Schwann cells are a type of glial cell, such as those that serve to stabilize and nourish nerve fibers in the peripheral nervous system. They also wrap around the axons of medullary nerve fibers, providing them with insulating myelin. In inflammatory demyelinating diseases in the peripheral nervous system, the myelin of the cells is destroyed and neurological deficits occur.
What is the Schwann cell?
The medical term Schwann cells refers to one of about ten special forms of glial cells. All glial cells are located in nerve tissue. They assume length dimensions of up to 100 µm and coat the axon of nerve fibers. Schwann cells exclusively cover peripheral nerve fibers. In vertebrates, they even wrap themselves around the axon of a nerve cell several times for this purpose. Like all other glial cells, Schwann cells primarily perform supporting and insulating functions. The German physiologist and anatomist Theodor Schwann gave the cells their name in the 19th century. Schwann supporting cells are exclusively a part of the peripheral nervous system and are not found in the central nervous system. The same is true for the peripheral glial cell types of mantle cells, motor teloglia, and Müller cells. Thus, glial support cells of the central nervous system must be distinguished from peripheral glial support cells such as Schwann cells. Neuroglia and radial glia, for example, fall into this group. Within the central nervous system, the oligodendrocytes fulfill exactly the same function as the Schwann cells in the peripheral nervous system. Unlike those in the central nervous system, glial cells in the peripheral nervous system may be able to recover from injury.
Anatomy and structure
Schwann cells are composed primarily of cytoplasm and a nucleus. The nucleus and cytoplasm of the Schwann cell are located in its outer region. This outer region is also called the neurolemm or Schwann’s sheath. Surrounding the neurolemma is the so-called basal lamina. This is an apparently homogeneous layer of proteins that forms the base of epithelial cells. This basal lamina connects the neurolemm with the connective tissue of a surrounding nerve fiber. In the peripheral nervous system, Schwann cells are extremely close to each other. Nevertheless, there is always an interruption between two neighboring Schwann cells, which establishes saltatory conduction and serves to optimize conduction velocities. These interruptions are called Ranvier poker rings. These poker rings are spaced between 0.2 and 1.5 millimeters apart. The distance between the stoking rings is also called internode or internodal segment by neurologists. Some interruptions of the myelin layer also run at an angle and are then called Schmidt-Lantermann notches.
Function and tasks
Peripheral nervous system Schwann cells particularly perform support functions and stabilize nerves. Apart from this, like all other glial cells, they also nourish nerve fibers-in this case, those of the peripheral nervous system. However, these vital tasks are not their only ones. In addition to support and nutritional functions, they also perform insulating functions in conjunction with medullary fibers. They produce slices of insulating myelin. The Schwann cells attach to the axons of medullary nerve fibers, and the myelin generated in the process gives rise to rapidly conducting nerves. Myelin is a fatty protein substance that prevents the migration of electrical excitations. The bioelectrics of the nervous system would not function without insulating myelin, because the excitation potentials would ever dissipate into the area surrounding the nerve fibers. With myelin, Schwann cells also protect nerve conductors from excitations that do not affect them. Insulation increases the capacity and conduction velocity of axons. Thus, by producing myelin, glial cells ultimately ensure that the body’s stimulus transmissions run smoothly. The frictionless transmission of stimuli is indispensable for numerous bodily functions. The body’s reflexes, for example, would be inconceivable without fast-conducting nerve fibers. The same is true for the perceptual processing of the sensory system. If a sensory perception did not reach the brain rapidly via fast conducting nerve fibers, then every impression of one’s own environment would be time-delayed.However, in addition to the medullary, fast-working fibers, the nervous system also embraces medullary, slower-working nerve fibers. These medullary nerve fibers in turn supply cytoplasm to the Schwann cells.
Diseases
In connection with Schwann cells, demyelinating diseases in particular play a role. These diseases are also called demyelinating diseases by neurologists and destroy the myelin of the nervous system. If several nerve cells are affected by demyelination, then a focal image is seen on MRI. The best known demyelinating disease is the inflammatory autoimmune disease multiple sclerosis. In this disease, the immune system mistakenly recognizes the body’s own and healthy tissue of the nervous system as a threat and attacks this tissue. This results in inflammation that destroys the myelin sheath of the nervous system. In the peripheral nervous system, this destruction corresponds to demyelination of the Schwann cells that wrap the peripheral axons. Miller-Fisher syndrome is also an inflammatory demyelinating disease. It exclusively affects the peripheral nervous system. In addition to absent reflexes, paralysis and movement disorders often occur symptomatically. Other demyelinating diseases include Balo’s disease, funicular myelosis, and neuromyelitis optica. However, in addition to demyelinating and inflammatory diseases, toxic processes can also injure or destroy myelin. After each demyelination, the transmission of stimuli is disturbed. Depending on how many axons are affected and where the affected axons are located, neurologically more or less severe deficits may occur. Injury to an axon or nerve fiber itself can also cause demyelination.