Myelinated Neuron Diagram

Myelin sheath is a layer of lipoprotein which is found on the outer surface of the nerve fibres or axons of myelinated neurons. It generally wraps the axon process of neurons diametrically. 

• The myelin sheath is discontinuous, interrupted by the presence of Nodes.
• The process by which the synthesis and deposition of the myelin sheath occurs in our nerve cells is known as Myelinogenesis.
• The myelin sheath is thick and provides insulation and protection to the neurons from the surrounding substances.
• Nervous system of the human body contains about 14 billion nerve cells or neurons.

Key Terms: Neurons, Nerve, Fibres, Organs, Myelinated, Medullated, Nodes, Schwann, Myelin Sheath, Axon, Dendron, Impulses, 

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What is a Neuron?

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The structural and functional unit of the nervous system is called neuron. 

• The neuron is made up of cell body or perikaryon or cyton and processes which are mainly made up of axon and dendron. 
• The neurons play a main role in the transportation of nerve impulses. 
• Sensory neurons transfer nerve impulses from the receptor to the central nervous system 
• The motor neuron carries response from the central nervous system to the effector organs.

Read More: Nervous System Diseases

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What are Nerve Fibres?

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The long process of the neuron is called the nerve fibre. In the grey matter of the nervous system the nerve fibre or avon is naked or uncovered. According to the structure, nerve fibres are of two types. 

1. Myelinated or Medullated nerve fibre
2. Non-myelinated or Non-medullated nerve fibre

Nerve Fibre

Myelinated or Medullated Nerve fibre 

Almost all nerve fibre of the central nervous system of mammals are medullated nerve fibre. This type of nerve fibre generally constitutes with three types of materials: 

(i) Axis cylinder: Axis cylinder originates from the cell body of nerve cells, which is a direct continuation of nerve cell protoplasm. 

• The thin covering of an axon is called an axolemma. 
• Neoplasm of axon is known as axoplasm. 
• It contains mitochondria and neurofibrils of the cell body. 
• Outside the axolemma there is a covering of a protein lipid complex that surrounds the axon, called myelin sheath.

(ii) Myelin sheath: In the white matter the axon is covered by a membrane made of protein and lipid material. This is called myelin medullary sheath.

(iii) Neurilemma: In the peripheral nervous system, the nerve fibre receives a second covering over and around the myelin sheath. This is called neurilemma. 

Functions of neurilemma-

• It protects the nerve fibre.
• It supplies nutrition to nerve fibre.
• It helps in the regeneration of injured nerves.

Non-myelinated or Non-medullated nerve fibre

Non-myelinated or non-medullated nеrvе fibrеs arе a type of nerve fibre prеsеnt in thе nеrvous systеm. Thеy arе also known as unmyelinated nеrvе fibrеs.

• Non-myelinated nеrvе fibrеs lack thе protеctivе covеring known as thе myеlin sheath that myelinated nеrvе fibrеs havе.
• Thеy arе instеad surroundеd by Schwann cеlls that do not crеatе a continuous covеring. 

1. Nodes of Ranvier : In the peripheral nerve, the myelin sheath is broken up at regular distances of 1 mm

• Neurilemma comes in contact with the axon through these broken parts. These parts are called the Nodes of Ranvier. 
• The Nodes of Ranvier help in the saltatory conduction of nerve impulse.

2. Schwann nucleus and Schwann cell: In the peripheral nerve, there are oval shaped nuclei beneath the neurilemma. These are called Schwann nuclei.The portion of the nerve fibre between the two adjacent Nodes of Ranvier with a Schwann nucleus is called a Schwann Cell.

Read More: Sense organs

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Classification of Nerve Fibres

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The nerve fibres can be classified as follows:

1. On the basis of structure — myelinated and unmyelinated nerve fibres
2. On the basis of function — motor and sensory nerve fibres
3. On the basis of origin — spinal and cranial nerve fibres
4. On the basis of distribution — somatic and autonomic nerve fibres

Read More: Neural Control and Coordination Important Question

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What is Myelin Sheath?

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In the white matter the axon is covered by a membrane made of protein and lipid material This is called myelin or medullary sheath. In the central nervous system, it is formed from oligodendroglia and in the peripheral nervous system, it is formed from Schwann cells.

Functions

(a) It insulates nerve fibre so that nerve impulse cannot spread to adjacent Tissues. 

(b) It supplies nutrition to nerve fibre.

(c) It protects axoplasm.

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Myelinogenesis

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Myelinogenesis is a physiological process of formation of myelin sheaths and its subsequent deposition on the axon body of neural cells (neurons). 

• All the nerve fibres of the peripheral nervous system are myelinated. 
• In the central nervous system, Myelin sheaths are synthesised by oligodendrocytes, when the immature oligodendrocytes extend their processes to ensheath nearby axons.
• In the peripheral nervous system, the myelin sheaths are synthesised by the Schwann Cells.
• Myelinogenesis usually begins in the pre-natal life and continues throughout postnatal development. Myelinogenesis occurs throughout life to support memory and learning. 

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Things to Remember

• A neuron is the structural and functional unit of the Nervous system. 
• A nerve fibre is the axon process of a neuron. Several nerve fibres together form the nervous bundle.
• The myelin sheath is a lipoprotein substance encompassing the axon of a neuron diametrically. 
• The myelin sheath is interrupted by the presence of Nodes of Ranvier.
• The process of synthesis of the myelin sheath is known as Myelinogenesis. 
• In the central nervous system, the myelin sheath is formed from oligodendrocytes while in the peripheral nervous system it is formed from Schwann cells.