There are three main types of muscle tissue - skeletal, smooth, and cardiac. Muscle fibers are multinucleate cells formed from the fusion of individual embryonic muscle cells. Within each fiber are many parallel myofibrils composed of repeating sarcomere units. A sarcomere contains overlapping actin and myosin filaments. When an action potential reaches the neuromuscular junction, acetylcholine is released causing calcium ions to enter the fiber and allow the myosin heads to bind to actin, pulling the filaments together and contracting the muscle fiber. ATP provides energy to break the binding and reset the filaments for the next contraction.

1. cle

2. There are 3 types of muscle, that vary slightly in structure and properties:
skeletal (voluntary), smooth (involuntary) and cardiac.

3. End view of a muscle fibre
muscle fibres are ‘full’ of
long parallel protein
structures
- the myofibrils

4. The cell membrane (sarcolemma) of the muscle fibre links with the
sarcoplasmic reticulum, which extends throughout the muscle fibre

5. Muscle tissue is not
made up of
individual ‘cells’, but
giant muscle fibres
As embryonic muscle
tissue differentiates,
individual cells fuse
together, creating
multinucleate
structures, the muscle
fibres

6. Within a muscle fibre
are many long,
banded structures, the
myofibrils.
These myofibrils
extend the whole
length of a muscle
fibre.
Myofibrils have a
regular repeating
pattern. Each repeating
‘unit’ is called a
sarcomere

7. A sarcomere is composed of 2 overlapping types of fibrous proteins,
actin and myosin

8. Muscle fibre
contractions are
controlled from
the CNS by
neurons that
synapse at
neuromuscular
junctions

9. A neuromuscular junction is a synapse
Acetylcholine (Ach) is the neurotransmitter

11. The arrival of an impulse releases Calcium ions
allowing myosin/ actin cross links to form
actin
myosin

12. An action
potential is
transmitted to
the muscle
fibre’s
sarcolemma and
spreads
throughout the
muscle fibre
along its
sarcoplasmic
reticulum

13. A muscle contraction is
caused by the interlocking
actin and myosin fibres
sliding over one another,
shortening the muscle.
The arrival of a nerve
impulse, and its spread
throughout the muscle
fibre causes this ‘sliding’
contraction

14. Myosin molecules have a head
and ‘tail’, and occur in
‘bundles’ or filaments
Actin molecules are globular
and occur in chains
In a resting muscle, any reaction between actin & myosin is prevented by
tropomyosin, which blocks actin’s binding site

15. when a nerve impulse stimulates a muscle
to contract....
The action potential spreads throughout the muscle
fibre, along its sarcoplasmic reticulum
Releasing Calcium ions into the cytoplasm
Calcium ions allow myosin ‘heads’ to form cross
links with actin
The myosin molecule pulls the actin molecule ‘back’,
shortening the overall length of the fibre
ATP provides the energy to release the myosin head
and change its angle, ready to bind again

16. So long as the actin binding sites are ‘open’, myosin will
continue to bind, contract and move the actin fibres along.
This process requires energy as ATP

17. 1. Stages in muscle contraction
The muscle fibre is at rest;
Myosin is prevented from forming cross links with
actin

18. 2. Stages in muscle contraction
When Calcium ions
are present, actin sites
are ‘unblocked’

19. 3. Stages in muscle contraction
Cross- bridges
can form

20. 4. Stages in muscle contraction
The myosin head pulls the actin ‘back’

21. Summary
• Energy provided by ATP is needed for any contraction to occur
• a muscle is always “ready” to contract, but this is prevented (or ’inhibited’) by
a lack of Ca2+
ions
• Ca2+
must be present to unblock actin’s binding sites.
• AFTER a contraction, Ca2+
is pumped back into the sarcoplasmic reticulum
• So, in the absence of Ca2+
, the muscle relaxes

22. Animations
•Sliding filaments
•Actin and myosin binding model