Expansion
Endoneurium around axons, perineurium around fascicles, epineurium around the nerve
Expansion
From inside out:
- Endoneurium: surrounds each axon and its Schwann cell
- Perineurium: surrounds each fascicle; forms the blood-nerve barrier
- Epineurium: surrounds the whole nerve, carrying the vasa nervorum
Seddon classification of injury maps onto these layers:
- Neurapraxia: conduction block with an intact axon, usually from compression or ischaemia. Full recovery in days to weeks
- Axonotmesis: axon disrupted but endoneurial tube intact. Wallerian degeneration distally, then regeneration along the preserved tube at about 1 mm per day, with good recovery
- Neurotmesis: complete transection including connective tissue. Regeneration is disorganised and may form a neuroma; surgical repair is required
Wallerian degeneration begins within days: the distal axon and its myelin break down and are cleared by macrophages, while Schwann cells proliferate to form the guiding bands of Bungner.
This anatomy explains why a crush injury recovers well while a laceration may not, and why recovery time can be predicted from the distance to the target muscle.