The authors focused on the surface layers of the neocortex. To allow direct observation of the area, they implanted a glass window over the two areas of the visual cortex in four- to six-week-old mice. These mice express fluorescent protein in neocortical neurons, allowing Lee et al. to track the location and morphology of these neurons using two-photon microscopy. Time-lapse images of six pyramidal neurons and eight nonpyramidal neurons in 13 mice were taken over the course of four to ten weeks. The length of dendritic branch tips was measured over time to evaluate physical changes in the neurons.
Many studies have focused on pyramidal neurons--excitatory neurons that promote neuron firing--but few have focused on the possible structural dynamics of a range of different neuron types. Surprisingly, the only neurons they saw growing were the so-called interneurons, which contain the neurotransmitter GABA and inhibit the activity of cortical neurons. The pyramidal neurons showed no structural changes in individual branch tips, but the inhibitory neurons showed dynamic changes, with one branch tip undergoing dramatic remodeling. "Within as little as two weeks," the authors note, "this branch tip more than doubled its length and exited the imaging volume." One nonpyramidal neuron even showed a few new branch tip additions.
The authors conclude that the cortic al neurons have different dynamic properties. The branch tips of interneurons in the adult neocortex can grow, retract, and sprout new additions--without experimental manipulations. It remains to be seen whether the structural plasticity seen here underlies observed functional reorganizations. Probing this question will depend on determining what kinds of structural changes might be expected, figuring out how to detect them, and then interpreting the changes. Studying the responses of axonal arbors connected to the nonpyramidal dendrites, for example, may prove instructive. Based on these results, direct observation of specific neurons in a local pathway should yield promising results.