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Found in the nervous system are glial cells that provide many supporting functions, including forming myelin sheaths around a neuron's axon. Schwann cells, or neurolemmocytes, discovered by Theodore Schwann, are glial cells found in the peripheral nervous system that wrap around the axon of a neuron multiple times until a myelin sheath is formed. Myelin sheaths are formed in segments, leaving gaps in-between, and give the white matter below the cerebral cortex its white appearance. The sheaths function to insulate an axon, send messages throughout the brain quickly, and prevent interference from other messages being sent. Damage to myelin sheaths in the central nervous system causes numerous impairments and this is evident in the neurological disease multiple sclerosis.
In order for a myelin sheath to be formed around an axon in the peripheral nervous system, numerous Schwann cells line up along the length of the axon and surround it with a pair of plasma membranes called the mesaxon. These surrounding membranes wrap around the axon numerous times, creating a thick layer that insulates the axon. A Schwann cell wraps around a segment of the axon, usually covering between 0.15 and 1.5 mm in length and creating small gaps in-between each segment called the node of Ranvier. If an axon is thick, a longer area of the axon will be myelinated by a Schwann cell. The number of Schwann cells needed to form a myelin sheath varies and, because nerves can be quiet long in the peripheral nervous system, a few hundred Schwann cells may be required.
The importance of myelin sheaths was discovered as a result of the neurological disease multiple sclerosis. Patients suffering from multiple sclerosis show areas of damaged tissue in the brain. This occurs when the immune system attacks a particular protein in the myelin sheaths that surround axons in the central nervous system. Myelin sheaths in the central nervous system are formed by different glial cells called oligondendrocytes, and, when damage occurs to these sheaths, patients suffer from various motor and sensory impairments.
German physiologist Theodore Schwann was the first to discover Schwann cells and propose a cell theory. Schwann’s cell theory states that cells are the smallest structural and functional units in a living organism, and that cells can only originate from cells that already exist. It also asserts that all life forms, both plant and animal, are made up of cells.