nerve. internal epineurium – loose, occupies space between fascicles. represents up to 50% of cross-sectional area of nerve trunk. thicker where nerve crosses joint. well-developed vascular plexus runs within epineurium. Perineurium – surrounds each fascicle. thin, dense, multilayered connective tissue. blood-nerve barrier: tight basement membranes within perineurium protect endoneurial space. tensile strength of perineurium maintains intrafascicular pressures. vascular structures traverse perineurium obliquely to enter endoneurial space. Endoneurium – surrounds individual myelinated nerve fiber or group of unmyelinated nerve fibers. delicate collagenous matrix with fibroblasts, mast cells, and capillary network Nerve fibers all axons are bundled together into fascicles: fascicles are located within internal epineurium. bounded by perineurium. fascicles are often grouped together into grouped fascicles. can be easily divided along internal epineurial planes. major peripheral nerves contain many grouped fascicles. there is constant redistribution of fascicular organization along peripheral nerve (interfascicular plexuses allow for interconnections). fascicles are more numerous and smaller where nerve crosses joint. smaller fascicles and more internal epineurium between them allows for increased protection of nerve fibers. up to 500 Schwann cells may myelinate single axon. peripheral nerves contain both myelinated and unmyelinated fibers (in average 4:1) traveling within each fascicle. Myelination process: Unmyelinated fibers: - multiple transport mechanisms: Anterograde transport: fast slow – speed is high but frequent prolonged stops (average speed is slow) all cellular proteins and neurotransmitters are produced in cell body; cell body may be at significant distance from terminal axon. Retrograde fast transport (removes breakdown products from distal axon back to cell body). Bibliography for ch. “PNS” → follow this
A15.%20PNS%20anatomy,%20histology,%20physiology.docx