A healthy person's brain has thousands, or maybe millions, of times as many synapses - contact points that relay signals from one nerve cell, or neuron, to the next - as there are stars in the Milky Way. In a blog entry not so long ago, I wrote:
In a sense, "you" are your synapses. They're the defining features of the brain circuits that fire up or chill out to generate every thought that passes through your mind and every flicker of emotion or glimmer of recollection you experience. You wouldn't want to leave home without them. Some of us get no choice. Massive synapse loss accompanies neurodegenerative diseases from Alzheimer's to Parkinson's to multiple sclerosis.
In a paradigm-shifting discovery just published in Nature, Stanford researchers found that a common but mysterious class of non-neuronal brain cells, called astrocytes because of their star-like shape, actively refine nerve-cell circuits throughout life by selectively eliminating synapses. much as a sculptor chisels away excess bits of rock to create an artwork.
Astrocytes, which account for between one-third and half of all the cells in our brains, belong to a general category of brain cells collectively called "glia," from the Greek term for glue. That's because until not long ago, neuroscientists (note the bias in that term) figured that these seemingly passive cells weren't much more than packing peanuts for the "really cool" cells, neurons, that hog the lion's share of attention.
Neurons, which compose merely 10 percent of all the cells in a healthy human brain, are so celebrated they've lent their name to brain science: neurology. But a team led by pioneering Stanford gliologist (I made that term up) Ben Barres, MD, PhD, has shown in the new study that astrocytes play a starring role in sculpting the very synaptic connections that are central to consciousness, cognition, motion and emotion.
As I once noted:
[Barres] - who trained as a practicing neurologist before circling back for his PhD - couldn't help noticing, in autopsies, that the brains of people with "neurological" disorders invariably showed obvious signs of glial disarray. So he resolved to devote his career to the study of glial cells...
Thus began a decades-long efforts to unveil the manifold functions of astrocytes and their glial comrades-in-arms (described in detail in this Stanford Medicine article). Barres and his colleagues have already shown that astrocytes in particular (there are two other glial cell types) play a critical role in determining exactly where and when new synapses are generated. With the finding that astrocytes also single out outmoded or downright counterproductive synapses and gobble them up, it's increasingly certain that substantial remodeling of brain circuits takes place in the adult brain and that astrocytes are master sculptors of its constantly evolving synaptic architecture. This holds huge implications, according to my press release on the new study:
The findings also raise the question of whether deficits and excesses in this astrocytic function could underlie, respectively, the loss of this remodeling capacity in old age or the wholesale destruction of synapses that erupts in neurodegenerative disorders, such as Alzheimer's and Parkinson's disease.
Previously: Protein known for initiating immune response may set our brains up for neurodegenerative disorders, Unsung brain-cell population implicated in variety of autism and Brain imaging, and the 'image management' cells that make it possible
Photo by Ed Bierman