Schwann cells are specialized glial cells found exclusively in the peripheral nervous system (PNS). They surround nerve cells, providing support and maintaining the health and function of peripheral nerves, which transmit signals between the brain and spinal cord and the rest of the body.
Forming Myelin Sheaths
Schwann cells form the myelin sheath around axons. Myelination occurs when a Schwann cell wraps its plasma membrane concentrically around a single axon multiple times. This creates a compact, multi-layered insulating layer rich in lipids and cholesterol.
This insulation is not continuous along the entire axon; instead, small gaps known as Nodes of Ranvier exist. These nodes allow for saltatory conduction, a type of electrical impulse transmission where the signal “jumps” from one Node of Ranvier to the next. This significantly increases the speed of nerve impulse conduction. Myelinated nerve cells can transfer signals up to 10 times faster than unmyelinated nerves, enabling rapid communication for functions like muscle movement and sensory perception.
Supporting Unmyelinated Nerves
While many axons in the peripheral nervous system are myelinated, many are not. Schwann cells provide support for these unmyelinated nerve fibers. These non-myelinating Schwann cells, sometimes called Remak cells, ensheath multiple small-diameter axons within invaginations of their plasma membrane.
Although they do not form an insulating myelin sheath, non-myelinating Schwann cells are associated with these axons. They provide physical protection, metabolic assistance, and trophic support by supplying nutrients and removing waste products. This support aids the health and survival of unmyelinated nerve fibers, which transmit slower, continuous signals such as those related to pain and temperature.
Aiding Nerve Repair
Schwann cells are involved in the repair and regeneration of damaged peripheral nerves. Following an injury, Schwann cells at the site transform into a “repair” phenotype. These repair Schwann cells clear cellular debris, including degenerated axons and myelin, often in cooperation with macrophages.
The transformed Schwann cells then align themselves to form structures called “Bands of Büngner”. These bands act as a guide tube, providing a physical pathway and scaffolding for regenerating axons to grow along. Repair Schwann cells also secrete various neurotrophic factors and growth cues, such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), and neurotrophin-3 (NT3), which promote the survival of injured neurons and encourage axonal elongation and regrowth back to their original targets.