1. They are specialised effectors that contract to cause movement
MUSCLES
Muscles are organs,
They are specialised effectors that contract to cause movement

2. There are three basic types of muscle:-
Found only in
heart tissue
Found in
voluntary muscle
tissue e.g. biceps
Found in
involuntary
muscle tissue
e.g. stomach,
artery walls

3. Skeletal Smooth Cardiac
Appearance
Striated, multinucleate fibres
Discrete uninucleate spindle shaped cells. No striations
Striated, branched fibres with intercalated discs between cells (discrete lines)
Distribution
Attached to bone found throughout the body
Lining of the gut & blood vessels, the iris & ciliary body in the eye.
Wall of the heart
Nervous control
Voluntary, conscious control
Involuntary, automatic control: reflex actions
Myogenic & involuntary control

5. skeletal muscle
Voluntary muscle that is under conscious control
It is also called striated muscle because of its striped appearance.

6. structure of skeletal muscle
Muscles are attached to bones by tendons and are made up of hundreds of specialised cells called muscle fibres, which are several cms long
the surface membrane of each muscle fibre is called the sarcolemma and is deeply folded to form transverse or T-tubules.

7. The cytoplasm is called the sarcoplasm and contains many mitochondria.
Many nuclei are found just below the sarcolemma, making muscle fibres multinucleate.
The sarcoplasm also contains long specialised units called myofibrils that run the length of the fibre.

8. Muscle
fibres

9. nuclei
sarcolemma

10. sarcolemma
sarcoplasm
myofibril
nuclei

11. Ultrastructure of myofibrils
Each myofibril consists of 2 types of proteins, often called myofilaments
Thick myofilaments are made of the large protein myosin, whilst
Thin myofilaments are composed of the smaller protein actin
Muscle fibre > myofibrils  myofilaments (actin & myosin)

12. (light) actin &
(dark) myosin
myofilaments

14. Myosin and actin filaments lie adjacent to one another.
Myosin filaments are linked by a thin disc, the M line, which runs perpendicular to the filaments
M line
Myosin protein filament

15. Z line
Actin filaments are linked in a similar way, held together by a disc called the Z line.
Actin protein
filament

16. Infoldings of sarcolemma
actin and myosin
filaments overlap
mitochondria
nucleus
sarcoplasm
Infoldings of sarcolemma
form T tubules

17. The actin filaments slot between the myosin filaments.
The thicker myosin filaments appear darker than the thinner actin and give the skeletal muscle its striated/banded pattern under the microscope.
The section of myofibril between 2 Z-lines forms the basic contractile unit and is called a sarcomere.

18. The part of the myofibril
containing only actin is called the I-band (isotropic)
containing myosin, including the overlap with actin, is called the A-band (anisotropic)
Within the A-band containing myosin only is called the H-zone
Detailed inspection shows a regular arrangement in the area of overlapping myosin and actin. Each myosin being surrounded by 6 actin filaments in a hexagonal pattern.

19. Z line
Z line
one sarcomere
H-band
M line
I-band
A-band

21. M

23. Muscle contraction sliding filament mechanism.
When a muscle contracts the actin filaments are pulled over the myosin filaments (towards the M line), reducing the length of each sarcomere and therefore the whole muscle.
This process, known as the
sliding filament mechanism.

24. How does this occur?
Bulbous heads extend from the myosin filaments, which are able to attach to receptor complementary receptor sites on the actin filaments.
receptor site
actin filament
myosin head
myosin filament

25. At rest the actin receptor sites are blocked by an accessory protein called tropomyosin, which prevents the myosin heads from binding.
tropomyosin
blocking actin
receptor sites

27. The sliding filament mechanism
1. An action potential arrives at the neuromuscular junction causing the release of neurotransmitter substance that crosses the synaptic cleft and depolarises the post synaptic membrane. The AP travels along the sarcoplasm through the T tubule system.
2. The AP causes calcium ion channels in the sarcoplasmic reticulum (ER of muscle fibres) to open.
3. Ca2+ stored in the sarcoplasmic reticulum diffuse into the sarcoplasm down their concentration gradient.
4. Ca2+ cause the tropomyosin to move, freeing the actin binding sites for the myosin heads to attach, forming actomyosin bridges.
5. When attached the myosin heads change shape, rotating back to an angle of 45o. This pulls the actin filaments over the adjacent myosin filaments (about 10nm).

28. 6. ATP attaches to each myosin head & is hydrolysed, releasing energy which causes the myosin head to detach from the stationary actin binding site and return to its original position.
7. The detached myosin heads repeat the process, attachment, rotation, release, about 5 times per second.
8. The cycle will continue as long as the muscle receives nervous stimulation and Ca2+ is present.
Each individual sarcomere (about 2.5mm) moves a very short distance, but the combined movement of all the sarcomeres lined end to end in a muscle fibre, all contracting at the same time, is considerable.

29. does not change in length
The movement of the actin filaments across the myosin filaments causes the following changes:
the sarcomere shortens,
the Z lines become closer together, so the
I bands become shorter
The H zone becomes shorter
However the A band (length of myosin)
does not change in length

30. H zone
Myosin heads

32. How an action potential causes a muscle to contract
The synapse between the axon of a motor neurone and a muscle is called the neuromuscular junction.
The part of the muscle fibre that forms the post-synaptic membrane is called the motor end plate.
It is infolded to increase the surface area for more neurotransmitter receptors.

33. striated muscle fibres are stimulated by motor neurones
innervated by a
motor neurone
Motor end plate

34. Motor end plate

36. motor end plate in section
myelin sheath
synaptic
cleft
vesicle
neurone axon
presynaptic
membrane
mitochondria
postsynaptic
membrane
sarcolemma
sarcoplasm
sarcoplasmic
reticulum
Myofibril contains repeating
dark and light bands, responsible
for the striped appearance
A band
Dark band
I band
Light band

37. The strength of the contraction depends on:
There are many neuromuscular junctions along each muscle fibre. Each receives an AP at the same time, ensuring the whole muscle contracts at the same time, rather than a wave gradually spreading across it.
The strength of the contraction depends on:
How long the muscle is stimulated
How many muscle fibres are stimulated and contracting.

39. REVIEW

40. the structure and function
Give an account of
the structure and function
of skeletal muscle