Download How Muscle Contracts - Mr-Corrente

How Muscle Contracts
Within each myofibril are a number of contractile units called sarcomeres. They are attached end to end within the
myofibril.
Each sarcomere is comprised of two types of protein myofilaments: myosin (thick filament) and actin (thin
filament). Myosin filaments are surrounded by actin filaments.
The thin actin filaments sliding over the thick myosin filaments are what causes muscle contraction (muscle
shortening to produce movement). This is called the sliding filament theory.
Sliding Filament Theory
Each myosin filaments contains tiny contractile elements called myosin bridges. Myosin bridges stick out at 45
degree angles from the myosin filament (think oars on a rowing boat). When a signal from the motor nerve
arrives, the myosin bridges attach themselves to the actin filaments. This is called cross bridge formation. The
myosin bridges continue to move forward, sliding the actin filaments closer together. Actin filaments moving
closer together = sarcomere shortening = myofibril shortening = muscle contraction.
A single “stroke” of the myosin bridges shortens the sarcomere by ~1%. The nervous system is capable of
activating up to 50 cross bridge formations / second. Because the sarcomeres are connected end to end, the
effect is even greater.
Sliding Filament Theory at Molecular Level
Step 1 – Motor nerve signal depolarizes muscle cell (- to +)
Step 2 – Depolarization causes calcium ions to be released from the sarcoplasmic reticulum
Step 3 – Calcium ions move bind to troponin to move tropomyosin away from actin binding sites
Step 4 – Cross bridge formation occurs (myosin binds to actin)
Step 5 – Myosin bridge moves (sliding actin) with stored energy.
Step 6 – Adenosine Diphosphate (ADP) and a Phosphate (P) leave the myosin bridge as the myosin head is moving
(power stroke) to make room for ATP
Step 7 – Bond between myosin and actin is broken when Adenosine Triphosphate (ATP) binds to myosin head.
Step 8 – ATP is broken down into ADP + P while the myosin head relaxes.
Step 9 – Stored energy from ATP breakdown readies myosin head to move again
Step 10 – Step Four to Nine repeat until motor signal leaves.
Step 11 - The muscle cell repolarizes and the calcium ions return to sarcoplasmic reticulum.
Step 12 - Tropomyosin covers actin binding sites. Muscle relaxes. Actin then returns to original position.
Optimal Joint Angle
If the sarcomeres are too far apart (stretched) or run into each other (contracted), the myosin stroke is
not efficient. For optimal cross bridge formation, the sarcomeres must be an optimal distance apart. At
this optimal distance (~0.002mm), maximal muscle force is produced. At a certain angle of joint
movement, the optimal distance occurs. Optimal joint angle = maximal force.