Question

Which of the following statements is correct regarding a myelinated nerve fibre?

(i) It is always associated with an axon.

(ii) It allows rapid conduction of nerve impulses.

(iii) It allows slow conduction of nerve impulses.

(iv) It has nodes of Ranvier.

A. Only (i) and (ii)
B. Only (i), (ii), and (iii)
C. Only (i), (ii), and (iv)
D. Only (i), (iii), and (iv)

Answer 1

Hint:
Myelinated retinal nerve fibre layers (MRNF) are retinal nerve fibres anterior to the lamina cribrosa that, unlike normal retinal nerve fibres, contain a myelin sheath. Clinically, they appear to be Gray-white well-demarcated spots with frayed borders on the anterior surface of the neurosensory retina.

Complete answer:
The myelinated nerve fibres are encircled with Schwann cells, which form a myelin sheath around the axon. The gaps between two adjacent myelin sheaths are known as nodes of Ranvier. This offers quicker transmission of electrical impulses through the axon. Myelin sheet acts to speed up impulses and serves as an insulator, which does not allow the current to flow through the segments that lie between the nodes of Ranvier. The nodes of Ranvier, being exposed to the ECF every 1 mm contain huge amounts of voltage-gated sodium channels. This enhances sodium’s ability to travel along the axon to re-create action potential. Because only the nodes of Ranvier are excited during conduction in myelinated fibres, sodium flux into the nerve is significantly less than in unmyelinated fibres, in which the whole membrane is involved. The conduction of impulses is too slow in unmyelinated fibres. Unmyelinated nerve fibres are the nerve fibres that do not include myelin sheath insulating the nerve axons.

Option ‘C’ is correct

Note:
Myelinated nerve fibre layer (mNFL) is a virtuous clinical entity that results from an embryologic developmental anomaly through which the focal regions of the retinal nerve fibre layer fail to lose their myelin sheath. Clinically, mNFL appears as separate white patches on the internal retinal surface.