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Lecture 10

CSB332H1 Lecture Notes - Lecture 10: Wallerian Degeneration, Reinnervation, Myocyte


Department
Cell and Systems Biology
Course Code
CSB332H1
Professor
Melanie Woodin
Lecture
10

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CSB332H1S L10; Feb. 13, 2012
Degeneration & Regeneration
Read: Ch 27
Degeneration happens when have neural injury
Need degeneration before regeneration
The neuron synapsing onto the motoneuron & its target are
also affected
Wallerian degeneration:
o Characteristic seq of events following severing of a
vertebrate peripheral nerve
Injury/Lesion Distal axon degenerates
o Schwann cells (had formed myelin sheath) dedifferentiate,
proliferate, and w invading macrophages & microglia
from blood, phagocytize the axonal & myelin debris
recycle nutrients
o Cell body & nucleus of injured neuron swell
Several hrs later:
o Axon sprouts near proximal region & begins to regenerate
o Schwann cells preserve endoneural tube so axon can
grow back thru
If a connection is reestablished cell body resumes its normal
position
o Axon regenerates along the column of Schwann cells
If a connection not reestablished cell often dies rapidly
After axotomy:
o Degeneration of nerve terminal & distal segment of axon
o Macrophages & microglia invade & phagocytize the debris
o Presynaptic terminals retract
Due to retrograde signals following axotomy
reduce synaptic transmission onto this axon
Neurons that synapse onto damaged cell are also affected
Most often synapse becomes weakened
o Either due to decrease in amt of transmitter released or in
the density of postsynaptic recs
Retrograde signals cause many presynaptic terminals to
retract
If neuron regenerates & re-innervates target synaptic inputs
recover
Recording from muscle fibre, using ionophoresis (inject
charged substance on muscle) ,move ACh pipette across
muscle fibre
Focus on blue line
If pipette ACh directly on end-plate large response
o Even if pipette 1 mm away no response
Since high conc of ACh in end-plate region
Denervate
o Amplitude of end-plate stays same, also in neighbouring
regions
o Response all across muscle
Fibrilation:
o Denervated muscle shows spontaneous, asynchronous
contractions
o Also in heart & skeletal muscles since also have AChRs
Fibrilations originated in end-plate regions from changes in
membrane (not ACh)
o Resistant to Na channels
Denervated muscle does so because of super-sensitive to ACh
(1000x)
o Super-sensitivity to ACh increase in AChRs
Increases chance of having motoneuron innervate it
W more time muscle begins to atrophy & waste away (if
reinnervation doesn’t happen)
o Need a lot of physical therapy to activate muscles & avoid
atrophy
Area sensitive to ACh increases until almost uniform
distribution of ACh across muscle surface
Effects of Denervation on Postsynaptic Cells
Distribution & turnover of AChRs often studied by labeling
them w radioactive α-bungarotoxin binds to AChRs
These are new Rs, not simply Rs diffused from synaptic region
o Viewed by increase in radioactivity
o Can use antagonists
Apprearance of new AChRs is blocked by prot synthesis
inhibitors
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