The function of the myelin sheath in nerve fibers is to insulate axons and enhance the speed of electrical impulses along the nerve. Composed of fatty substances, it wraps around the axon in segments, leaving gaps known as nodes of Ranvier. This arrangement allows for saltatory conduction, where impulses jump from one node to the next, significantly increasing transmission speed. Proper myelination is crucial for efficient communication within the nervous system and for overall neurological health.