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

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

The playas

atp adenosine triphosphate energy currency of biological systems

myosin protein thick filament head and tail – looks like golf club myosin head has attachment site for actin

actin protein thin filament has a binding site for the myosin head

troponin protein has a binding site for calcium

tropomyosin stringy looking cord-like structure that covers the binding site on actin

Calcium ions release of Ca2+ facilitates the interaction of myosin and actin molecules

Let’s diagram it …

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. called cross bridge formation.

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 – ATP binds to myosin “head”. Myosin releases actin. Step 5 – ATP hydrolysed to ADP + Pi. Myosin head ‘cocks’ or ‘spring loads’ to high energy conformation. Step 6 – Myosin head forms a ‘cross-bridge’ on the active site of the actin filament.

Sliding Filament at Molecular Level Step 7 – Pi released from myosin. The cross bridge pulls actin, which slides over the myosin – known as the ‘Power Stroke’ creating mechanical energy. Step 8 – The release of ADP completes the cross-bridge movement. Step 9 – ATP attaches to myosin, breaking the actin-myosin crossbridge. 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.

Q. How can we use this info to explain why it’s hardest to do a “bicep curl” at the very beginning of the action and again towards the end???

A. At the beginning of the curl, the elbow is in full extension, therefore, the sarcomeres are stretched out too far. At the end of the curl, the elbow is in full flexion, therefore, the cross-bridges are interfering with each other.

force throughout bicep curl

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.