Download The sliding filament model

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.