SN5
SHORT NOTE
5M
★ High-yield
Describe the changes following section of a peripheral nerve. Add a note on regeneration.
Definition
Wallerian degeneration = the anterograde degeneration of the axon and myelin sheath distal to the site of a nerve injury.
Changes After Nerve Section
| Site | Change | Timing |
|---|---|---|
| Distal segment | Wallerian degeneration — axon fragments, myelin breaks into droplets, debris removed by Schwann cells and macrophages | Begins 24 h, complete by 1 week |
| Proximal segment | Retrograde (traumatic) degeneration — back to one or two nodes | Few days |
| Cell body | Chromatolysis — Nissl granules disperse, cell swells, nucleus moves to periphery | 2 days – 3 weeks |
| Muscle | Denervation atrophy, fibrillation | Weeks |
Regeneration
Schwann cells proliferate → Form bands of Büngner (guiding tubes) → Axonal sprouts grow from proximal stump → One sprout enters the tube and grows at 1 mm/day → Reaches the end organ → Remyelination and functional recovery
- Rate about 1 mm/day (roughly 1 inch per month)
- Recovery is never complete — conduction velocity stays below normal because internodes are shorter
Peripheral VS Central Nerve
| Feature | Peripheral nerve | CNS |
|---|---|---|
| Myelin formed by | Schwann cell | Oligodendrocyte |
| Regeneration | Occurs | Does not occur |
| Reason | Schwann cells provide guiding tubes and growth factors | Glial scar; oligodendrocytes release inhibitors (Nogo, MAG) |
Classification of Nerve Injury (seddon)
| Type | Lesion | Recovery |
|---|---|---|
| Neurapraxia | Conduction block only; no degeneration | Complete, in days to weeks |
| Axonotmesis | Axon severed; sheath intact | Good — regeneration along intact tubes |
| Neurotmesis | Complete section of nerve and sheath | Poor without surgical repair; neuroma may form |
Applied Aspects
- Saturday night palsy — radial nerve neurapraxia from pressure; recovers fully
- Nerve grafting and microsurgical repair improve outcome in neurotmesis
- Tinel sign — tapping over the regenerating nerve produces tingling distally; used to follow the advancing front of regeneration
All questions in Nerve & Muscle Physiology
- Q1Define action potential. Describe its phases and ionic basis. Add a note on the refractory period. ★
- Q2Describe the structure of the neuromuscular junction and the steps of neuromuscular transmission. Add a note on the drugs acting on it. ★
- Q3Describe the sliding filament mechanism of skeletal muscle contraction. Explain excitation–contraction coupling and the role of calcium. ★
- Q4Classify nerve fibres. Describe the mechanism of impulse conduction and compare saltatory with continuous conduction. Add a note on nerve degeneration and regeneration. ★
- Q5Compare the structure and properties of skeletal, cardiac and smooth muscle. Describe the mechanism of smooth muscle contraction. ★
- SN1Write a short note on myasthenia gravis. ★
- SN2Write a short note on rigor mortis. ★
- SN3Define motor unit. Describe the innervation ratio and the size principle. ★
- SN4Differentiate between isotonic and isometric contraction.
- SN5Describe the changes following section of a peripheral nerve. Add a note on regeneration. ★
- SN6Describe the length–tension relationship of skeletal muscle and give its significance.
- SN7Write a short note on muscle fatigue. Add a note on oxygen debt.