1. 13.8 Muscles are effectors which enable movement to be carried out

2. Muscle
Is responsible for almost all the movements in animals
3 types

3. Muscles & the Skeleton
Skeletal muscles cause the skeleton to move at joints
They are attached to skeleton by tendons.
Tendons transmit muscle force to the bone.
Tendons are made of collagen fibres & are very strong & stiff

4. Antagonistic Muscle Action
Muscles are either contracted or relaxed
When contracted the muscle exerts a pulling force, causing it to shorten
Since muscles can only pull (not push), they work in pairs called antagonistic muscles
The muscle that bends the joint is called the flexor muscle
The muscle that straightens the joint is called the extensor muscle

5. Elbow Joint
The best known example of antagonistic muscles are the bicep & triceps muscles

6. Muscle Structure Bicep Muscle
A single muscle e.g. biceps contains approx 1000 muscle fibres.
These fibres run the whole length of the muscle
Muscle fibres are joined together at the tendons

7. Muscle Structure Each muscle fibre is actually a single muscle cell
This cell is approx 100 m in diameter & a few cm long
These giant cells have many nuclei
Their cytoplasm is packed full of myofibrils
These are bundles of protein filaments that cause contraction
Sarcoplasm (muscle cytoplasm) also contains mitochondria to provide energy for contraction

8. Muscle Structure
The E.M shows that each myofibril is made up of repeating dark & light bands
In the middle of the dark band is the M-line
In the middle of the light band is the Z-line
The repeating unit from one Z-line to the next is called the sarcomere

9. Muscle Structure
A very high resolution E.M reveals that each myofibril is made up of parallel filaments.
There are 2 kinds of filament called thick & thin filaments.
These 2 filaments are linked at intervals called cross bridges, which actually stick out from the thick filaments

10. The Thick Filament (Myosin)
Consists of the protein called myosin.
A myosin molecule is shaped a bit like a golf club, but with 2 heads.
The heads stick out to form the cross bridge
Many of these myosin molecules stick together to form a thick filament

11. Thin Filament (Actin)
The thin filament consists of a protein called actin.
The thin filament also contains tropomyosin.
This protein is involved in the control of muscle contraction

12. Sarcomere = the basic contractile unit

13. The Sarcomere

14. I Band = actin filaments

16. Anatomy of a Sarcomere The thick filaments produce the dark A band.
The thin filaments extend in each direction from the Z line.
Where they do not overlap the thick filaments, they create the light I band.
The H zone is that portion of the A band where the thick and thin filaments do not overlap.
The entire array of thick and thin filaments between the Z lines is called a sarcomere

17. Sarcomere shortens when muscle contracts
Shortening of the sarcomeres in a myofibril produces the shortening of the myofibril
And, in turn, of the muscle fibre of which it is a part

18. Mechanism of muscle contraction
The above micrographs show that the sarcomere gets shorter when the muscle contracts
The light (I) bands become shorter
The dark bands (A) bands stay the same length

19. The Sliding Filament Theory
So, when the muscle contracts, sarcomeres become smaller
However the filaments do not change in length.
Instead they slide past each other (overlap)
So actin filaments slide between myosin filaments
and the zone of overlap is larger

20. What makes the filaments slide past each other?
Energy for the movement comes from splitting ATP
ATPase that does this is located in the myosin cross bridge head.
These cross bridges attach to actin.
The energy from the ATP causes the angle of the myosin head to change.
So they are able to cause the actin filament to slide relative to the myosin.
This movement reduces the sarcomere length.

21. The Cross Bridge Cycle The cross bridge cycle has 4 steps
It is analogues to 4 steps in rowing a boat

22. Step 1
The Cross bridge swings out from the myosin filament & attaches to the actin filament.
Put Oars in water

23. Step 2 – The Power Stroke
The cross bridge changes shape & rotates through 45 degrees
Causes the filaments to slide.
Energy from ATP is used for this power stroke
ADP + Pi are released
Pull oars through water

24. Step 3 A new ATP molecule binds to myosin
The Cross bridge detaches from the thin filament
Push oars out of water

25. Step 4 The Cross bridge changes back to its original shape
This occurs while it is detached (to ensure the actin filament is not pushed back again).
It is now ready for a new cycle, but further along the actin filament
Push oars into starting position

26. Repetition of the cycle
One ATP molecule is split by each cross bridge in each cycle.
This takes only a few milliseconds
During a contraction 1000’s of cross bridges in each sarcomere go through this cycle.
However the cross bridges are all out of synch, so there are always many cross bridges attached at any one time to maintain force.

27. Control of Muscle Contraction
How is the cross bridge cycle switched off in a relaxed muscle?
This is where the regulatory protein on the actin filament, tropomyosin is involved.
Actin filaments have myosin binding sites.
These binding sites are blocked by tropomyosin in relaxed muscle.
When Ca2+ bind tropomyosin is displaced and the myosin binding sites are uncovered.
So myosin & actin can now bind together to start the cross bridge cycle

28. Tropomyosin, Ca2+ & ATP Ca2+ causes tropomyosin to be displaced.
No longer blocks myosin binding site
Power stoke can begin.
Ca2+ also active myosin molecules to breakdown ATP
So energy is released to begin contraction

29. Neuromuscular junction: Note Ach = Acetylcholine

30. Sarcoplasmic
Reticulum

31. Sequence of events
1. An action potential arrives at the end of a motor neurone, at the neuromuscular junction.
2. This causes the release of the neurotransmitter acetylcholine.
3 This initiates an action potential in the muscle cell membrane (Sarcolemma).
4. This action potential is carried quickly into the large muscle cell by invaginations in the cell membrane called T-tubules.

32. Sequence of events
5. The action potential causes the sarcoplasmic reticulum to release its store of calcium into the myofibrils.
6. Ca2+ causes tropomoysin to be displaced uncovering myosin binding sites on actin.
7. Myosin cross bridges can now attach and the cross bridge cycle can take place.
Relaxation is the reverse of these steps