Question

How does the diameter of an axon affect the speed of an impulse?

Answer 1

Axon or nerve fibre is a long, slender projection of a nerve cell that carries away nerve impulse from the cell body. The function of the axon is to transfer information to other neurons, glands and muscles.

Complete answer:
A nerve cell has a long threadlike part of a nerve cell along which conduction of impulse takes place from cell body to other cells. Most axons have their diameter usually microscopic like that of 1µm. The largest mammalian axons can have a diameter of upto $20\mu m$. The axon of a squid that is specialized to conduct signals at high speed or very rapidly has a diameter of axon of about 1mm that is almost the size of a small lead of pencil.

Larger conduction of impulse in axons with large diameter is seen that means they conduct impulse at a higher speed, they send signals faster. Now, let’s see how axon transfers the signals. The signals travel down the axon in the form of action potential when the axon depolarizes and repolarizes. There are sodium and potassium ions that help the impulse to travel down quickly and jump to the next cell after terminating at the synaptic contacts.
When the diameter of the axon is large the flow of ions face less resistance and they can easily flow at a faster rate. When the diameter of the axon is not so large the ions flow may face something that can bounce them back.
So larger the diameter of the axon faster is the speed of impulse conduction.

Note:
Axons may be myelinated and unmyelinated, those enclosed in a myelin sheath have a higher speed of impulse transmission. Some axons have so high speed of nerve impulse conduction that they conduct at a rate of 90 metres per second.