Myelin and Neurilemma Sheaths

 Describe in brief the  myelin sheath and neurilemma?

Myelin Sheath

• It is a laminated sheath made up of lipoproteins.
• Myelin sheath is formed by consecutive wrappings of Schwann cell membrane around the axon of a neuron.
• Its outer surface is encased in neurilemmal sheath.
• It is interrupted at the nodes of Ranvier.
•  In case of peripheral nerves the myelin sheath is formed by the Schwann cells and in the central nervous system the sheath is derived from oligodendrocytes. Its functions are:
• Function:
  • Protects and insulates the nerve fibres.
  • It also increases the rate of conduction of nerve inpulse  (saltatory conduction).

 Neurilemma sheath

• The  outer cell membrane along with the cytoplasm and the nuclei of the Schwann cells which lie outside the myelin sheath are collectively called the neurilemma.
• The neurilemma is only present in the peripheral nervous system. It is present in both myelinated and non-myelinated fibers.
• It is absent in the central nervous system due to the lack of Schwann cells. 
• The neurilemma is important for the regeneration process of nerves.  The lack of neurilemma  may account  for the inability of the neurons to  regenerate in the central nervous system.

All  myelinated and non-myelinated  nerve fibres in PNS are surrounded by the neurilemmal sheath but only myelinated fiberes are surrounded by  myelin sheath. Axons less than 1µm in diameter are unmyelinated.

Describe the Myelinated and Unmyelinated Nerve Fibers.

• Axons or nerve fibers  may be myelinated or unmyelinated.
• In central nervous system the myelin is formed by oligodendrocytes and in peripheral nervous sytem by Schwann cells.
• The spaces between adjacent Schwann cells, where the axon is exposed are known as nodes of Ranvier.
• In myelinated nerves the speed of nerve impulse transmission is faster as the propagation of nerve impulse occurs from one node of Ranvier to the next node (Saltatory Conduction) which increases the conduction velocity. therefore speed of nerve impulse conduction is more in myelinated than in unmyelinated  nerve fibers.
• All somatic nerve fibers >1µm in diameter and all preganglionic autonomic nerve fibers are myelinated nerve fibers.
• All somatic  nerve fibers < 1µm in  diameter and all postganglionic autonomic nerve fibers are Unmyelinated.

When does myelination begins?

Myelination begins as early as fourth month of fetal life.

Name the parts of an axon where myelin sheath is absent.

 The following sites are devoid of myelin sheath:

• Nodes Of Ranvier (Gaps of exposed axon between the adjacent Schwann  cells).
• Proximal part of the axon, close to the cell body.
• Near the termination of axon before it divides.

Can a non-myelinated nerve regenerate?

 A non-myelinated nerve also possesses neurilemmal sheath which is essential for regenearation, therefore it can regenerate.