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What determines the conduction velocity of a nerve action potential?

  • diameter of axon
  • presence of myelin


What are schwann cells

  • Glial cells surrounding axons, are myelinated or non-myelinated
  • Function:
    • trophic factor release
    • support regneration
    • antigen presentation


Define the structural organization of a nerve

  • Endoneurium: encases axon and their gial cells 
  • Perineum: encases group of axons into a fascicle
  • Epineurium
    • EpiFascicular - internal - surrounds group of fascicles
    • Epineurial - external - surrounds nerve trunk
  • Mesoneurium - external vascular network and permits gliding


What is Wallerian degeneration

Distal degeneration of axon leaving behind scaffold of schwann cells

Proximal degeneration varies with degree of injury


Classify nerve injury

  1. Neuropraxia - focal segmental demyelination. Tinels (-), Recovery complete days-wks
  2. Axonotmesis - axon damaged + Wallerian degenration, Tinels (+) progresses, Recovery complete wk-mths
  3. Axonotmesis - axon, endoneurium damaged, Tinels (+) progresses, Recovery variable
  4. Axonotmesis - axon endoneurium perineurium damaged, Tinels + BUT no progression, No recovery=> NIC
  5. Neurotmesis - complete never transection, Tinels + no progression, Neuroma
  6. Mixed nerve injury


How do nerves heal

  • Injury occurs then events occur at body and stumps
  • @ Cell body
    • cell swells and NT production reduces
    • 2wks later, cell returns to producing structural proteins and axoplasm flow begins again
  • @ proximal stump
    • minimal wallerian degeneration to netx proximal node of ranvier
    • Axonal spouts with filopdia and growth cones start wihtin 24hrs
    • Filopodia attracted to trophic factors of distal stump. Once in endoneurial scaffold, optimal location for growth and beomes parent nerve
    • RLS in neuroregeneratio is axonal transport
    • 4-400mm/day.
    • Regeneration 1mm/day after 30day delay for clearance of cellular debris 
  • @ Distal stump
    • Schwann cells phagocytose axoplasm/myelin and macrophage clear debris = empty endoneurial conduits
    • conduits shrink if no growth wihtin a month
    • Bands of bunger are scaffold of schwann cells ready for neuronal ingrowth


What are systemic factors that inhibit nerve healing

  • Vitamin deficiency
  • Gout (colchicine inhibit tubulin)
  • Alcohol
  • DM


What occurs to the end organs when nerve injury occurs

  • Motor nerve injury
    • Motor end plate increases number of AchR wihtin weeks of denervation
    • = Denervation supersensitivity - lower memrbane potential and prone ot fibrillations
    • Once reinnervated, AchR# normalize
    • can babysit with another motor or sensory nerve until correct nerve reaches target
    • at 12mths, muscle fibrosis limits function
  • Sensory nerve injury
    • Pacinian and Merkel degenerate but regain fx if renervated
    • Meissner degeneratio is permenant after 6mth
      • If delay of reinnervation >12mth, lose 2PD but may regain protective sensation


What is an EMG and what information does it provide

  • a depiction of muscle  electrical activity by measuring electrical potential diff b/w 2sites
  • MUAP : motor unit outcome potential where MU is muscle, NMJ, axon, Ventral horn
  • MUAP activity described 
    • at rest, at needle insertion, volunteer activity
    • amplitude, duration, rise time, phases
  • Normal muscle shows small bursts of activity w needle insertion then stops. Abnormal muscle continues to fibrillate w needle insertion


What is a NCS and what information does it provide

  • describes the elctrical activity from peripheral nerves between sensory and motor nerves to end organ targets
  • SNAP and CMAP
  • SNAP amplitude for peak to peak area = # of nerve fibers stimulated
  • CMAP amplitude = # of muscle fibers stimulated
  • Measures of NCS include
    • amplitude, conduction velsocity, duratio, configuration
  • drop in amplitude =  axon degeneration of SNAP/CMAP = wallerian degeneration
  • drop in cv = Demyelination
  • variation based on stimulation from distal to proximal = conduction block


What is the purpose of intraop NCS?

To assess a NIC

  • determine if any axons are viable through NIC
  • If NAP are detected, an external and internal neurolysis will provide beenfit
  • if no NAP detected, indicates need for NIC resection and reconstruction


How long o dyou wait fo NCS and EMG studies following nerve injury?

  • 6weeks


  • takes 6 wks for muscle AchR# to increase to demonstrate fibrillations if in fact nerve degeneration has occured
  • takes 4 wks for wallerian degenration to occur


List nerve repair types

  • Epineurial - aligned via vasa nervosa
  • Group Fascicular (inner epineural)
  • Fascicular (perineural)
  • End to side repair
  • Nerve transfer
  • Fibrin glue
  • Laser


List techniques to facilitate approaprite coaptation of fascicular group

  • Anatomic landmarks - fascicle size, position, orientation, vasa nervosa 
  • Ecltrical Stimulation - possible up to 3days post injury
  • Histochemical staining - Acestease Motor, CA Sensiry - possible up to 9 days post injury


What are poor prognostic indicators for nerve repair result


  • Multi-level
  • Avulsion/Crush/traction
  • Mixed nerve < (Worse than pure sensory/pure motor)
  • Concomitant ST vascular injury
  • Proximal
  • Devascularization


  • Delayed
  • Tension


  • >40
  • Compliance


How do nerve grafts heal

Same as skin grafts

  • Imbibition - diffusion -  for 3days
  • Inosculation - vascular reconnection day 3-6
  • Revascularization - day 6


What are types of nerve grafts

  • Trunk grafts: large grafts interposed in segment
    • get central fibrosis due ot prolonged time fo rrevascularization - not used
  • Cable grafts - multiple small segements to interpose - less fibrosis as greate SA for graft survival
  • InterFascicular graft - poor if gap >6cm
  • Free vascularized "graft" - larger # of myelinated axons- improved reinnervation if radiated/scarred bed
  • Nerve conduits - if <3cm


What are sources of nerve grafts

  • Autogenous
    • for defects >5cm
    • Nonvascularized
      • MABC, MBC, LABC, PIN, sural, med/lat CFN
      • portion fo damaged nerve
    • Vascularized
      • radial, ulnar, sural artery & nerve, deep peroneal-dorsalis pedis
  • Allografts
    • acts as scaffold
    • <30mm, 1-2mm diameter
  • Conduits
    • synthetic - silicone, PGA
    • biologic - vein


What is axogen

  • Axogen = decellularized cadaveric peripheral nerve scaffold
  • Processed: decellularized, radiated, enzyme degradation
  • keep at -40


How do you manage nerve repairs post-opeatively

  • Splint 3 weeks
  • Follow tinels sign
    • irritability of regenerating nerves not yet insulated by schwann cells
  • Outcomes
    • Motor - based on MRC
    • Sensory
      • deep pain
      • superficial pain and sensibility
      • superficial pain and touch
      • localization and some 2PD
      • complete recovery w 2PD (4-6mm)


What is a neuroma

Response of a nerve followiing trasnsection- sprouting axons regenrated outside of epineurium

Occurs only at the proximal end


What is  a glioma


Response ot nerve transection in the distal stump


Why is a neuroma painful

  • continuous chemical or mechanical irritation of axons
  • persistent activiation of axon through DRG


What are treatment options for a neuroma


  • PT, desensitization, medications


  • repair nerve +/- graft
  • place nerve end into muscle
  • resect and allow retraction into soft tissue
  • resect nerve and close over epineurium
  • relocate neroma away from pressure point