"Our results suggest that long-term meditators have white-matter fibers that are either more numerous, more dense or more insulated throughout the brain. We also found that the normal age-related decline of white-matter tissue is considerably reduced in active meditation practitioners.
It is possible that actively meditating, especially over a long period of time, can induce changes on a micro-anatomical level.
Meditation, however, might not only cause changes in brain anatomy by inducing growth but also by preventing reduction. That is, if practiced regularly and over years, meditation may slow down aging-related brain atrophy, perhaps by positively affecting the immune system.
[However], it's possible that meditators might have brains that are fundamentally different to begin with. For example, a particular brain anatomy may have drawn an individual to meditation or helped maintain an ongoing practice — meaning that the enhanced fiber connectivity in meditators constitutes a predisposition towards meditation, rather than being the consequence of the practice.
Meditation appears to be a powerful mental exercise with the potential to change the physical structure of the brain at large. Collecting evidence that active, frequent and regular meditation practices cause alterations of white-matter fiber tracts that are profound and sustainable may become relevant for patient populations suffering from axonal demyelination and white-matter atrophy."
~ Eileen Luders, a visiting assistant professor at the UCLA Laboratory of Neuro Imaging, quoted in "Is Meditation the Push-Up for the Brain?" by Mark Wheeler, UCLA Newsroom, July 13, 2001