Glial Cells—the Other Brain

Oxygen generates the energy that fuels cells and, in the process, releases free radicals (molecules that are missing an electron). Antioxidants help to defend against free radicals. Glutathione, one of the most powerful antioxidants in the body, is packed in highest concentration inside glia, specifically astrocytes. Thus, astrocytes can survive toxins that can produce high enough levels of reactive oxygen to kill neurons instantly. Astrocytes release antioxidants in an attempt to protect neurons that are in danger. (Fields, R. Douglas, PhD. The Other Brain. p 104-106. NY: Simon & Schuster, 2009.)

The first major glial cell to be discovered, astrocytes are two to ten times more plentiful than neurons. There may be as many different types of astrocytes as there are neurons. They are found only in the brain, spinal cord, and optic nerve (the eye forms during embryonic development as a swelling growing out from the brain and so is part of the brain). They delivery energy to neurons, remove waste products, and provide a physical matrix for structural support. Astrocytes also assist in healing of neurons but they also form a glial scar around the site of injury, which repels growth. (Fields, R. Douglas, PhD. The Other Brain. p 45-47, 80-95. NY: Simon & Schuster, 2009.)

Astrocytes pass messages on among themselves using calcium ion instead of electrical signals. While neurons communicate through synaptic connections in linear circuits like telephones, astrocytes communicate by broadcasting signals widely like cell phones. (Fields, R. Douglas, PhD. The Other Brain. p 55-57. NY: Simon & Schuster, 2009.)

Astrocytomas, a form of brain cancer, account for 25% of all brain cancers. (Fields, R. Douglas, PhD. The Other Brain. p 74-75. NY: Simon & Schuster, 2009.)

The most common form of glial tumors, astrocytomas, most often grow in the frontal lobes. (Schramm, Derek D., PhD. The Creative Brain. p 14-16. CA: Institute for Natural Resources, Health Update. 2007.)

Not all axons are myelinated: only those that must carry information at high speeds and over long distances. Myelinated axons in the human brain are as small as one-thousandth of a millimeter in diameter. Axons in the brain and spinal cord are myelinated by oligodendrocytes. Axons in the rest of the body that are myelinated, are wrapped by Schwann glial cells. (Fields, R. Douglas, PhD. The Other Brain. p 40-41. NY: Simon & Schuster, 2009.)

Microglia and astrocytes act together as sentinels. They watch for bacteria and viruses that infect the brain and then mobilize to fight the invading microbes. (Fields, R. Douglas, PhD. The Other Brain. p 65-66. NY: Simon & Schuster, 2009.)

Glial cells (non-neural supporting cells found in the brain) make up an estimated 90% of the brain. They produce a substance vital to strong communications between neurons. (Brynie, Faith Hickman. 101 Questions Your Brain Has Asked About Itself But Couldn’t Answer, Until Now. p 34. CT: Millbrook Press, 1998.)

2-year study by Vladimir Parpura and Philip Haydon: Neurons make up only 10 percent of the brain's cells, yet that's the part most research has always focused on. There's 90 percent of the brain yet to learn about—an uncharted area. (Neurons, Not Only Brain Cells in Signal Transmission. Iowa State University, July 2000.)

Brain cancer typically involves glial cells (rarely involves neurons). Mature neurons don’t undergo cell division so are not in a position to become cancerous (e.g., cancer is a failure of brakes that stop cellular division thus leading to runaway growth of cells into tumors). Glia do divide and multiply. Cell division is so intricately regulated that it typically requires several failures in the control process to unleash runaway cell division, one reason there will never be the equivalent of a vaccine to cure all cancer. (Fields, R. Douglas, PhD. The Other Brain. p 69-72. NY: Simon & Schuster, 2009.)

Glial cells communicate with one another using calcium waves and an intracellular diffusion of chemical messengers (unlike the serial flow of information along neuronal chains.They are also able to release neurotransmitters and other signal molecules that allows them to coordinate activities across neuronal networks.(Fields, R. Douglas and Beth Stevens-Graham. New Insights into Neuron-Glia CommunicationScience. 2002; 298(5593); 556-562. Accessed 2007.)

French National Centre for Scientific Research studies: glial cells, formerly believed to be just supports for neurons, release molecules that bind to neuronal receptors and facilitate the transmission of information. (Glial cells: essential to brain communication,2006.)

Glia means glue in Greek. Glial cells outnumber neurons by perhaps 10 to 1. They are now believed capable of encoding and transmitting information on their own. The number of elements involved in information transfer, along with their interactions, represents a number likely greater then the number of particles in the known universe. (Restak, Richard. Mysteries of the Mind. p 11-12. Washington, DC: National Geographic, 2000.)

The name Neuroglia comes from the Latin for “neuron glue.” Santiago Ramon y Cajal, professor of Anatomy at Zaragoza, Spain and the most renowned neuroanatomist of the 20th Century, called them “spider cells” because of the many protoplasmic legs extending in all directions from their corpulent cell body. Others scientists thought glial cells resembled starts and called them “astrocytes” (that name prevails today for one of the four major types of glial cells now recognized). Glial cells lack both wire-like axons and root-like dendrites. (Fields, R. Douglas, PhD. The Other Brain. p 7-11. NY: Simon & Schuster, 2009.)

In the fetus, glial cells form the scaffolding that regulates the survival and differentiation of neurons. Glial cells allow the rest of the nervous system to develop. (Jessen, Kristjan R. Cells in Focus: Glial Cells. Int J Biochem Cell Biology. 36:1861-1867, 2004.)

  • Astrocytes (play a role in normal mental function in a healthy brain)
  • Microglia (protect the brain from injury and disease)
  • Oligodendrocytes (form myelin insulation on axons in the brain and spinal cord)
  • Schwann (form myelin insulation on selected axons outside the brain and spinal cord)

(Fields, R. Douglas, PhD. The Other Brain. pp 20-38. NY: Simon & Schuster, 2009.)

Glial cells perform many essential functions in the brain. Acting as “cleaner-up” cells, they are peptide factories that move around macrophage-like, sometimes destroying and sometimes nurturing nerve endings. (Pert, Candace, PhD. Molecules of Emotion. p 289. NY: Scribner, 1997.)

Glial cells cannot fire electrical impulses as do neurons, so do not have a wire-like axon for sending impulses over long distances or bushy dendrites for receiving them through thousands of synapses.

Memories are not bottled up inside neurons. They are stored in the connections between neurons linked by synapses. With new experiences, new connections are made between neurons and others are lost. In a sense, memores are not stores inside matter but in the spaces between them. (Fields, R. Douglas, PhD. The Other Brain. p 17. NY: Simon & Schuster, 2009.)

As their name implies (Latin and\or Greek for glue), glia help keep things together. Questions about the ratio of glial cells to neurons abound. Some have said a 9:1 ratio; others that 80% are glias. Recently, neurophysiologist Suzana Herculano-Houzel and colleagues have developed a new technique for estimating numbers of glial cells. An interesting finding is that rather than an overall constant glia-neuron ratio, the numbers change in different parts of the brain (e.g., there are much higher numbers of glial cells in the cerebral cortex as compared to the cerebellum). Regardless of the true glia to neuron ratio, scientists have already shown that glia are, functionally, the brain’s other half: some watch for bacteria and viruses that infect the brain and then mobilize to fight the invaders, others form myelin insulation to coat neuronal axons, while still others secrete food for neurons. (Source)

Glial cells outnumber neurons by at least 6 to 1 but the ratio differs in different parts of the nervous system. The ratio can be 100 glials to 1 neuron along nerves in the white matter tracts in the brain; in the frontal cortex the ratio is 4 to 1. Interestingly, whales and dolphins have 7 glials for every neuron in their gigantic forebrains. (Fields, R. Douglas, PhD. The Other Brain. p 24. NY: Simon & Schuster, 2009.)

This protein is part of the cellular skeleton of astrocytes. GFAP can increase in astrocytes as a result of many different brain stresses and disorders. For example, too much GFAP causes astrocytes to become choked with Roesenthal Fibers, a hallmark of Alexander disease. (Fields, R. Douglas, PhD. The Other Brain. p 35-38. NY: Simon & Schuster, 2009.)

Glioblastomas account for 51% of all brain cancers. (Fields, R. Douglas, PhD. The Other Brain. p 74-75. NY: Simon & Schuster, 2009.)

The uncontrolled proliferation of glia can result in gliomas. One of every two primary brain tumors and one of every five primary spinal cord tumors are composed of glial cells. The most common form of glial tumors, astrocytomas, most often grow in the frontal lobes. (Schramm, Derek D., PhD. The Creative Brain. p 14-16. CA: Institute for Natural Resources, Health Update. 2007.)

Glial cells are the inflammation brokers in your brain. When stress, a toxin pollutant, or a destructive food additive (e.g., MSG, aspartame) enter your brain they induce excitotoxic reactions that inflame brain cells. This has implications for multiple and early immunizations for children (e.g., adjuvant added to vaccine). (Source)

Glial cells appear to be particularly important in mental health, in maintaining balance and setting the general tone of excitability in the brain. (Fields, R. Douglas, PhD. The Other Brain. p 133-161. NY: Simon & Schuster, 2009.)

Microglia or “microglue cells” constitute 5-20 percent of the entire glial population in the brain (e.g., nearly one microglial cell for every neuron). They are the smallest and most dynamic of all glia, able to transform into a highly mobile amoeboid cell whenever infection or injury is detected. Microglia orginate from the same embryonic line that gives rise to other immune system cells in the body. They enter the part of the embryo that will become the brain at a very early stage in development and grow up with the brain. They have a powerful ability to resupply their ranks by rapid cell division (something neurons cannot do). (Fields, R. Douglas, PhD. The Other Brain. p 41-44. NY: Simon & Schuster, 2009.)

Microglia can track down and devour bacteria, viruses, and cellular debris. They may also attack invaders with chemical agents (e.g., glutamate, cytokines, reactive oxygen, nitrogen species) that can be harmful to neurons in high concentrations.(Fields, R. Douglas, PhD. The Other Brain. p 44. NY: Simon & Schuster, 2009.)

Glial cells seem to aid in the migratory process (of neurons). They give rise to fibers that extend toward the brain’s surface. By climbing the glial trail, neurons find their homes. (See also Segregation.) (LeDoux, Joseph. Synaptic Self, How Our Brains Become Who We Are. p 69. NY: Penguin Books, 2002.)

Neurons are fed and guided by glial cells that form a path along which the neurons migrate. After the neurons reach their destination one type of glial cell controls metabolism, another coats the axons with myelin and controls speed of information conduction. (Ratey, John J., MD. A User’s Guide to the Brain. p 24. NY: Vintage Books, 2002.)

In the fetus, glial cells form the scaffolding that regulates the survival and differentiation of neurons. Glial cells allow the rest of the nervous system to develop. (Jessen, Kristjan R. Cells in Focus: Glial Cells. Int J Biochem Cell Biology. 36:1861-1867, 2004.)

The trillion glia (ten times as many as there are neurons) in the brain, busy multitaskers, come in several forms:

  • Radial glia: serve as ladders or scaffolding in the embryonic brain
  • Microglia: serve as the brain's immune system
  • Schwann cells and Oligodendroctes: form insulating sleeves around neurons to keep their electric signals from diffusing.

(Zimmer, Carl. The Dark Matter of the Human Brain. Discover Presents the Brain. p 62-63. NY: Fall 2010.)

Musicians are at risk for deafness due to loud music. Sixty percent of inductees into the rock and roll Hall of Fame have lost hearing due to loud music. Thirty-seven percent of rock musicians are partially deaf. Intense noise or chronic exposure to loud sound damages hair cells in the inner ear as well as in the inferior colliculus, the part of the brain where neurons carrying impulses from the hair cells make their first connection. Brain cells die from overstimulation and then astrocytes digest away the damaged synapses. (Fields, R. Douglas, PhD. The Other Brain. p 98-100. NY: Simon & Schuster, 2009.)

Myelin insulation is formed by oligodendrocytes in the brain and spinal cord, and by Schwannglials in the nervous system outside the brain and spinal cord. (Fields, R. Douglas, PhD. The Other Brain. p 22. NY: Simon & Schuster, 2009.)

The key difference between vertebrates and invertebrates is that vertebrates have myelin-forming glial cells (invertebrates do not). The complex, centralized nervous system of the vertebrates, with myelin insulation wrapped around axons like electrical tape is wrapped around wire, is a fundamental difference that separates these two great groups of creatures. (Fields, R. Douglas, PhD. The Other Brain. p 38-40. NY: Simon & Schuster, 2009.)

Glial cells are involved with both brain health and disease. Diseases of glia (e.g., demyelinating diseases) are also neurodegenerative. When glial cells die in diseases such as multiple sclerosis, often their neural partners die, as well. Glia are involved in both beneficial and detrimental ways in neurodegenerative diseases such as Parkinson’s amyotrophic lateral sclerosis, and Alzheimer’s. Sometimes glia are the targets of infectious organisms (e.g., HIV/AIDS, mad cow disease). (Fields, R. Douglas, PhD. The Other Brain. p 65-67. NY: Simon & Schuster, 2009.)

Glial cells have sensors that can detect a large number of neuronal signaling molecules, including all the various neurotransmitters neurons use for synaptic communication. Glial cells monitor neuronal communication. (Fields, R. Douglas, PhD.The Other Brain. p 57-58. NY: Simon & Schuster, 2009.)

Refer to Neurons and Neurotransmitters for additional information.

NGF are powerful proteins (medicines) secreted by Schwann cells (glia) and designed to rescue neurons from death.(Fields, R. Douglas, PhD. The Other Brain. p 88-90. NY: Simon & Schuster, 2009.)

Glial cells are constantly secreting substances (e.g., peptides) that can nourish neurons or that can weaken/destroy them. (Pert, Candace, PhD. Your Body is Your Subconscious Mind. Audio Cassettes. CO: Sounds True, 2000.)

Glial cells are many times more numerous as compared to neurons. In the brain and spinal cord, neurons exist only in close proximity to glial cells, which comprise about half of the brain’s volume. The attrition rate of mature glial cells (that do divide—unlike neurons) is negligible because of their ability to rapidly proliferate. (Schramm, Derek D., PhD. The Creative Brain. p 4-6. CA: Institute for Natural Resources, Health Update. 2007.)

These glial cells are seen almost everywhere in the brain, but are especially numerous in white matter tracks. The name means “stuffy dendrites” or “short branches.” They are responsible for making the myuelin sheath on axons in the brain. (Fields, R. Douglas, PhD. The Other Brain. p 32-36. NY: Simon & Schuster, 2009.)

About 5% of brain cancers are oligodendrogliomas. They typically occur in the middle decades of life and often involve the frontal and temporal lobes of the brain. (Fields, R. Douglas, PhD. The Other Brain. p 74-75. NY: Simon & Schuster, 2009.)

Glial cells have the ability to intensify and prolong sensations of neuropathic and other types of pain. They are also able to undermine the pain-controlling properties of morphine and other opioids. Glial activation can release substances that intensify pain by enhancing the excitability of neurons near the site. Under basal conditions, glial cells influence on pain is usually minimal. (Watkins, L. R., et al. Glaa as the “Bad Guys:” Implications for Improving Clinical Pain Control and the Clinical Utility of Opioids. 21:131-146. Brain Behav. Immun. 2007.)

Prion disease (e.g., Kuru, CJD or Cruetzfeldt-Jakob disease and vCJD, mad cow, scrapie, bovine spongiform encephalopathy or BSE) is a disease of glia as much as of neurons. Astrocytes infected with the diseased prior release cytokines and neurotoxic agents and neurons die as a result. When oligodendrocytes becomes diseased, the myelin sheath insulating axons suffers damage. In response to prion infection, microglia seek out and selectively kill neurons in prion disease. Note: PrP is a normal protein in neurons that becomes mutated and infectious in prion disease.(Fields, R. Douglas, PhD. The Other Brain. p 124-130. NY: Simon & Schuster, 2009.)

Schwann cells attach to neuronal axons and form a layer of electrical insulation, myelin, around large-diameter axons. Or they embed several small axons inside themselves much like a bun wrapped around several hotdogs, to provide structural support for the axonsThere are three types of Schwann cells: nonmyelinating, terminal, and myelinating that form myelin insulation on some axons (all outside the brain and spinal cord). (Fields, R. Douglas, PhD. The Other Brain. p 15, 24, 30. NY: Simon & Schuster, 2009.)

Microglia and astrocytes act together as sentinels. They watch for bacteria and viruses that infect the brain and then mobilize to fight the invading microbes. (Fields, R. Douglas, PhD. The Other Brain. p 65-66. NY: Simon & Schuster, 2009.)

Spinal cord injury tends to be permanent, sudden, and profoundly life-altering. Three out of four people in wheelchairs are men or boys, victims of male attraction to fast cars, motorcycles, sports, and a propensity to engage in violence. The initial death of neurons and oligodendrocytes results from the blow to the spinal cord that sheared open cells, and to the release of neurotransmitters (e.g., glutamate) spilled in topic levels from damaged neurons. The disrupted blood flow from vascular damage kills other cells, as well. A second wave of cell death results from toxic conditions in the damaged region caused by microglia and astrocytes battling the injury. There are no Schwann cells to help with repair as there are in the peripheral nervous system. (Fields, R. Douglas, PhD. The Other Brain. p 80-90. NY: Simon & Schuster, 2009.)

There are four main types of glial cells. Schwann cells in nerves and oligodendrocytes in the brain and spinal cord, both of which form myelin insulation on axons. Plus, astrocytes and microglia. The microglia protect the brain from injury and disease, their own body guard. On average there are about 6 glial cells for every neuron, although they come in differing ratios in different parts of the brain. (Fields, R. Douglas, PhD. Pg 23. NY: Simon & Schuster, 2010)

The key difference between vertebrates and invertebrates is that vertebrates have myelin-forming glial cells (invertebrates do not). The complex, centralized nervous system of the vertebrates, with myelin insulation wrapped around axons like electrical tape is wrapped around wire, is a fundamental difference that separates these two great groups of creatures. (Fields, R. Douglas, PhD. The Other Brain. p 38-40. NY: Simon & Schuster, 2009.)

Viruses can attack and infect cells in the brain, spinal cord, and nervous system. For example:

  • HIV – infects glia (not neurons) especially microglia and astrocytes and can lead to dementia in the central nervous system; infects Schwann cells in the peripheral nervous system
  • Polio – infects and kills motor neurons in the spinal cord
  • Herpes – infects sensory neurons (Type 1 above the waist, Type 2 below the waist)

(Fields, R. Douglas, PhD. The Other Brain. p 128-132. NY: Simon & Schuster, 2009.)

Astrocytes respond to visual stimulation and participate in vision by controlling neurons. (Fields, R. Douglas, PhD. The Other Brain. p 49-50. NY: Simon & Schuster, 2009.)

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