Question

layer of insulation on axons of certain neurons that helps speed nerve impulse transmission
myelin sheath
muscle cell
hair cell
schwann cell
muscle fiber

Explanation:

The myelin sheath is the insulating layer on axons of certain neurons that speeds nerve - impulse transmission. Muscle cells, hair cells, Schwann cells (while related as myelin - forming cells in the PNS, not the insulation itself), muscle fibers are not this insulating layer.

Answer:

myelin sheath