1. Muscles
3.5.3 Skeletal muscles are stimulated to contract by nerves and act as effectors

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

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

4. Muscle Structure Each muscle fibre is actually a single muscle cell
This cell is approx 100 μm 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

5. Sarcomere = the basic contractile unit

6. 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

7. 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

8. 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

9. 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

10. The Sarcomere

12. I Band = actin filaments

14. 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

15. 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

16. 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

17. The Sliding Filament Theory
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

18. 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 heads.
The energy from the ATP causes the angle of the myosin head to change.
The myosin heads can attach to actin.
Movement of the myosin heads and them attaching and detaching from actin causes the filaments to slide relative to one another.
This movement reduces the sarcomere length.

19. 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.

20. The Cross Bridge Cycle

21. The cycle begins with ATP binding to the myosin head
The cycle begins with ATP binding to the myosin head. This causes the myosin head to be released from actin.

22. The Cross Bridge Cycle

23. 2. The ATP molecule is then hydrolysed while the myosin head is unattached. The ADP & Pi formed remain bound to the myosin head.

24. The Cross Bridge Cycle

25. 3. The energy released by the hydrolysis of ATP is absorbed by the myosin
This causes the myosin head to change shape (places it in energised state or cocked state – also called the recovery stroke)
It then binds to the actin filament.

26. The Cross Bridge Cycle

27. 4-5. The ADP and Pi are then released from the myosin head
Result = Power stroke occurs (the myosin head changes shape)
This draws the actin filament over the myosin filament.

28. The Cross Bridge Cycle

29. The cycle begins again when the next ATP binds to the myosin head
The cycle begins again when the next ATP binds to the myosin head. Causing the myosin head to be released from actin.

30. Control of Muscle Contraction
How is the cross bridge cycle switched off in a relaxed muscle?
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

31. Tropomyosin, Ca2+ & ATP Ca2+ causes tropomyosin to be displaced.
So it no longer blocks the myosin binding site
So myosin and actin can bind together allowing cross bridge cycling

32. Neuromuscular junction: Note Ach = Acetylcholine

33. Sarcoplasmic
Reticulum

34. 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.

35. 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