A high speed of conduction along axons is very important for the survival of organisms, particularly when quicker response times allow escape from a predator

Question

Many factors affect speed of conduction, including temperature, myelination, and the diameter of the axon. Discuss how myelination and diameter affect the cable properties of a neuron and affect conduction speeds.

What will be an ideal response?

Answer 1

Answer: To understand how myelination and axon diameter affect conduction speeds, it is important to understand the basics of how current flows along an axon. Positively charged particles enter the cell via channels during an action potential. They then flow through the intracellular fluid up and down the axon. Outside the axon, positive charges are also flowing, but in the opposite direction. To complete the circuit, it is necessary for some charges to cross the membrane, flowing out of the cell. To achieve the greatest speed of conduction, it is important to have low resistance to flow inside and outside the cell and high resistance across the membrane. This means most of the charge will flow down the axon, rather than out of it. Additionally, we have to think about capacitance. When charges flow across a membrane, they first build up on opposite sides of the membrane, then are able to flow across. If it takes a long time to store up the charge, it will take a longer time for the current to continue flowing down the axon. A large diameter axon will provide a very low internal resistance to flow. A myelinated axon will have a very high resistance to flow across the membrane, and also a very small capacitance. This allows for very high speeds of conduction.