Muscle Contraction and the Sliding Filament Theory

Learning Objective:• To be able to explain the sliding filament

theory using appropriate terminology.

Structure of a MuscleSkeletal muscle is made up of bundles of

muscle fibres.Fibres are bound together by connective

tissue through which run blood vessels and nerves.

Each muscle fibre contains:Numerous myofibrils,Many mitochondria,An extensive network of interconnecting tubes

called the sarcoplasmic reticulum,Many nuclei.

Structure of a Muscle

The Microstructure of a MuscleMuscle fibres contain thread like myofibrils.Myofibrils consist of repeated units called sarcomeres. Each sarcomere is divided from the next by Z lines

and contains two protein filaments: the thin actin filaments and the thick myosin filaments.

Muscle contraction occurs as the sarcomere shortens due to myosin filaments pulling the actin filaments inwards.

This results in the actin and myosin filaments being drawn closer together and the whole muscle shortening.

Sliding Filament TheoryIn its most basic form:Muscle contraction occurs when the myosin

and actin filaments slide across each other.What actually occurs is myosin pulls the actin

in.This shortens the muscle and is a muscle

contraction.

Sarcomere and Muscle ContractionDuring muscle contraction in the sarcomere:• The Z lines come closer together,• The width of the I band (area of actin not overlapping

myosin) decreases,• The width of the H zone (area of myosin not

overlapping with actin) decreases,• No change in the width of the A bands (myosin).

How does it all happen?Video clip explaining the sliding filament theory.1. A nerve impulse arrives at the

neuromuscular junction (place where motor neurone links to muscle fibre).

2. At rest tropomyosin (a protein that winds around actin) strands prevent myosin from attaching to and pulling actin.

3. At rest the sarcoplasmic reticulum (a network of channels that spread over the surface of the myofibril) stores large amounts of calcium ions.

How does it happen?4. To initiate a muscle contraction the

electrical nerve impulse travels into a myofibril via the transverse tubules and triggers the release of calcium ions from the sarcoplasmic reticulum.

5. The calcium ions bind to troponin (a globular protein on actin) causing it to change shape.

6. As it does this it moves tropomyosin and frees up the actin binding site for myosin.

7. Myosin binds to actin (forming a cross bridge), activating myosin ATP-ase (an enzyme).

8. This enzyme causes ATP to breakdown and energy is released.

9. The energy causes the myosin head to pull the actin filament along (known as the power stroke).

10.Myosin detatches and reattaches further along pulling actin in with a ratchet-like mechanism.

This process of muscular contraction can last for as long as there is adequate ATP and Ca+ stores. Once the impulse stops the Ca+ is pumped back to the sarcoplasmic reticulum and the actin returns to its resting position causing the muscle to lengthen and relax. Another clip to explain the sliding filament theory

Or act it out…Starring:Sophie ActinMatt MyosinRory and Ross ( the Calcium brothers)Becki and Charlotte (the Tropomyosin sister’s)

On a night out…Matt Myosin likes Sophie Actin.Unfortunately Sophie Actin is with her two mates,

the Tropomyosin sisters.Matt’s friends Ross and Rory Calcium admirably

take one for the team and go to chat up the Tropomyosin sisters.

With some smooth words they move the Tropomyosin out of the way, thus freeing Sophie Actin for Matt Myosin to head in for the pull!!

After a bit of ‘action’ it’s getting late so the Calcium brothers head home meaning that the Tropomyosin sisters return forcing Matt Myosin and Amy Actin to go their separate ways.

HomeworkExplain the sliding filament theory in your

own words.You can use your text book and you tube to

help (just type muscle contraction or sliding filament theory in).

You must include the following terminology: Muscle fibre, myofibril, actin, myosin,

sarcomere, I Band, A Band, H Zone, Z Lines, Sarcoplasmic reticulum, troponin, tropomyosin, transverse tubules, calcium ions, ATP, myosin ATP-ase.