Final answer:
The myelin sheath is mainly composed of lipids to effectively insulate axons and is low in protein concentration. Its structure is critical for rapid signal transmission and is primarily found in the white matter of the brain and spinal cord.
Step-by-step explanation:
The myelin sheath has a very low concentration of protein because its primary function is to insulate axons and facilitate the rapid transmission of electrical signals along the axon. The myelin sheath is lipid-rich, formed by the tightly wrapped plasma membrane of glial cells; specifically, oligodendrocytes in the CNS and Schwann cells in the PNS. Proteins do exist in the myelin sheath, but their role is mainly to hold the layers of the glial cell membrane closely together rather than provide the insulating property, which is largely due to the high lipid content.
Misconceptions such as 'all myelin sheaths are located in the gray matter' should be corrected as myelin actually occurs in the white matter of the brain and spinal cord, not the gray matter. Diseases like multiple sclerosis highlight the importance of the myelin sheath, as the loss of this insulation can severely disrupt nervous system functions.