Q.What is the difference between a myelinated and an unmyelinated nerve fibre? Name the cells that form the myelin sheath in the peripheral and central nervous systems respectively, and explain the significance of the nodes of Ranvier.

A neuron is built from a cell body (cyton), dendrites that receive signals, and an axon (often myelinated) that carries signals away to synaptic terminals — described here in place of a drawn, labelled figure.

(a) Structure of a neuron:

Since this format cannot render an actual drawing, the structure is described part by part, matching what a correctly labelled diagram would show:

• Cyton (cell body/soma): the main body of the neuron, containing the nucleus and cytoplasm. The cytoplasm contains characteristic granular structures called Nissl granules (rough endoplasmic reticulum with ribosomes), which are the site of protein synthesis.

• Dendrites: short, thin, branched processes that arise from the cyton. They receive nerve impulses from other neurons (or receptor cells) and conduct these impulses toward the cell body.

• Axon: a single, typically much longer process arising from the cyton (at a region called the axon hillock), which conducts the nerve impulse away from the cell body toward another neuron, muscle or gland.

  • Many axons are covered by a fatty, insulating myelin sheath, formed by wrapping Schwann cells (in the peripheral nervous system) around the axon.
  • The myelin sheath is interrupted at regular intervals by small gaps called Nodes of Ranvier, where the axon membrane is exposed. These nodes allow the nerve impulse to 'jump' from node to node (saltatory conduction), greatly increasing the speed of conduction compared to an unmyelinated axon. …