What is the primary function of a myelin sheath around an axon?

• A. To release neurotransmitters
• B. To integrate signals at the axon hillock
• C. To insulate and speed neural impulses
• D. To form the blood-brain barrier

Answer: (C)

The main idea is that myelin acts as insulation around the axon to speed up nerve signaling. Myelin wraps the axon in fatty layers, so the membrane path for ions is blocked most of the way. This insulation increases membrane resistance and lowers capacitance, so the electrical signal doesn’t leak as it travels. The action potential can then leap from one gap, the node of Ranvier, to the next in a process called saltatory conduction. This dramatically speeds up conduction without needing more energy. It also reduces the energy cost because fewer ions move across the membrane along the axon, meaning fewer pumps are needed to reset ion gradients. Neurotransmitter release happens at synapses, not along the axon shaft. The axon hillock is where signals are integrated to decide if an action potential starts, not a role for myelin. The blood-brain barrier is formed by endothelial cells with tight junctions, not by myelin.