House of Mind

"Biology gives you a brain. Life turns it into a mind" - Jeffrey Eugenides

  • 5th March
    2012
  • 05

List: Astrocyte Functions


Astrocytes

Astrocytes are a special kind of stellate-shaped brain cells that are found throughout the central nervous system and that play a supportive role for neurons. For a long time, astrocytes were thought of as merely providing “assistance” for neuron function and survival. However, the discovery that astrocytes express voltage-gated channels and neurotransmitter receptors suggests the possibility of an active role for astrocytes in neuronal communication. Astrocytes primarily originate come from either radial glia cells or from cells in the sub ventricular zone and can be visualized with glial fibrillary acidic protein (GFAP). Below is an image of GFAP staining for astrocytes. 

GFAP Stain

Other groups of glia: 

  • Oligodendrocytes: Provide myelin sheath in neurons present in the central nervous system (CNS). Each oligodendrocyte can myelinate multiple axons. 
  • Schwann Cells: Myelinate axons of neurons present in the peripheral nervous system (PNS). Schwan cells, however, only myelinate one axon. 
  • Microglia: Derived from bone marrow and function as antigen presenting cells. Microglia have phagocytic activity, which means they “eat up” (or clear) cellular debris and their roles are predominantly host defense. 

Astrocyte Functions:

  • Regulation of brain extracellular pH via secretion of acid into the extracellular space (aka potassium buffering). Other regulatory functions of astrocytes include limiting the rise of both extracellular potassium (K+) and pH during neural activity. In addition, astrocytes can take up potassium in a variety of ways: Na+-K+ exchange, K+-Cl- cotransport and other K+ channels characterized by distinct properties. 
  • Regulating the uptake of glutamate near the synaptic cleft. 
  • Astrocytes can serve as signaling elements within an astrocyte network, between astrocytes and blood vessels, and/or between astrocytes and neurons. For example, astrocytes can signal to other neurons via Ca+2 oscillations (otherwise known as calcium waves). These calcium waves can come about in two ways: they are either triggered by neural activity (such as activation of astrocyte glutamate receptors) or spontaneoulsy via calcium release from internal stores and activation of IP3 receptors. Astrocytes may also serve as neurotransmitter transporters and receptors as well as aiding in neurotransmitter catabolism.
  • Modulate synaptic and neural activity via “gliotransmission”. Known gliotransmitters (chemicals that can act on neighboring neurons, glial cells or vessels) include glutamate, cytokines, ATP, and D-serine.  As illustrated below, astrocyte processes govern the amount of neurotransmitter spillage around synapse, thus controlling lateral spread of excitation. 
  • Modulation of brain vascular tone (i.e. vasodilation/vasoconstriction) and promotion of neurovascular coupling. Basically, astrocytes regulate cerebral blood flow. Moreover, vascular tone depends on the release of vascular agents into the perivascular space. 
  • Control of synapse formation, stabilization and function as well as neurogenesis. These roles have been predominantly explored in the context of brain pathology and psychiatric disorders like ALS, Alzheimer’s, brain tumors, traumatic brain injury and ischemia. 

Sources: 

Chesler, Mitch. Properties of the brain, extracellular space and astrocyte function. Lecture given as part of the cellular neuroscience course. Fall 2009. 

Volterra A & Meldolesi J. 2005. Astrocytes, from brain glue to communication elements. Nature Reviews Neuroscience. 6 (8): 626-40. 

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