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

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

How is a rope like a muscle fibre?

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

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

Sarcomere

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

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

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.

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.

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

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

Sliding filament theory

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