APK 2105C Lecture Notes - Lecture 41: Myosin Head, Sarcomere, Skeletal Muscle
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 smaller—actin is more on top of the myosin
▪ H zone gets smaller—actin 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
it—in 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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