Question

Node of Ranvier occurs where
A.Nerve is covered with myelin sheath
B.Neurolemma is discontinuous
C.Neurilemma and myelin sheath are discontinuous
D.Myelin sheath is discontinuous

Answer 1

Hint: A nerve fibre can be best termed as an axon of a neuron covered with one or two sheaths. Nerve fibres perform function in conduction of nerve impulses. On the condition 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 known to occur in the white matter of the brain and spinal cord, and also in the cranial and spinal nerves. This type of nerve fibre is made up of a central axial cylinder which is covered by two sheaths. This cylinder is termed as neuraxis. Medullary or myelin sheath is present inner, thicker sheath that is made up of shining white fatty substance called myelin. The medullary sheath makes a continuous layer in the brain and spinal cord and is discontinuous at regular intervals in the peripheral nervous system. The point at the absence of medullary sheath are called nodes of Ranvier.

Now let us find solution from the options given:
Nerve is covered with myelin sheath: when nerve is covered with myelin sheath, they form the myelinated neurons. When at all the levels myelin sheath is present there is no space left uncovered so node of Ranvier is not present.
Neurolemma is discontinuous: neurolemma is defined as the cytoplasmic sheath that is composed of Schwann cells. This is present along the myelin sheath but the node of Ranvier does not have any role with Schwann cells.
Neurilemma and myelin sheath are discontinuous: to have a node of Ranvier myelin sheath should be absent but it is not necessary that neurilemma can be discontinuous.
Myelin sheath is discontinuous: when myelin sheath is discontinuous it leads to formation of the Node of Ranvier. The nodes of Ranvier function as a conducting pathway for the nerve impulse.

Hence, the correct answer is option (D)

Note: The myelin sheath was found and named by a German Pathological Anatomist Rudolf Virchow in 1854.
The node of Ranvier was named and found by a French pathologist and anatomist Louis Antoine Ranvier.