Responses of Peripheral Nerves to Injury Flashcards
Mildest nerve injury
Compression/low shear, or low pressure crush
Can get ischemia or mechanical deformation
What does ischemia do to an axon?
Capillary perfusion pressures exceeded from the pressure
Causes edema in the endoneurial space
O2 dependent ion channels become dysfunctional
Axoplasmic transport is disrupted
Prolonged ischemia can lead to death of support cells +/- the segment of axon
What does mechanical deformation do to an axon?
Can cause demyelination from myelin separating near the nodes
Higher and more focal deformation can cause axon tears
Tinel’s sign
If sensory axon has been disrupted
Tapping on the proximal cut edge will get tingling pins-and-needles feeling
Proximal stump is more irritable and brain perceives it not right!
What do Schwann cells do when an axon is cut?
Detect the injury and signal macrophages to come and clean up debris
De-differentiate, myelin is lost
Up-regulate some genes
Recruit, multiply, and re-differentiate
What does the renegeration process look like?
Cell body responds to retrograde signals sent from injury site (up-regulates expression of genes and production of building materials, etc)
Sprouts are produced at cut end (growth cone at tip of each with motile filopodia)
Bands of Bungner
Schwann cells differentiate and join, aligning to form pathways
At the distal end of a cut axon
Endoneurial tubes
The basal lamina (+/- adjacent endoneurial elements) remain or are repaired
3 things essential to regeneration
Interaction of growth cones with environment ***
Basal lamina and Schwann cells
Supportive environment
How fast to axons regrow?
1-4mm per day
What turns off regeneration?
Receptor-axon terminal interaction
Neuroma
A bundle of nerve fibers, connective tissue, and scar at the end of a cut axon
Happens if the gap is too wide
Can be quite sensitive/painful
12 month rule
Nerves need to reinnervate a muscle within 12 months or else there will be irreversible change to the muscle
Sensory end organs don’t have this 12 month rule
Neurapraxia
A type of peripheral nerve injury
The affected nerve cells have a localized conduction block
An AP cannot be propagated across a section of the nerve, yet the axons are still intact
Usually due to focal demyelination
How long (usually) does it take for neuropraxia to recover?
Rarely more than 3 months
