Path - Peripheral Nerve and Skeletal Muscle

Question 1

what does a motor unit consist of

Answer 1

• lower motor neuron (anterior horn of spinal cord or cranial nerve motor nucleus in the brain stem)
• axon
• muscle fibers

Question 2

what does a nerve consist of

Answer 2

numerous fibers grouped into fascicles by connective tissue sheaths

Question 3

what does a fascicle consist of

Answer 3

myelinated and unmyelinated nerve fibers

Question 4

what is a node of ranvier

Answer 4

a gap in the myelin sheath of a nerve

Question 5

define the three major connective tissue components of peripheral nerves:

epineurium, perineurium, and endoneurium

Answer 5

epineurium: enclosed entire nerve
perineurium: multi-layered, concentric connective tissue sheath that encloses each fascicle
endoneurium: surrounds individual nerve fibers

Question 6

damage targeted at the schwann cells manifests as what

Answer 6

segmental demyelination

primary involvement of schwann cell and loss of myelin

Question 7

damage targeted at the axons manifests as what

Answer 7

axonal degeneration

*may be followed by axonal regeneration and reinnervation of muscle

Question 8

describe wallerian degeneration

Answer 8

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

Question 9

describe segmental demyelination

Answer 9

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

Question 10

compare denervation atrophy and myopathy

Answer 10

denervation atrophy: what occurs after loss of an axon

myopathy: primary abnormality of the muscle fiber itself

Question 11

describe the damage and repair process that occurs after acute axonal injury

Answer 11

normal: checkerboard of mixed myofiber types

acute axonal injury: causes degeneration of axon/denervated nerve fibers

repair: reinnervation of muscle fibers

Question 12

what happens to the axon and myocytes in segemental demyelination?

Answer 12

they remain INTACT

Question 13

what replaces injured schwann cells

Answer 13

precursor cells inside the endoneurium

Question 14

what are “onion bulbs” and what are they seen in

Answer 14

concentric layers of schwann cytoplasm and redundant basement membrane surround thinly myelinated axons

• seen in:

segmental demyelination

chronic inflammatory demyelinating polyradiculoneuropathy

Question 15

describe the process of axonal degeneration

Answer 15

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

Question 16

describe the appearance of the fibers when regenerated after axonal degeneration

Answer 16

they are “angulated” fibers

atrophic fibers are smaller and triangular (cone) shape when denervation atrophy occurs

Question 17

compare color, action, strength, and physiology between type 1 and type 2 muscle fibers

Answer 17

1:
color: red
action: sustained force
strength: weight bearing
phys: slow twitch

2:

color: white
action: sudden movements
strength: purposeful movements
phys: fast twitch

Question 18

compare amount of lipids, glycogen, and mitochondria between type 1 and type 2 muscle fibers

Answer 18

1:

lipids: abundant
glycogen: scant
mitochondria: many

2:
lipids: scant
glycogen: abundant
mitochondria: few

Question 19

what is type grouping and when does it occur

Answer 19

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

Question 20

what causes type 2 fiber atrophy

Answer 20

inactivity or disuse

• limb fx
• pyramidal tract degeneration
• neurodegenerative dz
• glucocorticoid therapy

Question 21

describe the histological hallmarks of a regenerated muscle fiber cell

Answer 21

there is a large, centrally located nuclei with prominent nucleoli

cytoplasm laden with RNA that is RED

Question 22

clinical symptoms and pathogenesis of bell’s palsy

what diseases is bell’s associated with?

Answer 22

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

Question 23

what is neurogenic bladder and what can cause it

Answer 23

lack of bladder control due to brain, spinal cord, or nerve problem

• MS
• parkinson’s
• DM
• brain infection
• metal poisoning
• stroke
• spina bifida

Question 24

what antibodies are found with guillain-barre

Answer 24

anti-myelin antibodies

Question 25

describe how myelin is removed from the axon in guillain-barre

Answer 25

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

Question 26

endonurial fibrosis is associated with what dz process

Answer 26

lepromatous leprosy (hansen dz)

Question 27

varicella zoster virus affects what part of the PNS

Answer 27

sensory ganglia of the spinal cord and brainstem

Question 28

what neuropathy is seen with lyme dz

Answer 28

unilateral or bilateral facial nerve palsy

Question 29

what neuropathy can be seen with HIV

Answer 29

mononeuritis multiplex

can resemble guillain-barre

Question 30

what changes are seen in the nerves in DM neuropathy

Answer 30

segmental demyelination

decreased # of axons

relative loss of small myelinated fibers and unmyelinated fibers

Question 31

uremic neuropathy is associated with what dz process

Answer 31

renal failure

Question 32

how does thyroid dysfunction cause neuropathy?

Answer 32

hypothyroidism can cause compression mononeuropathies like carpal tunnel

Question 33

what vitamin deficiencies can cause neuropathy

Answer 33

B12, B1, B6, B9 (folate), vitamine E, copper, zinc