What substance coats some axons and increases efficiency in nerve impulse transmission?

Myelin is a fatty substance that wraps around the axons of certain neurons, forming an insulating layer known as the myelin sheath. This sheath significantly enhances the speed and efficiency of nerve impulse transmission. The myelin sheath facilitates saltatory conduction, allowing action potentials to jump from one node of Ranvier (gaps in the myelin) to another, which accelerates signal propagation along the axon. Without myelin, nerve impulses would travel much slower, leading to less efficient communication between neurons.

The other options do not serve the same function: axoplasm refers to the cytoplasm within an axon, neurotransmitters are chemical messengers involved in transmitting signals across synapses, and dendrites are the extensions of neurons that receive signals from other neurons. Therefore, myelin is specifically responsible for improving nerve impulse transmission efficiency.