Types of nerve fibers and their sheaths
Nerve fibers are made up of axons and glial cells. Their structure is different in central and peripheral nervous system. However, in both systems they can be either myelinated or unmyelinated. The Myelin Sheath is made up of lipoproteins, in which galactocerebroside predominates. More strongly myelinated fibers conduct impulses in a saltatory manner (from Latin salto, are - to jump) between the individual notches of Ranvier on the axon. This makes the impulse transmission faster than in unmyelinated or weakly myelinated fibers.
Nerve fibers in the central nervous system
In the CNS, nerve fibers are organized into tracts. They consist mainly of myelinated fibers. Myelin sheaths form oligodendrocytes, and one sheath can cover several axons. In addition to these, there are numerous unmyelinated junctions in the CNS that run freely between neurons.
Nerve fibers in the peripheral nervous system
In the PNS, nerve fibers are organized into nerves. They are surrounded by Schwann cells. The myelin sheath is formed by the insertion of the axon into the fold of the Schwann cell. The internal membranes fuse to form a mesaxon. By its rotation around the axon, it forms a multi-layered membrane. Its layers then merge. Remains of Schwann cell cytoplasm remain inside the sheath - Schmidt-Lantermann fissures. The myelin sheath of one axon consists of one Schwann cell. Between the individual Schwann cells are Ranvier's notches. Unmyelinated fibers' are sheathed by a Schwann cell that does not form myelin. Multiple axons may be involved in a Schwann cell. Schwann cells are tightly packed together so they do not form Ranvier's incisions.
Classification of nerve fibers according to conduction velocity
Conduction velocity is approximately directly proportional to axon thickness. We classify nerve fibers in the PNS into three types according to the speed of conduction. This classification also includes the peripheral branches of the projections of pseudounipolar neurons.
- A-fibers are myelinated: (radius, conduction velocity)
- Aα-fibers, Ia, Ib (12–20 μm, 70–120 ms-1) ensure proprioception and somatic momentum
- Aβ-fibers, II (5–12 μm, 30–70 ms-1) conduct touch and pressure information
- Aγ-fibers (3–6 μm, 15–30 ms-1) ensure the momentum of intrafusal muscle fibers
- Aδ-fibers, III (2–5 μm, 12–30 ms-1) convey messages about pain and temperature
- B-fibers (3 μm, 3–15 ms-1) are also myelinated and represent preganglionic autonomic fibers.
- IV C-fibers are unmyelinated and are both postganglionic sympathetic fibers (0.3–1.3 μm, 0.3–1.3 ms-1) and also are involved in the conduction of pain information (0.4–1.2 μm, 0.5–2 ms-1).
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References
- no. . Základy histologie. 7. edition. Jinočany : H & H, 1997. a LANGE medical book; pp. 502. ISBN 80-85787-37-7.
