05 December 2013

Astrocyte Might

For two centuries, scientists studying the brain focused on neurons and largely ignored neuroglia: a group of cell types that make up half our brain and spinal cord. Indeed, glial cells were considered little more than brain glue. But it’s now clear that they play an active role in brain function. Take astrocytes, for example (pictured). We now know that these cells, whose thousands of thread-like tendrils wrap around the junctions between neurons, can influence neural signaling. Researchers have also begun to explore whether astrocytes might explain why humans are more intelligent than other animals. One group showed that mice that received transplants of human astrocytes learned much more quickly than normal mice, suggesting that there may be something special about the human astrocyte (pictured right) – which is larger than the mouse version (left) – that contributes to the advanced computational abilities of the human brain.

Written by Daniel Cossins

Image by Maiken Nedegaard
University of Rochester, USA
Copyright Nedergaard Lab
Research published in Cell -Stem Cell, March 2013

Las neuronas del sistema nervioso central están sostenidas por algunas variedades de células no excitables que en conjunto se denominan neuroglia ( neuro = nervio; glia = pegamento). Las células en general son más pequeñas que las neuronas y las superan en 5 a 10 veces en número (50% del volumen del encéfalo y la médula espinal).

Hay cuatro tipos principales de células neurogliales, los astrocitos, los oligodendrocitos, la microglia y el epéndimo.

Astrocitos: Tienen cuerpos celulares pequeños con prolongaciones que se ramifican y extienden en todas direcciones. Existen dos tipos de astrocitos, los fibrosos y los protoplasmáticos. Los astrocitos fibrosos se encuentran principalmente en la sustancia blanca. Sus prolongaciones pasan entre las fibras nerviosas. Tienen prolongaciones largas, delgadas, lisas y no muy ramificadas. Contienen muchos filamentos en su citoplasma. Los astrocitos protoplasmáticos se encuentran en las sustancia gris, sus prolongaciones pasan también entre los cuerpos de las células nerviosas. Tienen prolongaciones más cortas, mas gruesas y ramificadas. El citoplasma contiene menos filamentos. Ambos, los fibrosos y los protoplasmáticos, proporcionan un marco de sostén, son aislantes eléctricos, limitan la diseminación de los neurotransmisores, captan iones de K+, almacenan glucógeno y tienen función fagocítica, ocupando el lugar de las neuronas muertas (gliosis de reemplazo).

Oligodendrocitos: Tienen cuerpos celulares pequeños y algunas prolongaciones delicadas, no hay filamentos en sus citoplasma. Se encuentran con frecuencia en hileras a lo largo de las fibras nerviosas o circundando los cuerpos de las células nerviosas. Las micrografías muestran que prolongaciones de un solo oligodendrocito se unen a las vainas de mielina de varias fibras. Sin embargo, sólo una prolongación se une a la mielina entre dos nodos de Ranvier adyacentes. Los oligodendrocitos son los responsables de la formación de la vaina de mielina de las fibras nerviosas del SNC. Se cree que influyen en el medio bioquímico de las neuronas.

Microglia: Son las células más pequeñas y se hallan dispersas en todo el SNC. En sus pequeños cuerpos celulares se originan prolongaciones ondulantes ramificadas que tienen numerosas proyecciones como espinas. Son inactivas en el SNC normal, proliferan en la enfermedad y son activamente fagocíticas (su citoplasma se llena con lípidos y restos celulares). Son acompañados por los monocitos de los vasos sanguíneos vecinos.

Epéndimo: Las células ependimales revisten las cavidades del encéfalo y el conducto central de la médula espinal. Forman una capa única de células cúbicas o cilíndricas que poseen microvellosidades y cilias. Las cilias son móviles y contribuyen al flujo de líquido cefaloraquídeo.

“… Glial cells, sometimes called neuroglia or simply glia (Greek γλία, γλοία "glue”; pronounced in English either /gliːə/ or/glaɪə/), are non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for neurons in the brain, and for neurons in other parts of the nervous system such as in the autonomous nervous system[1]. In the human brain, there is roughly one glia for every neuron with a ratio of about two neurons for every glia in the cerebral gray matter.[2]

As the Greek name implies, glia are commonly known as the glue of the nervous system; however, this is not fully accurate.Neuroscience currently identifies four main functions of glial cells: to surround neurons and hold them in place, to supplynutrients and oxygen to neurons, to insulate one neuron from another, and to destroy pathogens and remove dead neurons. For over a century, it was believed that they did not play any role in neurotransmission. That idea is now discredited;[3] they do modulate neurotransmission, although the mechanisms are not yet well understood.[3][4][5] …“

I scribbled together this diagram to try and make sense of the ANLSH - The astrocyte-neuron lactate shuttle hypothesis. The published literature isn’t entirely easy on this one. However, what I have gathered is that when neurons are firing more, they are using more energy. This will be reflected by the increased neurotransmitter (Glutamate; Glu) being released. Astrocytes at the synapse take up the glutamate once it has done its job, along with 3Na+ to maintain the electrochemical balance, but the excess Na+ must then be exported in an energy dependent manner. All in all, this import/export of Glu and Na+ leads to a corresponding energy consumption increase, with the neuron. As a a result, the astrocyte produces more Lactate, a by product of anaerobic glycolysis. The lactate is then shuttled out of the astrocyte and taken up by the active neurons to supplement their energy needs. The astrocyte also sends signals to local blood vessels to stimulate their dilation, increasing local blood flow, and increasing local oxygen and glucose to power the increased neuronal activity. All in all, the ANLSH and the associate physiology is a key concept in neuroenergetics, and underlies the interpretation of fMRI scans.

 Project Erythropoietin continues…

Giant Multipolar Neuron Smear

Tissue: Multipolar Nervous Tissue
Region (EOP on cell): Somal Cell Body
Structure (Thick Cell Process): Axon
Structure (Thin Cell Process): Dendrite
Structure (Small circular space in cell): Nucleus
Structure (Inside Nucleus): Nucleolus
Structure (Bodies around Nucleus): Nissl Bodies
Structure (Tiny bodies outside of cell): Neuroglia

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New discoveries on depression / able to cure memory dysfunction in ‘depressed’ rats by giving them doses of D-serine

[PRESS RELEASE 28 February 2012] During depression, the brain becomes less plastic and adaptable, and thus less able to perform certain tasks, like storing memories. Researchers at Karolinska Institutet have now traced the brain’s lower plasticity to reduced functionality in its support cells, and believe that learning more about these cells can pave the way for radical new therapies for…

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Sorry for the poor embed, but this is your brain on confocal microscopy.

This image is from Douglas Fields’ commentary in the latest issue Nature about the necessity of mapping glial cells alongside neurons in projects undertaken under the auspices of the NIH BRAIN Initiative.

While neurons are widely regarded as the basic unit of the nervous system, the function of neuroglia is still something of a mystery. Certain glial cells are known to be involved in supporting the activity of neurons in various ways, but they have also been increasingly investigated for their role in facilitating communication. Thus, if we ignore glial cells, our BRAIN initiative-supported maps will be missing a major componant.

The paper: 

Fields, D. (2013). Map the other brain. Nature, 501, 25–27. doi:10.1038/501025a