Which of the following is true concerning myelinated and unmyelinated axons?

[bobeanie95]

This question has been posted before but the answers have been inconclusive.

A. the amount of energy used by the Na/K ATPs ase is much less in the myelinated axons than in the unmyelinated axons
B. Myelinated axons can conduct many more action potentials per second that can unmyelinated axons
C. The size of the action potential depolarization is much greater in myelinated axons than in unmyelinated axons
D. Voltage gated potassium channels do not play a role in repolarization in unmyelinated axons

I thought the answer was B, but the correct answer is A. B is wrong according to the book because " the length of the refractory period (and hence the frequence of action potentials) is based on the characteristics of the voltage-gated sodium and potassium channels, which do not change.

However, later the book says that Schwann cells form myelin to increase the speed of conduction of APs along axon. If the speed of conduction is increase how does this not increase frequency of action potentials?

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[----x----]

You ask:

If the speed of conduction is increase how does this not increase frequency of action potentials?

Each individual action potential will propagate more quickly, but they are still limited in how many action potentials can be activated per second.

For example, let's say there's a gun that when shooting as fast as possible, shoots a projectile once every second, and launches it at 100 m/s. If you upgrade the gun so it shoots a projectile at 1000 m/s, it still is limited to shoot every second. It is just the projectile speed that increases, not the frequency of shots. The bullet will reach the target much quicker with the upgraded gun, but it still only shoots once every second.

Bringing it back to the axon, the myelin sheath is like that upgrade for the gun, it allows the action potential to travel more quickly. The refractory period remains constant (refractory period is the time when additional stimulus will not trigger a repeat action potential).

For more in-depth information, consider this free molecular biology textbook for a review. This is more than you need to know for the exam: http://www.ncbi.nlm.nih.gov/books/NBK21668/#_A6146_

 

[bobeanie95]

@----x---- Dope analogy! Thank you