In a fascinating study published in Nature, researchers found that astrocytes that exist in the brain to protect neurons take on a more toxic role following a brain injury.

In a healthy brain, astrocytes assist with developing and protecting the vast network of connected neurons.  Upon injury, microglia induce toxic activated A1 astrocytes that instead of protecting neurons instead act to remove dysfunctional and impaired neurons.  Importantly, the removal of damaged-but-still-functional neurons may contribute to a further decline in cognitive functioning, with some tests showing a 50% decline in neuronal activity together with weaker signalling.  Moheb Constandi of ScientificAmerican.com summarized this:

To determine whether A1 astrocytes retain either the building or repair functions, Liddelow and his colleagues grew retinal ganglion cells, which form the optic nerve, together with either A1 astrocytes or nonactivated astrocytes.  Petri dishes with A1 astrocytes added to them contained approximately 50 percent fewer synaptic connections compared with those holding nonactivated ones. And the remaining synapses were significantly weaker, producing smaller and less frequent signals than those in the dishes containing nonactivated astrocytes.

Together with removal of damaged debris, suppression of oligodendrocytes was also observed.  Oligodendrocytes are known to assist with the myelin sheath layer which is responsible for insulating axons to better transmit electrical signals.  Failing of this layer is observed in conditions like multiple sclerosis and are understood to contribute to headaches and migraines.

Read article at ScientificAmerican.com
Read journal article published in the Nature International Weekly Journal of Science (paid access).