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THE SLIDING FILAMENT THEORY Muscle Contraction LEARNING GOALS • I will understand how the muscle contracts. • I will understand the role proteins play in muscle contraction. • I will be able to explain the sliding filament theory of muscle contraction. REMEMBER THIS? INSIDE MYOFIBRILS... Within 1 sarcomere, there are many myosin & actin filaments stacked in both directions END TO END • 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. THE FILAMENTS SLIDING FILAMENT THEORY • Each myosin filament 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. Sliding Filament 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 and 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. Sliding Filament at Molecular Level 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. SFT VIDEO • A single “stroke” of the myosin bridges shortens the sarcomere by ~1%. • Why then does a muscle shorten by 1/3 during contraction? • The nervous system is capable of activating up to 50 cross bridge formations / second. • Because thousands of sarcomeres are connected end to end, the effect is even greater. 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. LEARNING GOALS • I will understand how the muscle contracts. • I will understand the role proteins play in muscle contraction. • I will be able to explain the sliding filament theory of muscle contraction.