Wallerian degeneration is a form of degeneration occurring in nerve fibers as a result of their division; - so called from Dr. Waller, who published an account of it in 1850.
Wallerian degeneration is distal degeneration of axons and their myelin sheaths secondary to injury to the neuron cell body. Macrophages/schwann cells digest the nerve.
Target tissue (i.e. the innervated muscle) is denervated, and thus atrophies