nervous system 2

Nerve Tissue & the Nervous system:

Myelinated and Unmyelinated Axons:
? In the PNS, all axons are enveloped by Schwann cells which provide both structural and metabolic support.
? Many axons with small diameter invaginate into one Schwann cell longitudinally and are simply surrounded by the cytoplasm of Schwann cells. They are called unmyelinated nerve fibres.
? Other axons, especially the ones with larger diameter, invaginate into the Schwann cell and are wrapped by concentric layers of the Schwann cell plasma membrane forming myelin sheath. These axons are called myelinated nerve fibres (Fig.1a ).
? There are gaps (areas of axon not covered by myelin) along the length of myelin sheath at regular intervals called nodes of Ranvier.
? In large myelinated axons, the nerve impulse jumps from node to node resulting in faster conduction (salutatory conduction).
? The segment of myelin between two nodes of Ranvier is called internode.
? The myelin of one internode is formed by a single Schwann cell.
? The myelin sheath shows cone-shaped clefts called Schmidt-Lantermann clefts. They are areas of remnants of cytoplasm of Schwann cells present within the myelin sheath.
? In the PNS, the myelin sheath of an individual axon is provided by many Schwann cells lying along the length of the axon.