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?

there are many myosin & actin filaments stacked in both directions INSIDE MYOFIBRILS... Within 1 sarcomere, there are many myosin & actin filaments stacked in both directions

End to end

They are attached end to end within the myofibril. 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.