How does an oligodendrocyte work?
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Wings for Life publishes “illustration guides” on regular basis to help non-scientists to understand, how the spinal cord works, how the injury alters it, and finally which different approaches scientists take to find a cure. This month we explore the oligodendrocytes.
Function and relevance for spinal cord research
Oligodendrocytes form the isolating sheath around the axons, which is essential for fast signal conduction. A traumatic spinal cord injury causes damage to those cells followed by the loss of isolation sheaths. As a result the conduction of electrical signals is massively impaired inside the axons. Due to its important role for the axonal function, many scientists focus on these cells in their work.
Oligodendrocytes are one of the members of the neuroglial population. Their main functions are to provide trophic and functional support and to form the insulation of the axons (the long projections of neurons; see last month's guide). Oligodendrocytes do this by creating the myelin sheath, a white and shiny fatty substance, which is composed by 80% of lipid and 20% of protein.
This particular composition confers to the oligodendrocyte’s capacity to isolate axons from each other and mostly to allow fast nerve signal conduction. In order to do so, the oligodendrocyte extends parts of its membrane to the axon and twists around it thereby forming a wrap of myelin sheaths around each axon. A single oligodendrocyte can extend its processes to 50 axons.
Getting really into it
An oligodendrocyte is divided into three parts: the cell body, the processes and the myelin segments
The cell body is the central part of the oligodendrocyte (same as for the neuron). It contains the nucleus of the cell (that carries all the genetic material) and organelles that allow protein synthesis and energy production. The cell body (green color) extends its processes (blue color), which form the myelin segments (violet color) that are covering the axons. The myelin segments are wrapped in concentric circles around the axons and insulate the axons just like the plastic wrapping around an electric cable. It takes several oligodendrocytes to fully insulate one axon.
Two main tasks: Velocity increase of information and support
The myelin sheaths facilitate rapid electrical conduction of the nerve signals from the cell body to the target at the end of the axon. How does this system work? Unlike the plastic insulation around an electrical cable the myelin sheath consists of myelin segments. Between each myelin segment is an empty space where the axon is not insulated. This “nude” area is called “node of Ranvier” and is actually the key element in the velocity of signal conduction.
The electrical impulse of a neuron starts from the cell body and jumps from one node of Ranvier to the next, creating what is defined as “saltatory conduction”. This phenomenon does not only increase the velocity of nerve conduction to an incredible 200 m/s (720 km/h), but also drastically decreases the energy consumption.
Finally oligodendrocytes provide trophic and mechanical support to the wrapped neurons. This tight connection provides the axons with stability and allows the oligodendrocytes to finally deliver energy to neurons/axons, which they have harvested from the blood vessels. Neurons have very high-energy consumption and this association supplies them with a constant flow of “ready to use” energy.
… next month
The next “Illustration Guide” will focus on the most abundant cell of the human brain: the astrocyte.
Text: Vieri Failli, Verena May, Rosi Lederer
Graphics: Vieri Failli