Muscle contraction
Syllabus 3.5.3
Toole page 184 -191
Aim
1. Explain the principles of the sliding
filament hypothesis of muscle
contraction.
Contraction of skeletal muscle
• When voluntary muscle contracts in response to nervous
stimulation, the actin (thin filaments) and myosin (thick
filaments) slide past each other.
• This is termed the sliding filament mechanism.
• The result is that the sarcomere shortens.
• A great deal of ATP is used in contraction.
• This occurs in a series of steps described as the ‘ratchet
mechanisms’.
The sliding filament hypothesis
of muscle contraction
1. Label the three structures shown in this diagram.
2. Describe the role of each structure.
The sliding filament hypothesis of
muscle contraction
Actin
•
Thin filament that is composed of a globular protein.
•
The molecules are arranged in long chains that are twisted around one another to
form a helical shaped strand.
•
An important myofibril that causes muscles to contract.
The sliding filament hypothesis of
muscle contraction
actin
troponin
•
When the muscle is required to contract troponin molecules bind to Calcium (Ca2+) ions.
•
The (Ca2+) ions enter the muscle when an action potential stimulates calcium ion channels to
open.
•
This binding causes the troponin to move slightly.
The sliding filament hypothesis of
muscle contraction
tropomyosin
actin
troponin
•
A protein that forms a long thin thread that wraps itself around the actin filaments.
•
At rest it is arranged in such a way that it covers the sites on the actin chain where
the heads of the myosin become attached.
The sliding filament hypothesis of
muscle contraction
myosin binding site
tropomyosin
actin
troponin
• The site at which the myosin heads bind to the
actin filament.
The sliding filament hypothesis of
muscle contraction
Ca2+
Ca2+
Ca2+
Ca2+
• Calcium ions are released from the sarcoplasmic
reticulum after stimulation from the T-tubules.
The sliding filament hypothesis of
muscle contraction
Ca2+
Ca2+
Ca2+
• The calcium ions bind to the troponin.
• It changes its shape.
Ca2+
The sliding filament hypothesis of
muscle contraction
Ca2+
Ca2+
Ca2+
Ca2+
The sliding filament hypothesis of
muscle contraction
Ca2+
Ca2+
Ca2+
Ca2+
The sliding filament hypothesis of
muscle contraction
Ca2+
Ca2+
• The troponin displaces the tropomyosin.
• This exposes the myosin binding sites.
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
The sliding filament hypothesis of muscle
contraction
Ca2+
Ca2+
Ca2+
• The bulbous heads of the myosin attach to the
binding sites on the actin filaments.
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
The sliding filament hypothesis of muscle
contraction
Ca2+
Ca2+
Ca2+
• The myosin heads change position to achieve a lower
energy state.
• This slides the actin filaments past the stationary myosin.
Ca2+
Pi Pi Pi A
Ca2+
Ca2+
Pi P i Pi A
Pi Pi Pi A
Pi Pi Pi A
Pi Pi Pi Ca
A 2+
Ca2+
Pi Pi Pi A
Ca2+
The sliding filament hypothesis of muscle
contraction
Pi Pi Pi A
Pi Pi Pi Ca
A 2+
Ca2+
• ATP binds to the bulbous heads.
Pi Pi Pi A
Ca2+
Pi Pi Pi A
Pi Pi Pi A
Pi Pi Pi A
Ca2+
Ca2+
Ca2+
• The ATP causes the myosin head to become
detached.
Pi
Pi Pi A
Pi
Pi
Pi Pi A
Pi Pi A
Ca2+
Ca2+
• The ATP is hydrolysed.
Ca2+
Pi
Pi Pi A
Pi
Pi
Pi Pi A
Pi Pi A
Ca2+
Ca2+
Ca2+
• The hydrolysis of ATP provides the energy to
“re-cock” the myosin heads.
Pi
Pi Pi A
Pi
Pi
Pi Pi A
Pi Pi A
Ca2+
Ca2+
Ca2+
Pi
Pi Pi A
Pi
Pi
Pi Pi A
Pi Pi A
Ca2+
Ca2+
Ca2+
• Calcium ions are re-absorbed back into the T
system.
Pi
Pi Pi A
Pi
Pi
Pi Pi A
Pi Pi A
Pi
Pi Pi A
Pi
Pi
Pi Pi A
Pi Pi A
• The troponin reverts to its normal shape.
Pi
Pi Pi A
Pi
Pi
Pi Pi A
Pi Pi A
Pi
Pi Pi A
Pi
Pi
Pi Pi A
Pi Pi A
• The tropomyosin move back to block the myosin
binding sites.
Pi
Pi Pi A
Pi
Pi
Pi Pi A
Pi Pi A
Pi
Pi
Pi Pi A
Pi Pi A
Pi Pi Pi A
Pi Pi Pi A
Pi Pi Pi A
Pi Pi Pi A
• Phosphocreatine regenerates ATP
Changes in a single sarcomere in relaxed and contracted myofibrils
Your task
• Using the PowerPoint, and the text books create an
active model (therefore it must move!) to represent the
sliding filament hypothesis of muscle contraction.
• Resources available are:
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Card
Paper
Art straws
String
Elastic bands
Wooden sticks (splints)
Foam packaging material