Page 93 - Headlines Histology2024_Neat

Types of peripheral nerves

Myelinated fiber

• Concentric layers of the Schwann cell plasma membrane forming myelin
sheath wrap the myelinated fibers.

• The process of myelination begins with the invagination of a single nerve
fiber into a Schwann cell.

• The Schwann cell plasma membrane fuses along the opining of the groove.
The site of fusion of the Schwann cell membrane is called the mesaxon.

• The mesaxon begins to rotate around the axon thus enveloping the axon in a
spiral layer of Schwann cell cytoplasm.

• As myelination continues, the cytoplasm is extruded and the inner layers of
plasma membrane fuse with each other so that several lipoprotein layers
that together form the myelin sheath surround the axon.

• Each Schwann cell covering only a segment of the axon called the
internode. Between the Schwann cells there are short intervals at which the
axon is not covered by a myelin sheath. This region is called node of
Ranvier.

• Residues of the cytoplasm of Schwann cells that are not completely
extruded during the process of myelination from what is called Schmidt-
lantermann clefts.

• In CNS, myelin sheaths are formed by oligodendroglia where the processes
of single cells make internodes for about 50 axons.

Functions of myelin sheath:
• Myelin sheath acts as an electric insulator.
• It prevents the nerve impulse from being transported along the entire
axon.
• 3. The impulse travels by jumping from one node to another. This mode of
conduction is called saltatory conduction that increases the conduction
velocity of axons.

93