PNS Anatomy/injury Flashcards
What do fibroblasts generate? Where are they located?
Generate collagen fibers. Located in all types of CT
What are the two types of peripheral nerve cell types?
Fibroblasts and Schwann cells
What do Schwann cells do?
Surround and support every axon in the PNS (doesn’t matter if they are myelinated or not)
How so Schwann cells produce myelin sheaths around axons?
Produce myelin sheath in segments
What do nerves contain?
Consists of fibers of different diameters, some myelinated some not. Some are motor, some are sensory
What are they two ways nerve fibers are classified!
- Conduction velocity
1. Fiber diameter
What type of nerve haas the fastest conduction velocity?
The A group which consists of myelinated sensory and motor fibers
How is the A fiber group subdivided?
Alpha, beta, gamma, and delta
What are the second fastest fiber type?
B fibers which are thinly myelinated (preganglionic autonomic fibers?)
What is the slowest fiber type?
C fibers which are unmyelimated.
What are the classifications of fiber diameter?
I, II, III (myelinated), IV (unmylinated)
What is the outermost wrapping of the neuron?
Epineurium
What is the middle wrapping of the neuron?
Perineurium
What is the innermost wrapping of the neuron?
Endoneurium
What does the epineurium mostly consist of?
Collagen fibers and fibroblasts
Where is the epineruium thickest? Thinnest?
Thick at the nerve trunk and thins out as the nerve branches towards its termination
What other structure is the epineurium continuous with at the level of the spinal nerve?
The dura mater
What protective function does the epineurium have?
Protects fasiculi from compressive forces
What does the perineurium surround?
Surrounds the fasicles
What does the perineurium consist of?
Collagen fibers and connective tissue cells
What type of junctions does the perineurium consist of and what do they do?
Tight junctions that closes off each fascicle from the fascicles around it
What is the function of the perineurium?
Maintains intrafasicular fluid pressure and accounts for elasticity and tensile strength of nerve
What does endoneurium surround?
Individual nerve fibers
What supplies the endo-, peri-, and epi- neurium with blood?
arterioles
What is Neurapraxia?
Transitory localized conduction block with no signs of denervation due to local damage to neural membrane
How long is full recovery from Neurapraxia?
Days to weeks
What is Axonotmesis?
Axon disruption but endoneurial sheath remains intact.
What type of degeneration takes place with Axonotmesis?`
Wallerian degeneration
What does recovery time for a peripheral nerve depend on?
distance necessary for regrowth. nerve grows 1mm/day 1in/month
What is Neurotmesis?
complete transection of nerve with disruption of axon and endoneurium
What type of degeneration takes place with Neurotmesis?
Wallerian degeneration
What are some causes of Neurotmesis?
Cut, crush, or severe stretch injury
3 types of compressive injuries?
Acute, intermittent, or chronic
What does iatrogenic mean?
caused by medical examination or treatment
In an acute compressive injury which is involved first sensory or motor loss?
Sensory involved before motor
What is Wallerian degeneration?
Process of degeneration that occurs following injury to an axon
What happens to the distal myelin and axon hours after the injury?
They begin to fragment and the neurotubules and neurofilaments become disarrayed
What type of swelling occurs along the axon distal to the injury?
Varicose swelling
HOw long after injury does it take for the axon to become discontinuous (no action potentials can propagate along the length)?
48-96 hours
When do Schwann cells activate and what do they do in the Wallerian degeneration process?
activate w/in 24 hrs of injury, undergo mitosis and help mast cells get rid of degenerated distal nerve segment and myelin
How is the myelin degenerated in Wallerian Degeneration?
phagocyizedby mast cells, aided by Schwann cells
How long does it take for the site of injury to be cleared?
1 week-several months post injury
What happens if there is severe injury and the endoneurium is disrupted?
capillaries are disrupted causing hemorrhage. Much inflammation follows and a dense fibrous scar can form
What do Schwann Cells do to facilitate correct nerve regrowth?
They form stacks to guide the growth of growth cones from the regenerating axons
What is chromatolysis?
Process by which nucleus in cell body becomes eccentric and and the Nissl substance breaks down
What can happen to the DRG’s with injury?
Injury can trigger apoptosis and 20-50% of DRG cells may die. More likely if injury is proximal
In what type of injury is functional recovery more likely?
Neruopraxia or Axonotmesis where endoneurial tube is intact
What happens to muscle with denervation?
Severe atrophy (~70% reduction in x-sectional area by two months) intramuscular fibrosis with proliferation of connective tissue
What can happen to motor units with reservation?
Can result in giant motor units where collaterals of a single alpha motor neuron innervate a large number of all the same type muscle fiber
How long will receptors wait to be reinnervated?
up to a year
5 things regeneration results in?
more axons due to collaterals smaller diameter initially Thinner myelin initially but eventually recovers more myelin segments slower conduction velocities
What is another name for Guillian-Barre?
Postinfectious Polyneuritis
Who does Guillian-Barre syndrome usually affect?
young adults-early middle age
When does Guillian-Barre syndrome usually appear?
After particular type of respiratory infection
What is physiologically happening with Guillian-Barre syndrome?
Lymphocytic sensitization to peripheral nerve antigen. Patchy difuse segmental demyelinization of peripheral nerves occurs
What are some symptoms of Guillian-Barre syndrome?
Severe ascending weakness (hours to days after)
areflexia
elevated CSF protein
possible cranial nerve and respiratory involvement (phrenic nerve)
mild sensory loss;disproportionate motor loss
How long does Guillian-Barre syndrome progress for? Stay stable? Recovery time
progress over 10 days, stay stable for 2 weeks, 6mo-2 year recovery time
How many people do not fully recover from Guillian-Barre syndrome?
10%
What are some indicators of incomplete recovery from Guillian-Barre syndrome?
more than 3 weeks from max weakness to initial improvement
reduced motor nerve conduction velocities
EMG evidence of denervation
respiratory support
What is CIPD?
Chronic Inflammatory Demyelinating Polyneuropathy
What is CIPD sometimes confused for
Guillian-Barre syndrome
What is CIPD?
A chronic progressive autoimmune mediated inflammation of peripheral myelin
What type of course does CIPD follow?
A relapsing/remitting course
IN CIPD, what type of innervation is involved (sensory/motor)?
Both equally
Do people return to prior fan with CIPD?
No
WHat are some treatments for CIPD?
Short term steroids
plasmapheresis
IV IGg
