Name: Chemical Coordination in Animals
Uploaded: 2019-07-19
Duration: 5 min 54 s
Description: There are different categories of hormones that regulate and stimulate different categories of functions in your body. Explore hormones and their types in yo...

Question

If myelinated neurones conduct impulses so fast, why do mammals have unmyelinated neurones? What is the likely function of giant axons in earthworms?

Accepted Answer

The speed of the electric impulse transmission across a neurone depends on two factors:

The neurone is myelinated or unmyelinated

Diameter of the neurone

The mammalian myelinated neurones conduct the electrical impulses much faster up to 100m/s compared to unmyelinated neurones which may conduct up to a slower rate of 0.5 m/s. This happens because of the presence of myelin sheaths that act as an insulator and do not allow impulses to pass across them. In this case, impulses are jumping from one node of Ranvier to the next through the saltatory mode of conduction and thereby a faster rate of transmission can be seen. Myelinated neurones are not found in all organisms. Mammals acquire it over time during evolution, so they also have unmyelinated neurones.

The conduction of neurones also increases with the diameter, that is, thick neurones conduct faster than thin. It is because of the increased surface area that increases the diffusion rate of the ions across the membrane and thereby transmission of the impulse. Earthworms have few giant unmyelinated axons only with thick diameters running across their body. It facilitates faster conduction of the impulses.

Mary Jones, Richard Fosbery, Dennis Taylor and, Jennifer Gregory Solutions for Chapter: Control and Coordination, Exercise 6: Question

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