What structure encases the fibers of neurons and increases the transmission speed of impulses?

The structure that encases the fibers of neurons and significantly increases the transmission speed of impulses is the myelin sheath. This fatty layer surrounds the axons of many neurons and serves as an insulating material. By providing this insulation, the myelin sheath allows electrical signals, or action potentials, to travel more quickly along the axon through a process known as saltatory conduction. This process occurs because myelinated axons have gaps, called nodes of Ranvier, where the action potential can jump from one node to the next, greatly enhancing transmission efficiency compared to unmyelinated axons.

In contrast, the axon terminal is a part of the neuron that is involved in transmitting signals to other neurons but does not impact the speed of impulse transmission along the axon itself. Dendrites are structures that receive signals from other neurons and play a key role in communication within the nervous system, but again, they do not affect the speed of impulse conduction along the axon. Lastly, a synapse is the junction between two neurons where communication occurs, and while it is essential for transmitting signals, it is not involved in the encasement or insulation of the axons. Therefore, the myelin sheath is critical for enhancing the speed of neural transmission