Flashcards in Degeneration and repair of the peripheral nervous system Deck (18)
What is significant about the population of neurones in our body?
We are born with all the neurones we will ever have.
What are the effects of ALS (motorneurone disease)?
- Causes degeneration of motor neurones.
- Leads to progressive paralysis.
What are the effects of Friedreich's ataxia?
- Causes sensory neurone degeneration.
- Leads to progressive loss of motor function.
What are the effects of poliomyelitis?
- Motor neurones are infected by the virus.
- Leads to progressive muscle weakness and paralysis.
What are the effects of syphilis?
- Causes degeneration of the sensory neurone dorsal root ganglia.
- Leads to loss of somatosensation (tabes dorsalis).
What are common demyelinating peripheral neuropathies?
- Charcot-Marie-Tooth disease: Causes muscle weakness, wasting and loss of sensation.
- Guillain-Barre syndrome: Autoimmune attack on myelin with similar symptoms to MS, but is transient.
What are the common mechanical causes of axon damage?
2. Trauma (compression/tearing) due to fractures
3. Trauma (compression/tearing) due to joint dislocation
4. Avulsion (pulling away) of spinal roots
What are the common causes of nerve compression?
1. Swelling (e.g. of local tendons in carpal tunnel syndrome)
3. Prolapsed intervertebral disc
What are the common causes of trauma-related nerve injuries?
What are the common causes of peripheral neuropathies?
Inadequate blood supply, e.g. frostbite, ischaemia, diabetes, leprosy...
What is the main difference between local and systemic nerve damage?
Systemic nerve damage produces bilateral loss of function while local nerve damage only causes unilateral loss of function.
What did River and Head show?
If a severed nerve is re-sutured, restoration of function is possible and nerves can regrow at rate of up to 1 mm/day.
What causes Wallerian degeneration?
Electrical/mechanical isolation of distal parts of axon from the soma of the neurone. This can occur as a result of compression/lesion.
What is the process of Wallerian degeneration?
1. Distal axon breaks down and myelin degenerates.
2. Schwann cells carry out phagocytosis to clear axon/myelin debris.
3. Schwann cells release neurotrophins that promote axon regeneration and guide the regenerating axon by chemotaxis.
4. Soma of cells undergo chromatolysis (dissolution of Nissl substance) to support axon growth.
5. As axon grows, Schwann cells form new myelin around new axon.
How do axons grow?
- They develop growth cones.
- These growth cones use contact and chemotaxis to guide the growth of axons.
- These cones use a combination of repel signals and attract signals to determine direction of growth.
What are the limitations of axon regeneration?
1. Incorrect connections are made. For example, motor neurones innervate skin and sensory neurones innervate muscle.
2. Regenerated neurones may not innervate structures in same way as originally.
3. Neurones may make connections to each other (neuroma).
What are the factors that affect axon regeneration?
1. Extent of damage: Extent of damage and disruption to connective tissue layers.
2. Speed of regrowth: Axons can only regrow at rate of 1 mm/day.
3. Time: Regrowth of axons rely on communication between axon and Schwann cells (via trophic factors). Target tissues also make trophic factors affecting growth. If axon doesn't reconnect within weeks, it atrophies and ceases to produce trophic factors. This prevents axon regrowth. This limits the distance of lesion.
4. Gaps: Axons are unable to grow across gaps. This means that 2 cut ends of a nerve need to be re-sutured or nerve graft needs to be used to bridge gap.
5. Accuracy: The greater the distance, the less accurate the axons are regenerated.