Biological Systems MCAT Practice Exam

What is the purpose of the myelin sheath?

1. To increase surface area for depolarization

2. To insulate and speed up conduction

3. To store neurotransmitters

4. To trigger impulses

The correct answer is: To insulate and speed up conduction

The myelin sheath serves a crucial role in the nervous system by insulating nerve fibers and enhancing the speed of electrical conduction along the axon. This insulation occurs because myelin is a lipoprotein substance that prevents the loss of electrical signals as they travel, leading to an efficient transmission of action potentials. The presence of the myelin sheath allows for saltatory conduction, where action potentials jump from one node of Ranvier (the gaps between myelin sheaths) to another. This significantly reduces the time it takes for an electrical signal to travel along the axon, enabling faster responses in the nervous system. Thus, the primary purpose of the myelin sheath is to both insulate the axon and increase the speed of conduction, making it vital for proper nervous system function. In contrast, other options do not accurately reflect the role of the myelin sheath; for instance, increasing surface area for depolarization is more relevant to the dendrites of neurons, storing neurotransmitters pertains to synaptic vesicles in nerve terminals, and triggering impulses is primarily the role of ion channels and the movement of ions across the membrane, rather than the myelin sheath itself.