Question

Node of Ranvier is a place where
A. Medulla sheath is discontinuous
B. Medullary sheath and neurilemma is discontinuous
C. Axolemma is absent
D. Axolemma is discontinuous

Answer 1

The gap between two adjacent myelin sheaths are called as nodes of Ranvier. The myelin allows the electric impulse to move down the axon.They are found in neurons of the body mainly on the axon and helps in the propagation of the action potential to travel rapidly along the fibres.

Complete answer: Let us first discover the structure of neurons.
A neuron is a microscopic structure composed of three major parts , i.e dendrites, axon and the cell body. The cell body contains cytoplasm with typical cell organelles and certain granular bodies called Nissl's granules. The fibres transmit impulse towards the body. The axon is a long distal fibre, the distal end which is branched. The axon transmits nerve impulses away from the cell body to a synapse or to a neuro- muscular junction. Each branch terminates as a bulb-like structure which is called a synaptic knob. These nobes consist of synaptic vesicles that contain chemicals called neurotransmitters.
Nodes of Ranvier are also called myelin sheath gaps which occur along the myelinated axon where the axolemma is exposed. They are usually uninsulated and highly enriched in ion channels and participate in the exchange of ions required to generate action potential. This conduction is also termed as saltatory conduction as the impulse "jumps" from one axon to the other. Thus, results in faster conduction of the action potential.
Axolemma is the cell membrane of the axon. It is responsible for maintaining the membrane potential of the axon and it contains the ion channels through which ions flow.
Neurilemma is the cytoplasmic sheath of Schwan cells that are present over the myelinated axon in a continuous manner. It is also present along the nodes of Ranvier.
Myelin sheath and neurilemma are discontinuous but it is not necessarily that neurilemma will be discontinuous.

Inferring from above information we can conclude that option A, Myelin sheath is discontinuous is the correct option.

Note: These interruptions on the axon in the myelin covering were first discovered by French histologist and pathologist Louis-Antoine Ranvier in 1878. He described nodes as constrictions. They are approximately 1 um wide and expose the neuron membrane with the external environment. The gaps in between are rich in sodium and chloride ions that are required to form an action potential.