Abi Starr
Chapter 11.2 Contraction of skeletal muscle
The process of skeletal muscle contraction is caused by the arrangement of the various proteins. This
process involves the actin and myosin fibres sliding past one another and is therefore called the sliding
filament mechanism.
Evidence for the sliding filament mechanism
Myofibrils appear darker where the actin and myosin filaments overlap and lighter where they do not. If
the sliding filament mechanism is correct, then there will be more overlap of actin and myosin in a
contracted muscle than in a relaxed one. When a muscle contracts, the following changes occur to a
sarcomere:
The I-band narrows
The Z-lines move closer together, or in other words, the sarcomere shortens
The H- zone becomes narrower
The A-band remains the same width. As the width of this band is determined by the length of the myosin
filaments, it follows that the myosin filaments have not become shorter. This discounts the theory that
muscle contraction is due to the filaments themselves shortening.
The three main proteins involved in the process:
Myosin – this is made of two types of protein
- A fibrous protein arranged into a filament made up of several hundred molecules (the
tail)
- A globular protein formed into two bulbous structures a one end (the head)
Actin – A globular protein whose molecules are arranged into long chains that are twisted
around one another to form a helical strand
Tropomyosin forms long thin threads that are wound around actin filaments
The sliding filament mechanism of muscle contraction
The hypothesis that actin and myosin filaments slide past one another during muscle contraction is
supported by the changes seen in the band pattern on myofibrils. The next question for the scientists
was: by what mechanism do the filament slide past one another? Clues lie in the shape of the proteins
involved.
In summary, the bulbous heads of the myosin filaments form cross-bridges with the actin filaments. They
do this by attaching themselves to binding sites on the actin filaments, and then flexing in unison, pulling
the actin filaments along the myosin filaments. They then become detached and, using ATP as a source
of energy, return to their original angle and re-attach themselves further along the actin filaments. This
process is repeated up to 100 times a second. The action in similar to the way a ratchet operates.
The following describes the sliding filament mechanism of muscle contraction in detail. The process is
continuous but, for ease of understanding, has been divided into stimulation, contraction and relaxation.
Muscle stimulation
An action potential reaches many neuromuscular junctions simultaneously, causing calcium ion
channels to open and calcium ions to move into the synaptic knob
The calcium ions cause the synaptic vesicles to fuse with the presynaptic membrane and release
their acetylcholine into the synaptic cleft
Page 1 of 2
Chapter 11.2 Contraction of skeletal muscle
The process of skeletal muscle contraction is caused by the arrangement of the various proteins. This
process involves the actin and myosin fibres sliding past one another and is therefore called the sliding
filament mechanism.
Evidence for the sliding filament mechanism
Myofibrils appear darker where the actin and myosin filaments overlap and lighter where they do not. If
the sliding filament mechanism is correct, then there will be more overlap of actin and myosin in a
contracted muscle than in a relaxed one. When a muscle contracts, the following changes occur to a
sarcomere:
The I-band narrows
The Z-lines move closer together, or in other words, the sarcomere shortens
The H- zone becomes narrower
The A-band remains the same width. As the width of this band is determined by the length of the myosin
filaments, it follows that the myosin filaments have not become shorter. This discounts the theory that
muscle contraction is due to the filaments themselves shortening.
The three main proteins involved in the process:
Myosin – this is made of two types of protein
- A fibrous protein arranged into a filament made up of several hundred molecules (the
tail)
- A globular protein formed into two bulbous structures a one end (the head)
Actin – A globular protein whose molecules are arranged into long chains that are twisted
around one another to form a helical strand
Tropomyosin forms long thin threads that are wound around actin filaments
The sliding filament mechanism of muscle contraction
The hypothesis that actin and myosin filaments slide past one another during muscle contraction is
supported by the changes seen in the band pattern on myofibrils. The next question for the scientists
was: by what mechanism do the filament slide past one another? Clues lie in the shape of the proteins
involved.
In summary, the bulbous heads of the myosin filaments form cross-bridges with the actin filaments. They
do this by attaching themselves to binding sites on the actin filaments, and then flexing in unison, pulling
the actin filaments along the myosin filaments. They then become detached and, using ATP as a source
of energy, return to their original angle and re-attach themselves further along the actin filaments. This
process is repeated up to 100 times a second. The action in similar to the way a ratchet operates.
The following describes the sliding filament mechanism of muscle contraction in detail. The process is
continuous but, for ease of understanding, has been divided into stimulation, contraction and relaxation.
Muscle stimulation
An action potential reaches many neuromuscular junctions simultaneously, causing calcium ion
channels to open and calcium ions to move into the synaptic knob
The calcium ions cause the synaptic vesicles to fuse with the presynaptic membrane and release
their acetylcholine into the synaptic cleft
Page 1 of 2
