Question

In medullated nerve fibre, nodes of Ranvier represent areas ______.
A. Swellings on axon
B. Myelin sheath touches axon
C. Swelling on cyton
D. Depressions found on cyton

Answer 1

A nerve fibre can be defined as an axon of a neuron covered with one or two sheaths. Nerve fibres help in the conduction of nerve impulses. On the basis of the number of sheaths, these are of two types-medullated or myelinated nerve fibres and non-medullated nerve fibres.

Complete answer:

Medullated or myelinated nerve fibres are found in the white matter of the brain and spinal cord, and also in the cranial and spinal nerves.
This type of nerve fibre is formed of a central axial cylinder which is covered by two sheaths. This cylinder is known as neuraxis.
Medullary or myelin sheath is inner, thicker sheath that is formed of shining white fatty substance called myelin. The medullary sheath forms a continuous layer in the brain and spinal cord and is discontinuous at regular intervals in the peripheral nervous system. The points of the absence of medullary sheath are called nodes of Ranvier that have only sodium ion channels. The part of the medullated nerve fibre present between the two adjacent nodes is called an internode.
This medullary sheath helps in forming an insulating coat and prevents the loss of energy during the conduction of nerve impulses.
Non-medullated nerve fibres are found in the grey matter of the brain and spinal cord. The autonomic and somatic nervous system is formed by the non-medullated nerve fibres. These fibres lack nodes of Ranvier.

So, the correct answer is B) Myelin sheath touches axon.

Note: Schwann or sheath cells are the tubular cells placed end to end. Each Schwann cell forms the medullary sheath of an internode around the neuraxis of PNS. The medullary sheath formed by these consist of alternating concentric layers of lipid and protein materials.