Peripheral Nervous System 1 Flashcards
What forms the myelin sheath in peripheral nerves?
Schwann cells wrapped around individual axon fibers form the myelin sheath in peripheral nerves.
What is the neurolemma?
The neurolemma is the outermost membrane of an individual nerve fiber in the peripheral nervous system.
What are the nodes of Ranvier, and what is their function?
Nodes of Ranvier are gaps between Schwann cells that allow saltatory conduction, enabling impulses to travel faster along myelinated fibers than unmyelinated ones.
What are the three connective tissue layers of axons, and what do they surround?
• Endoneurium: surrounds individual axons
• Perineurium: surrounds bundles of axons
• Epineurium: surrounds the entire peripheral nerve
What type of information do peripheral nerves typically carry?
Peripheral nerves typically carry both afferent (sensory/proprioception) and efferent (motor) information.
Do all peripheral nerves carry autonomic fibers?
No, only some peripheral nerves carry autonomic fibers.
List the 9 common causes of peripheral nerve damage.
- Direct trauma
- Infection (infective neuritis)
- Compression
- Injury repair complications
- Poisons/toxins
- Diseases causing peripheral neuropathies
- Birth trauma
- Loss of blood supply
- Extremes of temperature or radiation
What are examples of direct trauma causing PNS damage?
Severance or crush injuries (e.g. MVAs, industrial accidents), traction injuries (e.g. stinger/burner), forcible joint overextension, repeated small traumas.
Give examples of compression-related nerve injuries.
• Improper immobilization
• Poorly fitted crutches (should be 2 finger widths below axilla)
• Prolonged use of canes/wheelchairs
• Carpal tunnel syndrome, thoracic outlet syndrome, compartment syndrome
• Meralgia paresthetica
• Compression by tumors
• Prolonged postures or pressure at bony prominences
Name three examples of injury repair complications.
- Nerve tractioned or compressed by bony calluses (e.g. Colles fracture)
- Nerve caught in restrictive scar tissue
- Nerve entrapped in damaged joint or bone (e.g. shoulder dislocation)
Name poisons/toxins that can cause nerve damage.
Alcohol, chemical exposure (occupational/abuse).
List diseases that can cause peripheral neuropathies.
Diabetes, AIDS, polio, Hansen’s Disease (leprosy).
What is Wallerian degeneration?
A process occurring when an axon is cut, crushed, or severely compressed, involving degeneration of the distal axon and myelin sheath.
What happens during the degeneration phase of Wallerian degeneration?
• Distal axon separates from cell body
• Degenerates from injury site distally
• Small proximal segment also degenerates up to nearest node of Ranvier
What happens during the clearance phase?
• Myelin sheath and neurolemma degrade
• Macrophages, mast cells, Schwann cells clear debris
• Only the connective tissues (epineurium, perineurium, endoneurial sheath) remain
• The endoneurial sheath must remain intact for regeneration
What is required for nerve regeneration to occur?
The distal endoneurial sheath must be intact to guide regenerating axonal sprouts.
What occurs during the regeneration phase?
• Schwann cells migrate and reproduce
• Cell body shifts to repair mode
• Neuronal sprouting from proximal stump advances toward distal endoneurial tubes
What is gap repair, and how long does it take?
Gap repair is the initial healing phase of nerve injury, lasting 10–12 days, involving:
• Degeneration
• Clearance
• Neuronal sprouting and bridging the gap
What happens during growth and re-myelination?
• Neuronal sprouts grow into distal endoneurial • tubes
• Axons re-innervate target tissues
• Schwann cells re-myelinate the new axon
At what rate does axonal regeneration typically occur?
On average, 1 mm per day
What can go wrong during nerve regeneration?
• Sprouts fail to reach distal tubes (due to wide gaps, scar tissue, crush injuries)
• Formation of neuroma (dense fibrous mass with disorganized growth)
• Misrouting into incorrect endoneurial tubes (cross re-innervation)
• May require surgical repair, nerve grafts, or neuroma removal
What are the sensory effects of denervation?
• Sensory impulses can’t reach CNS
• Mild injuries may fully recover within 6–12 months
• Severe injuries may result in incomplete recovery
What are the motor effects of denervation?
• Muscle atrophy
• Regenerating motor axons must reach endplates to reform NMJs
• Can lead to motor unit rearrangement
What is neuropraxia?
Least severe nerve injury; temporary conduction block with no Wallerian degeneration
What causes neuropraxia?
Mild compression (e.g. Saturday night palsy, mild CTS), mechanical deformation or ischemia
What is the prognosis for neuropraxia?
• Full recovery expected
• May require rest, remyelination
• Sometimes leaves mild permanent damage
What is axonotmesis?
Loss of axon continuity but preserved endoneurial sheath; involves Wallerian degeneration
What causes axonotmesis?
• More severe compression
• Stretch injuries
• Neuritis, Lyme disease, diabetes, toxins, GBS
What is the prognosis for axonotmesis?
• Partial or full recovery
• Some permanent damage likely
• Requires immobilization or modified use
What is neurotmesis?
Most severe injury; complete axon and connective tissue disruption; Wallerian degeneration
What causes neurotmesis?
• Severance injuries (e.g. knives, amputations)
• Crush injuries, traction, industrial accidents
• Damages endoneurium, perineurium, and possibly epineurium
What is the prognosis for neurotmesis?
• Poor recovery
• Requires surgical repair, grafts
• Often results in permanent function loss
• Immobilization (3–6 weeks) and rehab needed
What is the Seddon Scale?
A classification system for nerve injuries, including:
- Neuropraxia – temporary conduction block
- Axonotmesis – axonal disruption with preserved sheath
- Neurotmesis – complete nerve and sheath disruption
