1. Muscle Contraction and the Sliding Filament Theory
Learning Objective:
To be able to explain the sliding filament theory using key terminology.

2. Structure of a Muscle
Skeletal muscle is made up of bundles of muscle fibres.
Fibres are bound together by connective tissue through which run blood vessels and nerves.

3. Structure of a Muscle Each muscle fibre contains: Numerous myofibrils,
Many mitochondria,
An extensive network of interconnecting tubes called the sarcoplasmic reticulum,
Many nuclei.

4. The Microstructure of a Muscle
Muscle 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.

6. The Microstructure of a Muscle
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.

7. Sliding Filament Theory
In 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.

8. Sarcomere and Muscle Contraction
During 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).

9. How does it all happen?
Video clip explaining the sliding filament theory.
A nerve impulse arrives at the neuromuscular junction (place where motor neurone links to muscle fibre).
At rest tropomyosin (a protein that winds around actin) strands prevent myosin from attaching to and pulling actin.
At rest the sarcoplasmic reticulum (a network of channels that spread over the surface of the myofibril) stores large amounts of calcium ions.

10. How does it happen?
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.
The calcium ions bind to troponin (a globular protein on actin) causing it to change shape.
As it does this it moves tropomyosin and frees up the actin binding site for myosin.
Myosin binds to actin (forming a cross bridge), activating myosin ATP-ase (an enzyme).

11. This enzyme causes ATP to breakdown and energy is released.
The energy causes the myosin head to pull the actin filament along (known as the power stroke).
Myosin detaches 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

12. Homework Explain the sliding filament theory in your own words.
First, try it from memory (omitting any key terms if you can’t remember them straight away)
Then, re-watch the YouTube clip and fill in any blanks.
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.

13. Initiation of a Muscle Contraction
Muscle motor units are stimulated by electrical signals sent from the brain down the spinal cord.
This signal reaches a motor neurone (the nerve cells which pass messages from the spinal cord to the muscle fibres)
The point at which the motor neurone meets the muscle fibre is the motor end plate and this forms the neuromuscular junction (junction between nerves and muscle, aka synaptic cleft)
Calcium ions enter the synaptic knob (the end of the motor neurone) and cause the release of acetylcholine.
The acetylcholine spreads across the synapse forming a bridge to allow the electrical impulse (action potential) to travel across.
The action potential triggers the release of more calcium ions which travel down the t-tubules to the sarcoplasmic reticulum, releasing calcium ions from here and beggining the sliding filament theory

14. 5 Stages of Muscle Contraction
There are 5 different stages of a muscle contraction.
Resting – when no impulse is sent from brain to muscle
Excitation – initiation of contraction as action potential is sent across neuromuscular junction
Contraction – sliding filament theory
Re-charge – the resynthesis of ATP to allow muscle contraction to be maintained
Relaxation – occurs when either the electrical signals from the brain stop being sent or when ATP stores run out.

15. Explain the physiology of a muscular contraction from nervous impulse to initiation of contraction. (6)

16. Outline how force is produced during the contraction phase of a muscle using sliding filament theory. (5)