What substance enhances the speed of signal transmission in nerve fibers?

Myelin is a fatty substance that forms a protective sheath around the axons of nerve fibers. This myelination is crucial for enhancing the speed of electrical signal transmission, or action potentials, along the nerve fibers. The presence of myelin allows for a process known as saltatory conduction, where the electrical impulses jump from one node of Ranvier (gaps in the myelin sheath) to another. This significantly increases the speed of signal transmission compared to unmyelinated fibers, where the signals must propagate continuously along the entire length of the axon.

In contrast, substances like serotonin, acetylcholine, and dopamine play various roles in neurotransmission and signaling in the nervous system but do not primarily function to enhance the speed of signal transmission in nerve fibers. Serotonin is largely involved in mood regulation and various functions in the central nervous system, while acetylcholine primarily functions as a neurotransmitter at neuromuscular junctions and in the parasympathetic nervous system. Dopamine is important in a range of neurological functions, including motivation and reward pathways. These neurotransmitters are more involved in chemical signaling and modulation rather than directly enhancing electrical conduction velocity in the nerve fibers.