Download Lecture2 - Physiology of Muscle contraction

Muscle Physiology
Dr Taha Sadig Ahmed
5/24/2017
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The Muscle Action Potential
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RMP = -90 mV ( same as in nerves )
Duration of AP = 1-5 ms ( longer
duration than nerve AP , which is
usually about 1 ms ) .
Conduction velocity (CV) in a muscle
fiber ( cell) = 3-5 m/s ( slower than
big nerves ) .
Amplitude
‫إرتفاعه‬
RMP= - 90 mV
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Muscle Contraction
There are 4 important muscle proteins :
A/ two contractile proteins that slide upon each other
during contraction :
(1) Actin
(2) Myosin
B/ And two regulatory proteins :
 Troponin  excitatory to contraction
 Tropomyosin  inhibitory to contraction
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The EPP at the motor endplate triggers a muscle AP
 The muscle AP spreads down
inside the muscle through the
Transverse Tubules
( T-tubules )
to reach the Sarcoplasmic
Reticulum (SR) .
 In the SR the muscle AP
opens calcium channels
( in the walls of the SR) 
calcium passively flows out
( by concentration gradient ) of
the SR into muscle
cytoplasm Ca++ combines
with Troponin  Tropomyosin
gets moved  Myosin heads
combines with Actin
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Skeletal muscle is made up of many cylinderical ,multinucleated muscle
cells ( fibers)
The fibers ( cell ) can be 10 to 100 ten micron in diameter , and can be
hundreds of centimeters long.
& is covered by a cell-membrane called Sarcolemma.
One muscle
cell ( fiber )
Sarcolemma
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Each cell contains between few hundreds to a few thousands
Myofibrils
The myofibril is striated
cand has
dark bands  called A-bands) and
light bands  called I-bands (I-bands).
Each Myofibril
Is made of 3000 Actin filaments
And 1500 Myosin filaments .
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Each myofibril
is striated
It is made of Sarcomeres
Z-lines
And each sarcomere is limited by two Z-lines (Z disk)
The sarcomere contains both the 
(i) A band and
(ii) I band light
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A-bands consist mainly of thick filament Myosin
 The ends of Actin are Z-Discs(Z-lines ).
I-bands consist of thin filament  Actin.
The part of the Myofibril lying between two Z-discs
is called Sarcomere .
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Sliding Filament Mechanism
When contraction takes place Actin & Myosin
slide upon each other ,
& the distance between two z-discs decreases
This is called Sliding Filament Mechanism .
Z-line come closer together
I-band gets smaller , and eventually
may disappear
A-band does not become smaller or bigger
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Actin is made of globular protein callled G-actin
G-actins are attached together to form F-actin strand ( chain )
Each two strands wind together to form double helix called Actin Filament
Tropomyosin lies in the groove between the F-actin strands to cover the active
sites on actin that bind the head of myosin
Troponin is attached to tropomyosin and to actin
Two Actin filaments
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Tropomyosin
covering
active sites on Actin
Groove between the 2
F-actin strands
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Attachment of Ca++ to Troponin initiates tcontraction
and when is activated by Ca++ it will move the Tropomyosin
away from the active sites on actinn & expose them for Myosin .
> then myosin head will immediately attach to these actin active
sites
> when the myosin head attaches to actin it forms a “ crossbridge”
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Myosin
Each 200 myosin molecules aggregate to form a
myosin filament , from the sides of which project
myosin heads in all directions .
Myosin
Each Myosin molecule has
(1) Head
( 2 ) Tail
(3) Hinge (joint )
Furthermore , each myosin head contains 
(1) ATP-binding site , &
(2) ATP-ase enzyme .
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Sliding Filaments
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 Attachment of Myosin to Actin activates the enzyme ATPase in the
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Myosin Head 
ATPase breaks down ATP releasing energy
This energy is used in the “Power Stroke ” to move the myosin head
 leading to pulling & dragging of actin 
 sliding of actin on myosin
The “ power stroke ” means tilting of the Myosin cross-bridge and
dragging ( pulling ) of actin filament
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Then , on order
to release the
head of Myosin
from Actin , a
new ATP is
needed to come
and combine
with the head
of Myosin .
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A new ATP binding to Myosin head
is essential for detachment of
Myosin from Actin
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The Cross-Bridge Cycle
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Q: What is Rigor Mortis ?
 Q: ATP is neede for 3 things : what are
they ?
 Q: Is muscle relaxation a passive or active
process ? Why ?
 Q: What happens to A-band and I-band
during contraction ?
 Q: Ca++ is needed in nerve & muscle :
when and where ?
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Why do we need the ATP in
contraction ?
ATP is needed for 3 things :
 (1) Power stroke.
 (2) Detachment of myosin from actin
active sites .
 (3) Pumping C++ back into the
Sarcoplasmic reticulum
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