Glial cells

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Appendix A endnote 2, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

The neurotransmitters excite or inhibit each neuron on the other end of a synapse, changing its rate of firing. [...] The transmission is made more or less efficient by glial cells.

Glial cells are found in the nervous system.[1][2][3][4][5] The main difference between a neuron and a glial cell is that a neuron has dendrites and an axon and therefore can generate an electrical signal (an “action potential”). They respond to neuronal activity themselves and communicate with other cells using chemicals (i.e., they release neurotransmitters in a nonspecific way). There are three general types of glial cells in the central nervous system (astrocytes, microglia, and oligodendrocytes) and they are more numerous than neurons. They wrap themselves around the synapses between neurons.

  • Oligodendrocytes help to wrap the axons of neurons which speeds their ability to convey information electrically (i.e., they create the myelin sheath around the axon of a neuron to speeds the movement of the action potential).
  • Astrocyctes are the most common cells in the central nervous system and regulate synaptic activity. Astrocytes wrap around the gaps between neurons, called synapses, and regulate the chemical environment during synaptic transmission to make neural communication more or less efficient. Astrocytes, are thought to help synchronize neural activity in the brain[6] (which helps to produce the brain states for sleep, thoughts, memories, and, of course, emotions). A single astrocyte wraps up to 140,000 synapses, and influence somewhere between 300 to 600 dendrites of four to six neuron bodies in a rat brain.[3] They help to clear excess neurotransmitters from the synaptic clef. They can also impair the ability of neurons to inhibit one another, which causes illness.[7]
  • Microglia modulate synaptic efficiency[3] by releasing cytokines and controlling inflammation in the brain. In this way, they sculpt the nervous system by killing neurons (in the name of metabolic efficiency). This can be helpful (e.g., neural pruning during childhood) or contribute to illness.

Some scientists call glia the “dark matter” of the nervous system because their function was not discovered until recently.

Notes on the Notes

  1. Salter, Michael W., and Simon Beggs. 2014. "Sublime microglia: expanding roles for the guardians of the CNS." Cell 158 (1): 15-24.
  2. Grace, Peter M., Mark R. Hutchinson, Steven F. Maier, and Linda R. Watkins. 2014. "Pathological pain and the neuroimmune interface." Nature Reviews Immunology 14 (4): 217-231.
  3. 3.0 3.1 3.2 Ji, Ru-Rong, Temugin Berta, and Maiken Nedergaard. 2013. "Glia and pain: is chronic pain a gliopathy?" PAIN 154 (supplement 1): S10-S28.
  4. Clarke, Laura E., and Ben A. Barres. 2013. "Emerging roles of astrocytes in neural circuit development." Nature Reviews Neuroscience 14 (5): 311-321.
  5. Fields, R. Douglas, Alfonso Araque, Heidi Johansen-Berg, Soo-Siang Lim, Gary Lynch, Klaus-Armin Nave, Maiken Nedergaard, Ray Perez, Terrence Sejnowski, and Hiroaki Wake. 2014. "Glial biology in learning and cognition." The Neuroscientist 20 (5): 426-431.
  6. Poskanzer, Kira E., and Rafael Yuste. 2016. "Astrocytes regulate cortical state switching in vivo." Proceedings of the National Academy of Sciences 113 (19): E2675-E2684.
  7. Robel, Stefanie, and Harald Sontheimer. 2016. "Glia as drivers of abnormal neuronal activity." Nature Neuroscience 19 (1): 28-33.