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A human brain contains billions of neurons. Neurons are probably the most important and best-known cells of the brain as they carry out the brain’s communication function. Less known are some trillions of support cells called glia, or glial cells. The glia may not be the stars of the show, but without them, neuron functions would be impossible.
The major types of glial cells in the brain include: oligodendrocytes, microglia, and astrocytes.
Oligodendrocytes are specialized cells with arm-like processes that wrap tightly around axons of neurons to form the myelin sheath. The myelin sheath acts like an electrical insulator around a wire. It helps to speed up the electrical signals that travel down an axon. Without oligodendrocytes, an action potential would propagate 30 times slower!
Microglia are special macrophages found only in the central nervous system. They wander through the brain tissue and phagocytize dead, injured cells and foreign invaders. High concentrations of microglia are an indication of infection, trauma or stroke.
Astrocytes are the most abundant and functionally diverse glia.
These star-shaped glial cells provide supportive frameworks to hold neurons in place. They provide neuron with nutrients such as lactate. They also produce growth factors that promote neuron growth and synapse formation. There is growing evidence that astrocytes can alter how a neuron is built by directing where to make synapses or dendritic spines.
Through their numerous processes, known as perivascular feet, astrocytes induce the endothelial cells of blood vessels to form tight junctions. These tight junctions are the basis of the blood brain barrier that restricts the passage of certain substances from the bloodstream to the brain tissue.
Astrocytes help to maintain the chemical composition of the extracellular fluid. They express membrane transporters for several neurotransmitters such as glutamate, ATP and GABA, and help to remove them from synaptic spaces.
Astrocytes also absorb potassium ions released by neurons at synapses. This helps to regulate potassium concentrations in the extracellular space. Abnormal accumulation of extracellular potassium is known to result in epileptic neuronal activity.
Another function of astrocytes is to form scar tissues to replace damaged tissues.
Recently, it has been shown that astrocytes can also communicate electrically with neurons and modify the signals they send and receive. In a manner similar to neurons, astrocytes can release transmitters, called gliotransmitters, upon stimulation. These open up a possibility that astrocytes maybe much more involved in the communication functions of the brain than we currently believe.
From a clinical viewpoint, neurons have little capacity for renewal and therefore rarely form tumors. On the contrary, glial cells are capable of dividing throughout life and are the primary source of brain tumors.