the myelin sheath is made from ________.
Answer:
The myelin sheath, which is a protective covering around nerve fibers, is mainly composed of two types of cells: oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS).
In the CNS, oligodendrocytes are responsible for producing and maintaining the myelin sheath. Oligodendrocytes extend their processes and wrap around multiple nerve fibers, forming multiple layers of myelin sheath segments along the axons. This insulation provided by the myelin sheath allows for faster and more efficient transmission of nerve impulses.
In the PNS, Schwann cells perform a similar function. Each Schwann cell wraps around a single axon, forming the myelin sheath. Unlike oligodendrocytes, Schwann cells only wrap around one segment of an axon, leaving small gaps called nodes of Ranvier between each segment. These nodes play a crucial role in the saltatory conduction of nerve impulses.
The myelin sheath is composed of lipids (fats) and proteins. The lipid-rich layers of the myelin sheath give it its characteristic white appearance, which is why areas of the CNS where these fibers are densely packed are referred to as white matter.
Overall, the myelin sheath plays a vital role in enhancing the speed and efficiency of nerve impulse conduction, enabling proper functioning of the nervous system.