Path - Peripheral Nerve and Skeletal Muscle Flashcards
what does a motor unit consist of
- lower motor neuron (anterior horn of spinal cord or cranial nerve motor nucleus in the brain stem)
- axon
- muscle fibers
what does a nerve consist of
numerous fibers grouped into fascicles by connective tissue sheaths
what does a fascicle consist of
myelinated and unmyelinated nerve fibers
what is a node of ranvier
a gap in the myelin sheath of a nerve
define the three major connective tissue components of peripheral nerves:
epineurium, perineurium, and endoneurium
epineurium: enclosed entire nerve
perineurium: multi-layered, concentric connective tissue sheath that encloses each fascicle
endoneurium: surrounds individual nerve fibers
damage targeted at the schwann cells manifests as what
segmental demyelination
primary involvement of schwann cell and loss of myelin
damage targeted at the axons manifests as what
axonal degeneration
*may be followed by axonal regeneration and reinnervation of muscle
describe wallerian degeneration
an active process of degeneration that results when a nerve fiber is cut or crushed and the part of the axon distal to the injury (i.e. farther from the neuron’s cell body) degenerates
describe segmental demyelination
random internodes of myelin are injured and then remyelinated by multiple schwann cells (while the axon and myocytes remain INTACT)
occurs when there is dysfunction of schwann ells or damage to the myelin sheath
compare denervation atrophy and myopathy
denervation atrophy: what occurs after loss of an axon
myopathy: primary abnormality of the muscle fiber itself
describe the damage and repair process that occurs after acute axonal injury
normal: checkerboard of mixed myofiber types
acute axonal injury: causes degeneration of axon/denervated nerve fibers
repair: reinnervation of muscle fibers
what happens to the axon and myocytes in segemental demyelination?
they remain INTACT
what replaces injured schwann cells
precursor cells inside the endoneurium
what are “onion bulbs” and what are they seen in
concentric layers of schwann cytoplasm and redundant basement membrane surround thinly myelinated axons
- seen in:
segmental demyelination
chronic inflammatory demyelinating polyradiculoneuropathy
describe the process of axonal degeneration
axonal injury occurs –> wallerian degeneration distal to injury –> distal axons begin to fragment and the myelin sheaths unravel and disintegrate into MYELIN OVOIDS –> macrophages recruited for clean up –> proximal stump of severed nerve shows degenerative changes in most distal 2-3 internodes –> regeneration
describe the appearance of the fibers when regenerated after axonal degeneration
they are “angulated” fibers
atrophic fibers are smaller and triangular (cone) shape when denervation atrophy occurs
compare color, action, strength, and physiology between type 1 and type 2 muscle fibers
1:
color: red
action: sustained force
strength: weight bearing
phys: slow twitch
2:
color: white
action: sudden movements
strength: purposeful movements
phys: fast twitch
compare amount of lipids, glycogen, and mitochondria between type 1 and type 2 muscle fibers
1:
lipids: abundant
glycogen: scant
mitochondria: many
2:
lipids: scant
glycogen: abundant
mitochondria: few
what is type grouping and when does it occur
when a patch of contiguous myocytes have the same histochemical type
- unaffected neighboring motor unit neurons extend sprouts to re-innervate the denervated myocytes and incorporate them into the healthy motor unit
occurs in re-innervation of muscle
what causes type 2 fiber atrophy
inactivity or disuse
- limb fx
- pyramidal tract degeneration
- neurodegenerative dz
- glucocorticoid therapy
describe the histological hallmarks of a regenerated muscle fiber cell
there is a large, centrally located nuclei with prominent nucleoli
cytoplasm laden with RNA that is RED
clinical symptoms and pathogenesis of bell’s palsy
what diseases is bell’s associated with?
CN VII mononeuropathy causing facial muscle paralysis
one sided facial droop within 48-72 hours, headache/neck pain, memory problems, balance issues, ipsilateral limb weakness/numbness
associated with URI and DM
what is neurogenic bladder and what can cause it
lack of bladder control due to brain, spinal cord, or nerve problem
- MS
- parkinson’s
- DM
- brain infection
- metal poisoning
- stroke
- spina bifida
what antibodies are found with guillain-barre
anti-myelin antibodies
describe how myelin is removed from the axon in guillain-barre
cytoplasmic processes of macropahges penetrate the basement membrane of schwann cells in the vicinity of the nodes of ranvier –> they extend b/w myelin lamellae –> strips myelin away from the axon
endonurial fibrosis is associated with what dz process
lepromatous leprosy (hansen dz)
varicella zoster virus affects what part of the PNS
sensory ganglia of the spinal cord and brainstem
what neuropathy is seen with lyme dz
unilateral or bilateral facial nerve palsy
what neuropathy can be seen with HIV
mononeuritis multiplex
can resemble guillain-barre
what changes are seen in the nerves in DM neuropathy
segmental demyelination
decreased # of axons
relative loss of small myelinated fibers and unmyelinated fibers
uremic neuropathy is associated with what dz process
renal failure
how does thyroid dysfunction cause neuropathy?
hypothyroidism can cause compression mononeuropathies like carpal tunnel
what vitamin deficiencies can cause neuropathy
B12, B1, B6, B9 (folate), vitamine E, copper, zinc
