Myosin and Actin Filaments: In resting muscle, actin does not bind to myosin due to the presence of troponin, which blocks the interaction.
Troponin: A protein associated with actin, measured after suspected heart attacks because it can be released into the blood when cardiac muscle is damaged.
Role of Calcium and Troponin
Calcium Release: Calcium is released from the sarcoplasmic reticulum (SR) during depolarization.
Calcium and Troponin Interaction: Calcium binds to troponin, facilitating the binding of myosin to actin by shifting the troponin and revealing binding sites on actin.
Key Idea: Calcium allows myosin to bind to actin.
ADP and ATP in the Cycle
ADP on Myosin: At rest, myosin is attached to ADP.
ATP Ejection: Myosin binds to actin causing ADP to be released, likened to jet propulsion.
Power Stroke
Myosin Power Stroke: The binding of myosin to actin causes a power stroke, moving the actin filament and resulting in muscle contraction.
Muscle Contraction Types:
Isotonic Contraction: Muscle shortening.
Eccentric Contraction: Muscle stretching where power stroke prevents actin ripping.
Releasing and Recocking Myosin
Myosin Detachment: ATP binds to myosin, causing it to detach from actin.
Hydrolysis of ATP: Converts ATP to ADP, recocking the myosin head for another cycle.
Key Idea: ATP is crucial for detaching myosin from actin.
Cycle Repetition
The cycle repeats thousands of times in typical muscle contraction.
Key Ingredients:
Calcium Availability: Determines the number of myosin heads involved and contraction strength.
ATP Availability: Determines the duration of the contraction cycle.
Rigor Mortis
Occurs when ATP is depleted after death, causing myosin heads to remain attached to actin, leading to muscle stiffness.
Does not last indefinitely as protein degradation eventually causes the body to lose rigidity.