APK 2105C Lecture Notes - Lecture 41: Myosin Head, Sarcomere, Skeletal Muscle

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Chapter 12, Lecture 2
Muscle Physiology
Sliding-filament mechanism of contraction
o Sarcomere = smallest functional unit of skeletal muscle contraction
The heads of the myosin face out towards Z line
H zone = bare zone
No heads of myosin
Actin thin myofilaments extend out to Z line
Z lines stain very darkly
I band is the surrounding actin around the Z line
o Actin and myosin slide past each other
Not decreasing in length
Have more overlap during contraction
o Changes with contracts
A band does NOT change length
Actin slides in closer to the midline (M line) of the sarcomere
Length of sarcomere decreases
Myofibril shortens
I band gets smalleractin is more on top of the myosin
H zone gets smalleractin is closing in towards the middle
The crossbridge cycle
o The does not occur
simultaneously for all
crossbridges
o Crossbridge = head portion of
myosin binding with actin to
create a bridge
Causes contraction
o Ca ions are NECESSARY for
contraction
Ca is stored in SR
o When tropomyosin is moved,
the head of myosin attaches
The head of myosin has
ADP and Pi attached to
itin an energized form
o Powerstroke
Push actin towards the
middle of the sarcomere
ADP is released
o Rigor
Myosin head is stuck in the position of pushing actin towards middle of
sarcomere
Myosin in low energy form
ATP comes in and binds to the myosin to move it back
o Occurs at every head of myosin
Does not occur at the same time for every cycle
Allows for progressive shortening
Synchronized manner but not all at the same time
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