Question

Which of the following has non – myelinated nerve fiber?
A. Optic nerves
B. Cranial nerves
C. Spinal nerves
D. Autonomic nerves

Answer 1

Hint: The neurons conduct nerve impulses throughout the nervous system. The neuroglia (‘glia’ is a Greek word which means glue) provides a structural, supporting, and metabolic role for the neurons. Neuroglia protects and nourishes the neurons.
They also maintain the surrounding interstitial fluid. It is due to this property that neuroglia is called the glue of the nervous system.

Complete answer: The nervous system is made of two types of cells.
These are Neurons – nerve cells and Neuroglia – glial cells.
Myelin can be produced by two types of neuroglia – Oligodendrocytes and Schwann cells (Neurolemmocytes).
1. Oligodendrocytes produce myelin in the neurons of the central nervous system and the
2. Schwann cells produce myelin in the neurons of the peripheral nervous system.
3. Myelin is composed of lipids and proteins.
4. It is a whitish substance, made of many concentric layers of the plasma membrane to collectively make the myelin sheath around the nerve axons.

Functions of myelin sheath include:
1. Separating the axons from the surrounding extracellular components.
2. Providing insulation to the axon.
3. Increasing the velocity of conduction of nerve impulse by Saltatory conduction of the nerve impulses.
4. Decreasing the metabolic energy used in the process of conduction of nerve impulse as the energy required to conduct the nerve impulse is less in myelinated fibres as compared to the unmyelinated fibres.

Saltatory conduction – Saltatory (origin – Latin, meaning – to hop or to leap) conduction is the propagation of nerve impulse(action potentials) along myelinated axons.
This conduction occurs from one node of Ranvier to another node, thereby increasing the speed of conduction of impulse.
Autonomic nerves are non – myelinated so that the conduction of nerve impulses is smooth.

Additional information: The diameter of nerve fibres also varies.
The myelinated nerve fibres have a diameter of and the non – myelinated nerve fibres have a diameter of less than in the central nervous system and less than in the peripheral nervous system.

So the correct answer is option D.

Note: Though non - myelination reduces the speed of conduction, the smooth conduction of impulse is an important requirement in the autonomic nerves.