1. The document describes the structure and function of striated muscle, including the microscopic structure of myofibrils and sarcomeres. 2. It explains the sliding filament theory of muscle contraction, where an action potential triggers the release of calcium ions which allow myosin to bind to actin, forming cross-bridges that pull the filaments together through a power stroke, shortening the muscle. 3. The contraction cycle involves myosin binding to actin binding sites after ATP splits, bending myosin and shortening the sarcomere, before detaching when new ATP binds.

1. Recap from last lesson
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2. Muscle contraction
11.2.5 Describe the structure of striated muscle fibres, including the myofibrils with
light and dark bands, mitochondria, the sarcoplasmic reticulum, nuclei and the
sarcolemma
11.2.6 Draw and label a diagram to show the structure of the sarcomere, including Z
lines, actin filaments, myosin filaments with heads, and the resultant light and dark
bands
11.2.7 Explain how skeletal muscles contract, including the release of calcium ions
from the sarcoplasmic reticulum, the formation of cross-bridges, the sliding of actin
and myosin

4. Types of muscle
• Skeletal or striated muscle
• Smooth Muscle
• Cardiac Muscle

5. Striated Muscle

6. Striated Muscle
• Also called skeletal muscle because it is
responsible for skeletal movement.
• Muscles made of thousands of cells called muscle
fibres
• Connective tissues, blood vessels and nerves
• Muscle cells contain multiple nuclei that lie just
inside the membrane called sarcolemma.
• Sarcolemma has multiple tunnel-like extensions
called T tubules (transverse tubules)

7. Striated Muscle
• Cytoplasm is called the sarcoplasm
• Sarcoplasm contains a large numbers of
glycosomes that store glycogen.
• Myoglobin is also found in the sarcoplasm
• Myofibrils run the length of the cell
• Myofibrils are the contractile elements of the
muscle

8. Myofibrils are made up of sarcomeres, and
sarcomeres are the units that allow movement.
• I bands only actin
• H band only myosin
You need to be able to draw and label these diagrams

9. Myofibril structure
Actin
• Thin filaments -(8nm in
diameter)
• Contains myosin-binding
sites
• Individual molecules for
helical structures
• Includes two regulatory
proteins, tropomyosin and
troponin
Myosin
• Thick filaments (16nm in
diameter)
• Contains myosin heads that
have actin binding sites
• Individual molecules form a
common shaft-like region with
outward protruding heads
• Heads are referred to as cross-
bridges and contain ATP-
binding sites and ATPase
enzymes

10. Step 1
Sliding Filament Theory
• Action potential reaches
neuromuscular junction.
• Neurotransmitter called
acetylcholine released into
the gap
• Ach binds to receptors on
Sarcolemma
• Sodium ion channels open
and Sarcolemma membrane
depolarizes.
• Muscle action potential
generated.

11. Step 2
Sliding Filament Theory
• Action potential
moves along T-
tubule.
• Calcium ions are
released into the
Sarcoplasm from
the Sarcoplasmic
reticulum

12. Step 3
Sliding Filament Theory
• Calcium ions
released into
sarcoplasm
• Calcium ions
bind to the
troponin on the
actin
myofilaments

13. Step 4
Sliding Filament Theory
• Calcium ions
bind to the
troponin on the
actin
myofilaments
• Myosin binding
sites are exposed
• Heads contain
ATPase which
splits ATP and
releases energy.

14. Step 5
Sliding Filament Theory
• Myosin binds to myosin binding
sites helped by tropomyosin
• Myosin-actin cross bridge rotate
to produce the power
• ATP binds resulting in the
detachment of the myosin from
the actin
• No further action potentials
cause calcium levels to drop.
Troponin/Tropomyosin complex
falls back into original shape –
blocking myosin binding sites
and muscle relaxes.

15. starter
• Troponin
• Tropmyosin
• Actin
• Myosin
• Calcium
• Sarcomere
• Sarcolemma
• Sarcoplasm

16. Very Simply – this happens

17. The contraction cycle
1. Myosin heads activated by the splitting of
ATP.
2. Myosin heads attach to myosin binding sites
on actin.
3. Cross bridge formed and ADP released,
energy lost and myosin bends towards the
centre of sarcomere.
4. Myosin binds to ATP and causes the head to
detach from the actin attachment site

18. Longer answer question
Use the mark scheme, your notes and simple
diagrams to illustrate what happens from start
to finish when a muscle contract.

19. Mark scheme for exam Q
muscles / fibres / myofibrils contain (repeating) units called sarcomeres;
muscle / sarcomeres contain actin filaments and myosin filaments;
actin fibres are thin and myosin fibres are thick;
arriving action potential causes release of Ca2+;
from sarcoplasmic / endoplasmic reticulum;
Ca2+ binds to troponin;
causing troponin and tropomyosin to move (on actin);
exposing binding sites on actin / for myosin;
ATP binds to myosin heads releasing them / breaking cross bridges;
ATP hydrolysed / split into ADP + Pi;
ATP / energy causes myosin heads to change shape / swivel / become
cocked;
myosin heads bind / form cross-bridges to (exposed) actin binding sites;
myosin heads swivel / move actin (releasing ADP + Pi);
myosin filaments move actin filaments towards centre of sarcomere;
sliding of filaments / actin and myosin shortens the sarcomere; 9 max
(Plus up to [2] for quality)