Photograph courtesy of Rafael Yuste, Columbia University, New York, USA.

Dendritic spines move; stubby, thin or mushroom alike. The question is what is the purpose of their relentless motility. Some clues have come from the observation that spines move during synaptogenesis and in response to synaptic stimulation. However, it is not clear whether this motility has any effect in the functional properties of the spine and the dendrite. To approach this problem, Majewska et al. have analysed the way in which diffusion into and out of the spine is altered in response to changes in the length of the spine neck using two independent strategies.

First, the authors transfected cortical cultured slices with enhanced green fluorescent protein, photobleached the fluorescence from individual spines, and measured the time constant for the recovery of the label. They observed a positive correlation between the time constant and the length of the spine. Furthermore, they studied the same population of spines at different time points to find out whether the continuous changes in length would have a similar dynamic effect on diffusion. Again, they found that a given change in neck length was accompanied by a proportional change in the time constant for diffusion.

Second, Majewska et al. evoked calcium transients in spines of neurons from acute slices by electrical stimulation. They found that the fast component of calcium decay showed a clear correlation to the length of the spine and that motility also had a dynamic effect on diffusion.

As one of the main functions of spines may be to act as biochemical compartments, the effect of length on calcium dynamics could have profound implications for synaptic function and plasticity. At the same time, the fact that bi-directional changes in length occur within minutes indicates that the efficacy of a given synapse might be subject to a more dynamic regulation than commonly thought. Moreover, their recent observation that Rho GTPases are involved in the regulation of spine length will enable us to determine how spine dynamics interact with synaptic activity.