What is the functional relationship of the peripheral nervous system to the central nervous system?

Mary complains that when she wakes up in the morning, her thumb and forefinger are always “asleep.” She mentions this condition to her physician, who asks Mary whether she sleeps with her wrists flexed. She replies that she does. The physician tells Mary that sleeping in that position may compress a portion of one of her peripheral nerves, producing her symptoms. Which nerve is involved?

Explain why damage to peripheral nerve fibers is often reversible, whereas damage to CNS fibers rarely is.