Myelinated Neuron Diagram

Myelin is a thick sheath of lipoprotein that insulates the nerve fibres of the myelinated neurons. It is not a continuous sheath and is absent at regular intervals. The region where the myelin sheath is absent is termed the node of Ranvier. Also, the segment of nerve fibre between the two nodes is termed the internode.

This myelin sheath is primarily responsible for the white colour of the nerve fibres.

Diagram of Myelinated Neuron

Neurons

A neuron is made up of three regions, namely the nerve cell body (soma), axon and dendrite. The axon and dendrite are the filamentous structures of the neuron. The former is a long process and the latter is a short process. The axon carries impulses from the soma to the axon terminal, which further passes the impulse to another neuron.

Classification of Nerve Fibres

The long process of a neuron is called the axon or nerve fibre. The nerve fibres can be classified as follows:

1. Depending on structure — myelinated and non-myelinated nerve fibres
2. Depending on function — motor and sensory nerve fibres
3. Depending on origin — spinal and cranial nerve fibres
4. Depending on distribution — somatic and autonomic nerve fibres

What is a Myelin Sheath?

Depending on the presence and absence of myelin sheath, the nerve fibres can be classified as myelinated and unmyelinated neurons.

Myelin sheath is a lipoprotein sheath made of concentric layers of proteins and alternating lipids. The lipid constituents include lecithin, cholesterol and cerebroside. The formation of this myelin sheath around the axon of a neuron is termed as ‘myelinogenesis’.

Myelinogenesis

This process starts during the fourth month of pregnancy in the peripheral nerves. It is completed only two years after birth.

This lipoprotein sheath is formed by Schwann cells in the neurilemma region. Neurilemma is nothing but the cytoplasmic region that surrounds the Schwann cells. Before the process of myelinogenesis, the Schwann cells of the neurilemma are found in close proximity with the axolemma. Also, this is the case in most unmyelinated neurons.

Later, the Schwann cells wrap and rotate in concentric layers. These layers fuse to form the myelin sheath. Still, the outermost neurilemma remains in the Schwann cell. The nucleus of these Schwann cells remains in between neurilemma and myelin sheath.

Likewise, in the central nervous system, the oligodendrocytes (neuroglial cells) create the myelin sheath. This is because neurilemma is absent in the central nervous system.

Functions of Myelin Sheath

• Myelin sheath is responsible for faster conduction of impulses through the nerve fibres. The impulses in the myelinated neurons jump from node to node in a transmission process called saltatory conduction. Thus the myelination of axons increases the speed of action potential.
• The sheath also provides a high insulating capacity. Because of this nature, myelin sheath controls the nerve impulse within a single nerve fibre and thus stops the stimulation of neighbouring nerve fibres. It is vital for normal sensory and motor functions.

Thus any damage to the myelin sheath in the nerve fibre is often associated with functional insufficiency. In non-myelinated neurons, neurilemma replaces the myelin sheath.

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