Rigor mortis occurs after death when muscles remain contracted without ATP. Muscle contraction occurs via the sliding filament theory where calcium ions cause actin and myosin filaments to bind via cross-bridges, using energy from ATP hydrolysis. After death, with no ATP to detach actin and myosin, the muscles remain contracted in a state known as rigor mortis.

1. Rigor mortis occurs after death. How do
the muscles remain contracted without
ATP?
Thinker & Communicator
Title: Muscle contraction
Date: 29 March 2023
 Understand the process of contraction of skeletal muscle in terms of the sliding filament
theory, including the role of actin, myosin, troponin, tropomyosin, calcium ions (Ca2+), ATP
and ATPase.
Homework: Complete
Class Notebook

2. First things first…!
 What types of muscle do you know?
Skeletal (conscious contraction)
Smooth (unconscious contraction)
Cardiac (unconscious contraction)

3. How is a rope like a muscle fibre?

5. The structure of striated skeletal
muscle.
 Each muscle is called a fibre. Each fibre
made up of a bundle of myofibrils.
 Each myofibril is made of myofilaments -
actin and myosin.
 The myofilaments are arranged so that
each myosin is surrounded by 6 actins

6. A bit of added strength!
 The muscle cells are fused together (fibres) and
share nuclei and cytoplasm called sarcoplasm.

7. Sarcomere

8. Microscopic structure

The lighter bands are called I
bands (isotropic). They are
lighter because the actin and
myosin filaments are not
overlapping
The darker bands are called A
bands (anisotropic). They are
darker because the actin and
myosin filaments are
overlapping
Task: Add labels to
your sarcomere and draw
a diagram to represent
the actin and myosin
filaments
H Zone
Note: STRIATED MUSCLE. : muscle tissue that is marked by transverse
dark and light bands

9. Let’s take a simpler look!
Thin!
Thick!

10. The structure of actin
 It consists of 2 threads wrapped around each
other. At each twist there is a binding site for
myosin. In a relaxed state, a molecule
called tropomyosin covers these sites.

11. The structure of myosin
 The filament consists of many myosin molecules. Each
molecule has a tail and a double globular head.
 The head attaches to the myosin binding sites on the
actin where the actin and myosin filaments overlap.
 These attachments are called cross-bridges. The heads
contain ATPase enzyme which releases energy from ATP
to power muscle contraction.

12. Muscle contraction
 Contraction occurs when an impulses from
a motor neurone reaches the synapse at
the junction with the muscle.

13. Sliding filament Hypothesis.
 Acetylcholine, a neurotransmitter substance, is released into the synapse,
diffuses across and attaches to specific receptors on the sarcolemma (the outer
membrane of the muscle fibre).
 The muscle sarcolemma is depolarised.
 Depolarisation spreads along the fibre.
 This causes calcium to be released from the sarcoplasmic reticulum into the
sarcoplasm.
 Calcium displaces tropomyosin, thus uncovering the myosin binding sites on the
actin filaments.
 ATP attached to the myosin heads cause them to flex and attach to the actin in
the overlapping areas.
 ATP is hydrolysed to ADP + P. The energy released causes the heads to alter their
angle to their tails. This pulls the actin filament past the myosin filament.
 The cross-bridges detach and reattach, this time further along the actin
filament.
https://www.youtube.com/watch?v=aUc3h6LvdJ
4

15. Sliding filament theory

16. Rigor mortis occurs after death. How do the
muscles remain contracted without ATP?
 Understand the process of contraction of skeletal muscle in terms of the sliding filament
theory, including the role of actin, myosin, troponin, tropomyosin, calcium ions (Ca2+), ATP
and ATPase.
ATP is only required to
relax and “reset” the
muscle, not to contract.
No ATP means muscle
remains contracted