Star-shaped brain cells once dismissed as packing turn out to talk back
Astrocytes, named for their star shape, were long seen as scaffolding for neurons. Since the mid-1990s researchers have watched them pass calcium waves across long distances and release their own chemical messengers. A single human astrocyte can wrap around as many as two million synapses at once.
These glial cells of the brain and spinal cord come in three main types. Fibrous astrocytes live in white matter, carry few organelles and send out long unbranched arms, some ending in feet pressed against capillaries. Protoplasmic astrocytes, the most common, sit in grey matter with short, heavily branched processes. Radial glia, arranged at right angles to the brain's fluid-filled ventricles, mostly exist during development to guide migrating neurons, though Müller cells in the retina and Bergmann glia in the cerebellum persist into adulthood. Many astrocytes are identified in the lab by a protein called GFAP.
Their job list is long. They store glycogen and can release glucose to neurons when demand spikes, and they hand over lactate as fuel. They sense glucose levels in the brain themselves, and when it runs low they can speed up stomach emptying. Densely packed potassium channels let them mop up the potassium neurons release when active, and they act as the central nervous system's main store and distributor of copper, manganese, zinc and iron.
They also shape signalling. The idea of a tripartite synapse treats a glial element as a third partner alongside the sending and receiving neurons. In the hippocampus astrocytes release ATP that breaks down to adenosine, dampening transmission. When neurons fire, the ATP they release prompts astrocytes to secrete a protein called leukemia inhibitory factor that pushes oligodendrocytes to build myelin. Their activity is tied to cerebral blood flow, which may be what fMRI scans actually register.
Energy accounting has shifted too. Early estimates gave astrocytes 5 percent of grey matter signalling energy against 95 percent for neurons. Once action potentials proved more efficient than assumed, the budget was redrawn with dendrites at 70 percent, axons at 15 and astrocytes at 7. After injury they fill damaged space with a glial scar, long seen as a barrier to regrowth, though their role in repair remains poorly understood.
Source: Astrocyte
