Max Planck Institute of Neurobiology, Martinsried, Germany

A neuroscientist asks whether neurons are enough when it comes to learning.

One of my main scientific interests is understanding how the brain adapts to and learns from experience. By ‘brain’ I normally mean networks of neurons, because the electrical impulses they produce are the common currency of sensation and perception; learning involves changes in the synaptic connections between these cells driven by sensory experience. But it’s beginning to seem that the brain’s plasticity depends on more than neurons alone.

A few months ago, I found myself debating with a colleague what role glial cells might play. I was sceptical. Some years ago, the various sorts of non-neuronal cells that go by the name of glia were thought of simply as a glue (which is what glia means in greek) that holds the brain together. More recently, though, these cells have been shown to form extensive networks important for regulating the local brain environment and to communicate with neurons.

Despite knowing all of this, I had not considered glia as serious players in neuronal plasticity. My thinking changed after reading two recent studies showing that glia not only change their activity after sensory stimulation (X. Wang et al. Nature Neurosci. 9, 816–823; 2006) but also influence the strength of synaptic connections on neurons via a secreted soluble protein (D. Stellwagen and R. Malenka, Nature 440, 1054–1059; 2006). Because the amount of secreted factor depended on the level of surrounding neuronal activity, these results may provide a glial link between sensory experience and synaptic plasticity. The challenge is now to show this directly in the intact brain. I am willing to give it a try — the glia may have changed my mind!