If a region is underutilized, its neighbors soon recruit the cells there to help with their own projects. Neurons that are destroyed are not, in general, replaced the way skin cells and bone cells and blood cells (among others) are. Nerve “regeneration” is still mainly a dream of bioengineers, not a normal feature of nervous systems, so the plasticity observed in many experiments must be due to neurons being reassigned to new tasks or required to take on additional workloads.
