Nerve damage and repair

Summary

Dr. Simon Friar discusses axonal loss and nerve regeneration in this video, covering key topics such as the basic anatomy of nerve connective tissue, Wallerian degeneration, and reinnervation. He explains classifications of nerve injuries, including neuropraxia, axonotmesis, and neurotmesis, detailing how each affects nerve function and recovery. Dr. Friar also explores how electromyography (EMG) signals change as nerves regenerate and muscles react. Through various stages of nerve trauma and regeneration, he emphasizes the roles of Schwann cells, axonal regrowth, and motor unit recruitment in nerve recovery.

Takeaways

• 🧠 The video covers axon loss and regeneration, starting with an overview of nerve anatomy and the stages of degeneration and reinnervation.
• 👨‍⚕️ Motor units consist of a motor neuron and all muscle fibers it innervates. These motor units combine to form a motor pool within a muscle.
• 🛡️ Nerves are structured into fascicles, with axons surrounded by Schwann cells and myelin sheaths, enclosed by connective tissue sheaths (endo-, peri-, and epineurium).
• 🔄 After a nerve lesion, Wallerian degeneration occurs where axons disintegrate distal to the injury site, followed by Schwann cells clearing debris and promoting regrowth.
• 💉 Different types of nerve injuries (neuropraxia, axonotmesis, neurotmesis) affect nerve recovery, with varying degrees of axon and myelin damage and regeneration potential.
• ⚡ Neuropraxia involves a temporary block in conduction, with no axon loss. Full recovery is expected without Wallerian degeneration.
• 🔧 In axonotmesis, axons and myelin are damaged but connective tissue sheaths are intact, allowing for guided regeneration.
• 🚨 Neurotmesis, the most severe injury, disrupts both axons and connective sheaths, leading to more difficult regeneration and uncertain recovery.
• 🔍 EMG tests help assess nerve regeneration, identifying fibrillations, positive sharp waves, and abnormal recruitment patterns during various stages of nerve injury.
• 📈 Reinnervation processes, such as terminal axon sprouting, can be observed on EMG as satellite potentials, which mature over time as myelination and nerve function improve.

Q & A

What is a motor unit and how is it structured?

A motor unit consists of a motor neuron and all the muscle fibers it innervates. In a muscle, there are many motor units, which collectively form the motor pool. Muscle fibers from different motor units are interdigitated, meaning they are mixed across the muscle rather than arranged in discrete patches.

What is the structure of a nerve fiber?

A nerve fiber consists of an axon surrounded by a Schwann cell and its myelin sheath. This structure is encased by the endoneurium. Groups of axons come together to form fascicles, which are surrounded by the perineurium and vasculature. The entire nerve is encased by the epineurium, which holds the fascicles together.

What happens during Wallerian degeneration and regeneration after a nerve injury?

Within 24 hours after a nerve lesion, the axon distal to the injury disintegrates, followed by the degradation of the myelin sheath. Schwann cells play a key role in clearing debris and forming tubes called Bungner bands, which secrete growth factors to attract the regrowth of the proximal axon. Axonal regrowth occurs at a rate of approximately 1 millimeter per day.

What is neuropraxia and how does it affect nerve function?

Neuropraxia is a physiological block of nerve conduction where the nerve is stunned but not permanently damaged. There is no axonal loss, so Wallerian degeneration does not occur. EMG findings may not show denervation signs, and recovery is typically good since the nerve has only been physiologically blocked.

How does axonotmesis differ from neurotmesis?

In axonotmesis, both the axon and myelin are disrupted, but the connective tissue sheaths remain intact. This allows for potential regeneration because the scaffolding for axonal regrowth is preserved. In neurotmesis, both the axon and connective tissue sheaths are damaged, making regeneration more difficult and the prognosis for recovery uncertain.

What role do Schwann cells play in nerve regeneration?

Schwann cells are crucial in clearing debris after a nerve injury and forming Bungner bands, which help guide axonal regrowth. They also secrete growth factors that attract the regenerating axon to the appropriate target.

What are fibrillations and positive sharp waves, and how do they relate to nerve injury?

Fibrillations and positive sharp waves are signs of denervation on EMG. They occur when muscle fibers become hyperexcitable due to upregulation of acetylcholine receptors, usually about 2.5 to 3.5 weeks after nerve injury. These findings indicate that Wallerian degeneration has occurred and the muscle is now denervated.

What is the significance of satellite potentials during nerve regeneration?

Satellite potentials are small, separate electrical signals seen on EMG during the early stages of nerve reinnervation. They indicate that surviving nerve fibers are sprouting new branches to reinnervate denervated muscle fibers. Initially, these potentials are unstable, but as reinnervation progresses, they stabilize and conduction speeds up.

What is the recruitment pattern of motor units and how does it change after nerve injury?

Normally, smaller motor units are recruited first, followed by larger ones, in an orderly fashion according to the size principle. After a nerve injury, recruitment becomes disordered because the brain tries to compensate for lost motor fibers, leading to a reduced interference pattern on EMG.

How does the EMG signal change in the chronic stage of nerve injury recovery?

In the chronic stage, motor units become larger and more polyphasic due to the summation of more muscle fibers contributing to the electrical signal. These units are recruited earlier than normal, and the interference pattern remains reduced, reflecting the incomplete reinnervation of the muscle.