The nervous system transmits information with exceptional speed and efficiency. This requires nerve fibers, or axons, to be well-insulated, much like an electrical wire. While the Schwann cell and the myelin sheath are fundamental to this process, they are distinct components often mistakenly used interchangeably. Understanding their individual roles—one as a living support unit and the other as a protective material—is necessary to grasp how the peripheral nervous system operates.
Schwann Cells: Defining the Glial Unit
Schwann cells are a specific type of glial cell, providing support and protection to neurons. They are the principal glia of the Peripheral Nervous System (PNS), which includes all nerves outside of the brain and spinal cord. Schwann cells function as a biological entity responsible for the maintenance and survival of the axons they surround. They exist in one of two main states.
Myelinating Schwann cells wrap tightly around large-diameter axons to produce a protective coating. Conversely, non-myelinating Schwann cells envelop multiple smaller, unmyelinated axons in bundles, providing metabolic support and cushioning without forming a thick sheath. In both forms, the Schwann cell’s primary function is to nurture the axon, produce the surrounding extracellular matrix, and support the nerve fiber’s health.
Myelin Sheath: Structure and Composition
The myelin sheath is a physical structure or material, not a living entity. It is a multilayered coating that serves as an effective electrical insulator for the axon. The sheath is composed primarily of a fatty substance, or lipid, along with a significant amount of protein.
This lipid-rich composition gives the sheath its characteristic whitish appearance, leading to bundles of myelinated axons being called “white matter.” The purpose of this physical insulation is to increase the speed of nerve impulse conduction. The myelin sheath enables transmission speeds up to 120 meters per second by forcing the electrical signal to jump from one small gap in the sheath to the next, a process called saltatory conduction.
The Core Difference: Cell vs. Product
The Schwann cell is the living producer, while the myelin sheath is the protective, insulating product it creates. A single Schwann cell wraps its own plasma membrane repeatedly around a segment of a peripheral axon. As the cell wraps, the cytoplasm and nucleus are squeezed to the outermost layer, leaving the tightly packed layers of the cell membrane to form the myelin sheath.
In the PNS, one myelinating Schwann cell is dedicated to insulating only a single segment of one axon, approximately 1 millimeter long. This differs from the Central Nervous System (CNS), where a different glial cell called an oligodendrocyte produces the myelin sheath.
Specialized Functions in Peripheral Repair
Beyond insulation, Schwann cells have a specialized function in repairing damaged nerves in the periphery. When a PNS axon is injured, the Schwann cells undergo changes. They dedifferentiate, shifting from maintenance to an active repair state, where they assist in the breakdown and clearance of the damaged axon and myelin debris.
These repair Schwann cells then proliferate and align to form a continuous structure known as the Band of Büngner. This band acts as a physical guide, creating a pathway that directs the regenerating tip of the damaged axon back toward its original target. By secreting neurotrophic factors and acting as a structural scaffold, the living Schwann cell provides the necessary chemical and physical support for successful nerve regeneration.