Explain the regeneration of the PNS and the CNS???

Peripheral neurons can regenerate very easily. They're normally sheathed in myelin, produced by Schwann cells. When a neuron is damaged, the portion downstream from the axon degenerates and is absorbed by the remaining Schwann cells (this process is called Wallerian Degeneration), and the myelin is cleared away. The Schwann cells stay in place, though, and provide a path for the regenerating axon to follow. Once it reaches its destination, it should be close enough that it can synapse with the nerve's original target (though it often is not perfect). PNS neurons usually regenerate at a rate of around 1mm a day, so complete regeneration depends on the length of the axon, and where it was injured.

The Central Nervous System doesn't do quite as well. CNS neurons are usually sheathed in oligodendrocytes. When the axon is damaged, it degenerates, but the debris and myelin doesn't get cleared away, and the oligodendrocytes themselves often undergo apoptosis. Since the debris is cleared away so slowly, and the oligodendrocytes don't leave a "trail of breadcrumbs" like Schwann cells do, the process is much slower (if it happens at all).

That's not to say that CNS regeneration is impossible. The brain contains a fair amount of neuronal progenitor cells that can sometimes re-form connections - it's not true regeneration, though. The idea that CNS neurons don't regenerate is based on old research. It was basically conducted in primates kept in bare cages. It's thought that the extreme lack of stimuli in their enclosures had a role in this, since synapses are reinforced by activity. The new research is pretty promising - with proper stimuli, repair is possible (even if not to the pre-injury level).