University:
Stanford University
Course:
MED 101 | Human Anatomy
Academic year:

2023
Views:

105

Pages:

1
Author:

larbi43100

‭ tructure and Formation of Myelinated Nerve‬ S ‭Fibers in the PNS‬ ‭ In these invaginations, the axolemma is in contact with the plasma membrane of the Schwann‬ ✪ ‭cell.Around the cells there is a basal layer = condensation of the connective tissue which passes in‬ ‭abridge above the saxons.‬ ‭ There may be 1 axon or several per invagination depending on the size of the axon.‬ ✪ ‭There‬‭are‬‭other‬‭invaginations‬‭containing‬‭collagen‬‭fibers‬‭which‬‭serve‬‭as‬‭structure,‬‭as‬‭supports‬‭for‬‭the‬ ‭Schwann cells, thus consolidating this fragile edifice of juxtaposition. longitudinal of cells.‬ ‭ A Schwann cell can accommodate up to 30 axes.‬ ✪ ‭These‬ ‭unmyelinated‬ ‭fibers‬ ‭with‬ ‭Schwann‬ ‭sheath‬ ‭do‬ ‭not‬ ‭contain,‬ ‭unlike‬ ‭others,‬ ‭constrictions‬ ‭and‬ ‭they have a small diameter of 0.5 to 3 μm.Alongside these fibers there are myelinated fibers.‬ -‭ Myelinated fibers with Schwann sheath.‬ ‭●‬ ‭One Schwann cell for one axon.‬ ‭●‬ ‭These are the fibers of the peripheral nerves, therefore the mixed sensory-motor nerves.‬ ‭●‬ ‭These fibers have a caliber > 3 μm.‬ I‭ n MO:‬ ‭●‬ ‭center:the axon which is the axis of the fiber.‬ ‭●‬ ‭Around the axon it is the myelin sheath, therefore translucent refractive structure which begins‬ ‭after the cone of emergence of the axon, continues throughout the axon and ends just before the‬ ‭terminal branches (terminal tree of the axon) which will form presynaptic elements.‬ ‭●‬ ‭There are constrictions therefore areas of periodic narrowing where there is interruption of the‬ ‭myelin sheath:These are the nodes of Ranvier. Depending on the fibers, they are spaced at‬ v‭ ariable intervals from 0.2 to 3mm.‬ ‭ ‬ ‭between2 constrictions it is a cylindrical segment except at the ends (just before the‬ ● ‭node of Ranvier) where there is a small dilation = bulb.‬ ‭●‬ ‭in the myelin sheath we can see Schmidt-Lanterman incisures which are‬ ‭lighter areas.‬ ‭●‬ ‭The structure at the periphery is the flattened cytoplasm of the Schwann cell except at the level‬ ‭of the nucleus where there is no more cytoplasm. There is one cell of S per segment.‬ ‭In ME:‬ ‭-‬‭This myelin sheath/Schwann sheath assembly belongs to the same structure.‬ ‭-‬‭The axon is limited by the axolemma within the vesicles containing the NT.‬ ‭-‬‭The myelin sheath is a laminated structure, lamellar in appearance with a‬ ‭alternation of dark protein bands/light lipid bands. These alternation correspondent to the‬ ‭winding of the S cell around the axon so the lipid bands are the membrane, the proteins are the‬ ‭cytoplasm so we have these tight juxtapositions of turns of turns which come from the‬ ‭progressive winding of the cytoplasm of the cell of S. The last turn of the turn, the most external,‬ ‭is the Schwann sheath which is not independent of the myelin sheath, it is a continuum.‬ ‭-‬‭At a certain level there is a sort of detachment of the sheets which makes the incisures of‬ ‭Schmidt-Lanterman therefore a little greater interposition of cytoplasm in certain places‬ ‭-‬‭between the turns of turns which correspond to the membranes.‬