The structures within the brain are made up of about 100 billion neurons, as well as trillions of support cells called glia. Neurons may be the more important cells in the brain that relay messages about what you're thinking, feeling, or doing. But they couldn't do it without a little help from their friends, the glial cells.
Each bead-like structure that is strung along an axon is a single oligodendrocyte.
Microglia process harmful bacteria and act as the brain's immune cells.
There are a few different types of glia in the brain: oligodendrocytes, microglia, and astrocytes. Each is needed to optimize brain function.
Oligodendrocytes are specialized cells that wrap tightly around axons to form the myelin sheath. The job of these cells is to
speed up the electrical signal (action potential) that travels down an axon. Without oligodendrocytes an action potential
would travel down an axon 30 times slower!
Microglia are special immune cells found only in the brain that can detect damaged or unhealthy neurons. They eat foreign invaders (bacteria and viruses), then display the chewed up parts on their cell surface to signal for help.
Astrocytes are star-shaped glia that hold neurons in place, get nutrients to them, and digest parts of dead neurons.
But because astrocytes cannot generate action potentials, they haven't received a lot of attention, until recently.
It has been discovered that astrocytes can indeed communicate with neurons and modify the signals they send or receive. That means astrocytes are much more involved than we thought in the processing of information, and in the signaling that occurs at the synapse.
Astrocytes can release gliotransmitters (like glutamate) by exocytosis to send signals to neighboring neurons.
Each astrocyte has its own territory (they don't overlap), and each may interact with several neurons and hundreds to thousands of synapses to properly integrate information.
"End-feet" connect to blood vessels in the brain. By signaling blood vessels to expand or narrow, astrocytes regulate local blood flow to provide oxygen and nutrients to neurons in need.
Astrocytes generate signals that are chemical rather than electrical. The details still need to be worked out, but astrocytes are somehow activated when the level of calcium ions increases inside the cell. This change in concentration signals the release (typically by exocytosis) of what are now called "gliotransmitters". These small molecules travel to a neighboring cell and deliver their message in a process very similar to that used by neurotransmitters.
Astrocytes are now thought to be involved in almost all aspects of brain function. Scientists want to know more about how
gliotransmitters can inhibit, stimulate, or fine-tune the action potentials fired by neurons. But astrocytes may even do more.
There is growing evidence that astrocytes can alter how a neuron is built by directing where to make synapses or dendritic spines. They can also attract new cells to their territory (like immune cells and perhaps even adult neural stem cells) to repair any damage.
Knowing more about astrocytes will also shed light on diseases in which communication between astrocytes and neurons is altered, including Alzheimer's disease, AIDS, brain cancer, and ALS (amyotrophic lateral sclerosis, also known as Lou Gehrig's disease).