Astrocyte functions: glutamate uptake, potassium buffering and the blood-brain barrier
Astrocytes are star-shaped glial cells (the supporting cells of the nervous system) that look after the chemical environment around neurones.
Remember
Astrocytes keep the neuronal environment stable: they clear excess glutamate (a neurotransmitter) and potassium, support energy supply and help form the blood-brain barrier (a highly selective barrier that regulates what is transported into the CNS).
Memory aid
astrocytes are the star-shaped housekeepers of the brain
Picture a housekeeper who feeds neurones, mops up glutamate and potassium and helps guard the blood-brain barrier.
Astrocytes are the most abundant glial cells. They look after the chemical environment around neurones, like star-shaped housekeepers (astro means star).
Astrocytes clear glutamate. Once a neurone has released this transmitter, astrocytes take it back up from the synaptic cleft (the tiny gap between two neurones). This reuptake prevents excitotoxicity, which is nerve cell damage from over-stimulation by glutamate. If astrocytes are disturbed, the transmitter lingers and signalling is disrupted.
Astrocytes also handle potassium, which leaks out of neurones every time they fire, so they take it up and buffer it. This is potassium buffering. If either potassium or glutamate builds up, neurones become hyperexcitable (they depolarise too easily), which promotes seizures.
They also support energy supply. Glycogen, the stored form of glucose, is held predominantly in astrocytes in the brain, and astrocytes are often described as passing lactate (a fuel made from glucose) to neurones.
They also help form the blood-brain barrier. The barrier itself is made of endothelial cells (the cells lining the capillaries) and is supported by astrocytes.
Astrocytes are not electrically excitable, so they do not fire action potentials. Other glial cells do different jobs: oligodendrocytes (which wrap axons in myelin), microglia (the immune cells, which engulf debris and microbes) and ependymal cells (which line the ventricles, the fluid-filled cavities of the brain, and help to make cerebrospinal fluid at the choroid plexus, where they meet blood vessels).
Key facts
- Glutamate
- astrocytes take it up from the synaptic cleft, which prevents excitotoxicity.
- Potassium
- astrocytes take up and buffer the potassium released when neurones fire.
- Energy
- most brain glycogen is stored in astrocytes, which can supply neurones with lactate.
- Blood-brain barrier
- astrocytes help to form it, and the barrier itself is made of endothelial cells.
- Not electrically excitable
- astrocytes do not fire action potentials.
Exam traps
- Myelinating axons is not an astrocyte job, because oligodendrocytes are the glial cells that insulate central nervous system axons with myelin.
- Phagocytosis is not the astrocyte role, because microglia are the immune cells that engulf debris and microbes.
- Making cerebrospinal fluid does not belong to astrocytes, because ependymal cells line the ventricles and, at the choroid plexus, help to make it.
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