Star-shaped glial cells that wrap synapses, buffer chemicals, feed neurons, control blood flow and help build the blood-brain barrier — and may take part in processing information.
Cells of the Nervous System
Intermediate
Astrocytes
The Editors
January 13, 2026
2 min read
In plain English
Astrocytes are star-shaped helper cells that wrap around synapses and blood vessels. They clear away used chemicals, pass energy to neurons, and help increase blood flow to busy parts of the brain. Some scientists think they help with brain signalling, too.
Intermediate level: Builds on the basics. Introduces the real vocabulary and the mechanisms behind it.
Concepts to know first
Astrocytes are the most abundant glial cells in many parts of the brain. Each has many fine branches, and a single human astrocyte can contact well over a hundred thousand synapses — more than in rodents, which has led some to speculate they contribute to the human brain's capacities.
Housekeeping
When neurons fire, potassium ions build up outside them and glutamate is released at synapses. Too much of either is dangerous: excess glutamate over-excites and can kill neurons (excitotoxicity). Astrocytes take up glutamate through transporter proteins and convert it to glutamine, which they hand back to neurons to recycle. They also buffer potassium, redistributing it through networks of astrocytes linked by gap junctions.
Energy and blood flow
Astrocytes store glycogen, the brain's only significant energy reserve, and according to the astrocyte–neuron lactate shuttle hypothesis they pass lactate to active neurons as fuel (the details are debated). Their endfeet wrap around blood vessels, where they help form and maintain the blood-brain barrier and contribute to neurovascular coupling — the local increase in blood flow when a brain region becomes active. That coupling is what functional MRI measures.
The tripartite synapse
Because astrocyte branches surround many synapses, researchers speak of the tripartite synapse: the presynaptic neuron, the postsynaptic neuron, and the astrocyte. Astrocytes respond to neurotransmitters with rises in internal calcium and can release signalling molecules of their own (so-called gliotransmitters). How much this shapes information processing in the living brain is an active and contested research area.
After injury
When the brain or spinal cord is injured, astrocytes become "reactive" and can form a glial scar. The scar walls off damage and limits inflammation, but it can also block axons from regrowing.
Everyday examples & things to try
Analogy
The bartender at a busy bar
At a synapse, neurons are the customers shouting orders. The astrocyte is the bartender: clearing away empty glasses (glutamate), restocking (energy), keeping the noise from spilling over to the next table (buffering) — and occasionally joining the conversation.
Key takeaways
- Astrocytes clear glutamate and potassium, protecting neurons from over-excitation.
- They link neurons to blood vessels, helping control blood flow and the blood-brain barrier.
- The 'tripartite synapse' idea gives astrocytes a possible role in signalling.
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