Cells of the Brain: yellow-neurons; orange-astrocytes; grey-oligodendrocytes; white-microglia.
 
Squeezing between tangles of dendrites and axons as they rush to kill the invader, microglia attack and devour any harmful organism. A microglia cell can transform from a latent solitary cell into a highly mobile cell when it detects the danger of infection or injury. There is nearly one microglia cell for every neuron. Each neuron has, in effect, its own private bodyguard.
— R. Douglas Fields, The Other Brain

Glia: any of the cells that hold nerve cells in place and help them work the way they should. The types of neuroglia include “oligodendrocytes,” “astrocytes,” (and) “microglia.” (NCIt) Glia provide physical and nutritional support for “neurons,” clean up brain debris, transport nutrients to neurons, digest parts of dead neurons, regulate the content of "extracellular" space, and provide insulation to neurons to increase (“synaptic transmission") velocity. (Chudler, 91)

Glia can move. Neurons are tethered into a fabric of synaptic connections in the brain. In contrast, the cellular tendrils of outcast glia (are) free to wander and probe at will through the tangled knotted network of nerve fibers in our brain. (Fields, 255) Make up two-thirds of all brain cells. (Goldberg, 40) Occupy essentially all the space in the "nervous system" not taken up by neurons. (The Brain-Charles Stevens, 15) Coordinate groups of neurons, regulate the excitability of neural networks, enhance or inhibit "synaptic strength," and probe through brain tissue to strip away “synapses” or clear away space for new ones to form. (Fields, 265) Support, nurture and maintain nerve cells and their environment in the brain. (Koch, 17) For decades considered little more than connective tissue that physically and perhaps nutritionally supported neurons. (Fields, 7) Clues (now) point to the possibility that all types of glia may sense and respond to electrical activity in neurons. (Fields, 23) They can detect information flow in neuronal “circuits” and influence it. They can communicate among themselves. (Fields, 250) (They) communicate through the slow spread of chemicals and ‘calcium waves,’ not by jolts of electricity. (Fields, 267) Some glia cells exhibit "propagating" calcium events, akin to "action potentials," except that they occur over seconds. (Koch, 17) They operate slowly and influence large territories of the brain. (Fields, 253) The possibility that glia might modulate “synaptic strength” and remodel connections in (the hippocampus), controlling human “memory” is inescapable. In any significant remodeling of our brain during development or “learning,” glia now seem to be the cells most likely to act. The ability of glia to move and secrete substances that remodel tissue, or stimulate neuron growth during development and repair of the brain, makes them ideally suited to rewire the neural circuits during learning in the healthy brain. (Fields, 265-266) Adjective: ‘glial.’ Also referred to as ‘glial cells’ and ‘neuroglia.’


Astrocytes: enormous bushy cells. (Fields, 253) Large, star-shaped cells that hold nerve cells in place and helps them develop and work the way they should. (NCIt) Found throughout the brain and spinal cord. Provide the energy source for neurons. Deliver the fuel in proportion to neuronal demand. (Fields, 22-23) Support neurons and ‘mop up’ “neurotransmitters” after the neurons ‘message’ has been transmitted. (Newsweek, March 2007, 43) Surround the synapse and regulate “(synaptic) transmission” by taking up and releasing neurotransmitters. Communicate with each other by using chemical messages. (Fields, Figure 26) The largest and most numerous neuroglial cells in the brain and spinal cord. Astrocytes are irregularly shaped with many long processes, including those with ‘end feet’ which form the glial membrane and directly and indirectly contribute to the “blood brain barrier.” They “regulate” the “extracellular” ionic and chemical environment. Reactive astrocytes (along with microglia) respond to injury. Astrocytes have … (“reuptake”) systems, “voltage-dependent” and “transmitter-gated ion channels,” and can release “neurotransmitters,” but their role in signaling is not well understood. (MeSH)

Bergmann Glia: neurons in the “cerebellum” are tightly ensheathed by (these) astrocytes. (Their) finger-like tendrils move. (Fields, 266)

Glial Fibrillary Acidic Protein (GFAP): tiny microscopic protein “filaments” inside astrocytes. Part of the cellular skeleton of astrocytes. Increases as a result of many different brain stresses and disorders, including “multiple sclerosis.” (Fields, 37) After injury, astrocytes proliferate and begin to express GFAP at high levels, a ‘cellular remodeling’ that allows astrocytes to change their shape and (motion). (Fields, 194) Encoded by the human GFAP gene. This protein may be involved in both neuronal maintenance and astrocyte morphology and motility. (NCIt)

Glial Trails: chemical trails that "migrating" cells follow. As the emerging brain expands in size, the traveling glia grow fibers that extend toward the brain surface. By crawling along the glial trail, young neurons find their way to their target. (LeDoux, 70)

Microglia: form of glial cell that scavenges debris in the nervous system. (Kolb, 84) Found throughout the brain and spinal cord. Protects the brain from injury and disease, making them central to recovery from brain and spinal cord injury. (Fields, 23) Originate in the "blood" as an offshoot of the "immune system" and "migrate" toward the brain. (Kolb, 84) Microglia are derived from different embryonic stock than that giving rise to neurons or other glia. They do not invade the brain, they grow up with it. (Fields, 43) There is evidence that they can apply (chemical) weapons to strip synaptic connections from neurons - not only in disease, but also in rewiring circuits in “learning.” May be able to unplug the connections between neurons. (Fields, 44) When brain cells are damaged, microglia invade the area to provide growth factors that aid in repair and to engulf and remove foreign matter and debris. (Kolb, 85) Equipped with powerful protein-dissolving “enzymes” to melt “extracellular matrix” proteins that stitch neurons together. Can race between tightly packed brain cells to the site of infection and kill invading organisms. Recently seen using these infection-fighting tools to strip synapses off neurons to rewire circuits after injury and during early life. They help us to rewire our neural circuits to enable us to learn. (Fields, 265)

Oligodendrocytes: found only inside the brain and spinal cord. Seen almost everywhere in the brain, but especially numerous in “white matter” “tracts.” (Fields, 33) "Myelinate" axons in the brain and spinal cord by sending out large, flat branches that enclose and separate adjacent axons. (Kolb, 85) Wrap myelin only around axons, never around dendrites, “cell bodies,” or other cells in the brain such as blood vessels. Myelinated axons in the human brain are as small as one-thousandth of a millimeter in diameter. (Fields, 40-41) Also referred to as ‘oligodendroglia.’

Schwann Cells: found outside the brain in the nerves of the body. (Fields, 24) Manufacture and maintain the fatty layer of myelin that sheathes the axons of the central nervous system. (Brain, Seymour Kety, 122) Provide structural and possibly physiological support for the axons. (Fields, 15) Coat the nerve fibers all along their length right up to the point where the nerve enters the “spinal cord” or brain, but do not cross that threshold. (Fields, 31) Must perform all the functions of the three types of glia recognized in the central nervous system, as well as forming the myelin electrical insulation around axons in the peripheral nerves. (Fields, 319)

Myelinating Schwann Cells: entwine around only large-diameter axons. Hundreds are attached in a series all along (the axon’s) full length like pearls on a necklace. (Fields, 31) Myelinating glia wrap dozens of layers of membrane around axons, insulating each one like electrical tape wrapped around a wire. (Fields, 39)

Non-Myelinating Schwann Cells: huge globular cells (that grasp) bunches of slender axons like a fistful of spaghetti. One cell engulfs a dozen or more small-diameter axons inside itself. (Fields, 31)

Terminal Schwann Cells: specialized cells that tightly surround a synapse onto “muscle.” (Fields, 321) Entire tip of the axon is completely engulfed. Seals off the nerve junction like ‘shrink wrap.’ Can sense and control information flow from nerve to muscle. (Fields, 32) Also referred to as ‘perisynaptic schwann cells.’