Muscle contraction 1 myosin ii molecules can

MUSCLE CONTRACTION
1. Myosin II molecules can associate to form myosin filaments
a. Globular heads- bind ATP
b. Double headed arrow
i. Arrow point to left and arrow points to right. Bare region in the middle between
the two arrows
2. Muscle contraction by sliding filament mechanism

a.
b. When myosin walks, they start walking toward the actin filaments which pulls the actin
filaments to the middle.
3. Sarcomeres
a. The contractile unit of the muscle
b. Contraction of a muscle is due to simultaneous shortening of all the sarcomeres
4. Myosin walks along actin by a series of conformational changes
a. Myosin head attached to actin filament at rest
b. When ATP binds to myosin for the first time, that doesn’t cause the muscle to contract.
i. ATP binding cause the release of actin from myosin
ii. Rigor mortis- what happens to muscles after an animal dies. The muscles turn
rock hard. Happens bc the muscles have run out of ATP so the myosin cant let go
of the actin
c. After ATP binds, actin releases from myosin. Then ATP hydrolyzes into ADP+Pi which
allows the myosin head to cock to that its posed to make the power stroke. Then the
myosin grabs the actin from the cocked position and pulls it
d. The power stroke is the myosin walking along the actin causing the muscle to contract
5. Muscle contraction
a. Triggered by a sudden rise in Ca2+ from the nervous system
b. Ca2+ is low inside the cell
c. Ca2+ is released in response to electrical stimulation of the neuromuscular junction
d. Calcium comes into the cell which causes the muscles to contract
e. Transverse tubules (formed from invagination of plasma membrane) inside the cell bring
the action potential to the cell
i. Also covered In sarcoplasmic reticulum which is full of calcium

f.
g. Voltage gated Ca2+ channels- allow calcium to flood the cell
h. Calcium induced calcium release- when you release calcium it causes more calcium to be
released
6. VIDEO: how a muscle contraction is signaled
a. When a neuron stimulates a muscle cell, an action potential sweeps over the plasma
membrane of the muscle cell
i. The action potential releases internal stores of calcium that flow thru the muscle
cell and trigger a contraction
b. Muscle cells architectures allows them to distribute calcium ions quickly throughout the
cytosol
c. T tubules crisscross the cell, when the cell is stimulated a wave of depolarization (actin
potential) spreads from the synapse over the plasma membrane and via the t tubules
deep into the cell
d. Voltage sensitive protein opens a calcium release channel in the adjacent sarcoplasmic
reticulum (the major calcium store in muscle cells) thereby releasing a burst of calcium
ions
e. Myofibril- a contractile bundle of muscle cells. Within the myofibril the calcium interacts
w protein filaments to trigger a contraction
f. Thin actin and thick myosin filaments are juxtaposed but cannot interact in the absence
of calcium
i. Myosin binding sites on the actin filaments are all covered by tropomyosin
g. Troponin- a calcium sensitive complex is attached to the end of each tropomyosin
molecule.
i. Tropomyosin blocks myosin binding site
ii. When calcium floods the cell, troponin binds to it moving tropomyosin off the
myosin binding sites
h. Opening the myosin binding site on the actin filaments allows the myosin motors to
crawl along the actin, resulting in a contraction of the muscle fiber
i. Calcium then quickly returned to the sarcoplasmic reticulum by the action of a
calcium pump
i. Without calcium, myosin releases actin and the filaments slide back to their original
positions

7.