Nerve regeneration begins about 20 days after nerve section and is complete in 3 months. The Schwann cells at the site of injury multiply and grow at the rate of up to 1 mm per day, forming a solid cord of elongated cells (band of Bungner) within the endoneurial tube.
The plasmalemma of the Schwann cells and adjacent basal lamina separate is creating an annular compartment between the Schwann cells and the endoneurium. Up to 100 axonal sprouts, each containing a neurofibril in it grows out in all directions from the proximal axon.
Some of them grow into the distal annular compartment. The daily rate of growth is up to 3-4 mm in the peripheral stump. Eventually all but one axonal sprout degenerate. The surviving fibril enlarges to fill the distal tube.
The Schwann cells in the Band of Bungner form myelin sheath around the reinnervating axonal sprout. The sheath begins to develop in about 15 days and is completed in one year. The Nissl substance and Golgi apparatus gradually reappear. The cell regains its normal size and the nucleus returns to its central position.
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According to Seddon’s classification, nerve injuries are of three types: (i) neuropraxia, which occurs due to minor nerve stretch or pressure causing ischemic injury to the nerve.
It results in conduction block without causing any structural damage; (ii) axonotemesis, which occurs due to excessive stress injury to the nerve. It results in Wallerian degeneration.
The basal lamina of Schwann cells and other sheaths are all intact, and (iii) neurotemesis, which as a result of penetrating injury to the nerve. All the sheaths are disrupted.