Mechanisms of muscle contraction and fatigue
1. describe the steps in the contraction cycle
2. explain how ATP is produced in muscle
3. describe the possible mechanisms of muscle fatigue
1. ATP hydrolysis
a. Myosin head include an ATPbinding site and an ATPase, an enzyme that
hydrolyzes ATP into ADP (adenosine diphosphate) and a phosphate group.
This hydrolysis reaction reorients and energizes the myosin head. Notice
that the products of ATP hydrolysisADP and a phosphate groupare still
attached to the myosin head.
2. attachment of myosin to actin to form crossbridges
a. The energized myosin head attaches to the myosinbinding site on actin
and releases the previously hydrolyzed phosphate group. When the myosin
heads attach to the actin during contraction, they are referred to as cross
3. power stroke
a. After the crossbridges form, the power stroke occurs. During the power
stroke, the site on the crossbridge where ADP is still bound opens. As a
result the crossbridge rotates and releases ADP. The crossbridge
generates force as it rotates toward the center of the sarcomere, sliding the
thin filament past the thick filaments toward the M line.
4. detachment of myosin from actin
a. At the end of the power stroke, the crossbridge remains firmly attached to
actin until it binds another molecule of ATP. As ATP binds to the ATP
binding site on the myosin head, the myosin head detaches from actin.
Production of ATP in muscle fibers:
• Unlike most of the other cells in the body, skeletal muscle cells often switch from
a low level of activity to a high level of activity.
• The amount of ATP present in the muscle cell itself is only enough to power
contraction for a few seconds. Therefore the muscle cell must make ATP itself.
• Muscle fibers have 3 ways of producing ATP: from creatine phosphate, anaerobic
respiration and aerobic respiration.
o Creatine phosphate: while muscle cells are relaxed they produce more than
the necessary amount of ATP. Most of the excess ATP is used to synthesize
creatine phosphate, an energy rich molecule that is found in muscle fibers.
The enzyme creatine kinase (CK) catalyzes the transfer of one the high
energy phosphate groups from ATP to creatine, forming creatine phosphate
and ADP. Creatine is a small, amino acidlike molecule that is synthesized
in the liver, kidneys, and pancreas and then transported to the muscle cells.
CK is 36x more plentiful than ATP in the sarcoplasm of a relaxed fiber. When contraction begins and the ADP level starts to rise, CK catalyzes the
transfer of a highenergy phosphate group from creatine phosphate back to
ADP. This direct phosphorylation reaction quickly generates new ATP
molecules. Since the formation of ATP from creatine phosphate occurs
very rapidly, creatine phosphate is the first source of energy when muscle
contraction begins. All together the stores of creatine phosphate and ATP
provide enough energy for muscles to contract maximally for about 15
o Anaerobic Cellular Respiration: series of ATPproducing reactions that for
not require oxygen. When muscle activity continues and the supply of
creatine phosphate is depleted, glucose is catabolized to gene