Muscle Contraction at the Cellular Level
Spend some time looking at the process of muscle contraction at a cellular level. Ask your tutor
questions on concepts you do not understand via email, discussion board or Collaborate.
Inspection of skeletal muscle under high power shows a repeating pattern of light and dark bands
along the length of the myofibrils. Electron microscopy allows the ultrastructure of the myofibrils to
be examined. The repeating units are, in fact, thousands of short segments called sarcomeres,
arranged end to end. Each one contains contractile filaments arranged between anchoring
membranes called Z lines. There are two types of contractile filaments - thin filaments and thick
filaments.
Figure 3.7: From Muscle to Myofibrils
Adapted from Seeley’s, Vanputte, Regan & Russo (2017)
Thick filaments consist of myosin molecules. Myosin looks similar to a golf club with long tails and a
head. Each myosin head contains binding sites for ATP andActin.
Figure 3.8: Thick Filament
Adapted from Seeley’s, Vanputte, Regan & Russo (2017)
EXT ISAP 20 Sem 2, 2018
, Thin filaments consist of three components (Figure 3.9).
Actin - contains a binding site for myosin
Troponin - contains a binding site for Ca2+
Tropomyosin -covers the myosin binding site on the actin molecule
Figure 3.9: Thin Filament
Adapted from Seeley’s, Vanputte, Regan & Russo (2017)
The thin filament extends from the Z lines at the ends of each sarcomere towards the centre, where
they partly overlap the myosin (thick filaments) (Figure 3.10).
Figure 3.10: Myofilament Arrangement
Adapted from Seeley’s, Vanputte, Regan & Russo (2017)
Actin and myosin interaction
Actin filaments have binding sites designed to interact with 'heads' that extend from the sides of the
myosin filaments. Once bonded together, the myosin heads will bend, or try to bend, to draw the
actin inwards, towards the centre of the sarcomere and shorten it. This is how tension is developed
in the muscle fibre.
The myosin heads will repeatedly make contact with actin binding sites, then bend, or try to bend,
then break free and snap back to position to bond with another site. If a muscle is able to shorten, it's
as if the myosin 'walks' along the actin.
The need for ATP
ATP is required for the bending of the myosin, and the release of myosin from actin, so that it is ready
for more bonding. You can remember this by thinking of rigor mortis - lack of ATP following death
makes muscles rigid for some hours, till the cells start to break down and the actin-myosin bonds are
released.
The role of calcium (Excitation-Contraction Coupling)
An action potential (AP) in the motor neuron is required to generate a corresponding AP in the
muscle fibre, and the AP rapidly spreads over the sarcolemma and deep into the cell via the T-
tubules. This triggers the sarcoplasmic reticulum to release its stored Ca2+, which quickly diffuses
throughout the sarcomeres and binds to troponin. Troponin in turn makes the tropomyosin move
out of the way, exposing the myosin binding site and allowing contraction to take place. This
sequence of events, from an action potential on the surface of the muscle cell to the interaction of
the myofilaments is called Excitation-Contraction Coupling (Figure: 3.11).
EXT ISAP 20 Sem 2, 2018
Spend some time looking at the process of muscle contraction at a cellular level. Ask your tutor
questions on concepts you do not understand via email, discussion board or Collaborate.
Inspection of skeletal muscle under high power shows a repeating pattern of light and dark bands
along the length of the myofibrils. Electron microscopy allows the ultrastructure of the myofibrils to
be examined. The repeating units are, in fact, thousands of short segments called sarcomeres,
arranged end to end. Each one contains contractile filaments arranged between anchoring
membranes called Z lines. There are two types of contractile filaments - thin filaments and thick
filaments.
Figure 3.7: From Muscle to Myofibrils
Adapted from Seeley’s, Vanputte, Regan & Russo (2017)
Thick filaments consist of myosin molecules. Myosin looks similar to a golf club with long tails and a
head. Each myosin head contains binding sites for ATP andActin.
Figure 3.8: Thick Filament
Adapted from Seeley’s, Vanputte, Regan & Russo (2017)
EXT ISAP 20 Sem 2, 2018
, Thin filaments consist of three components (Figure 3.9).
Actin - contains a binding site for myosin
Troponin - contains a binding site for Ca2+
Tropomyosin -covers the myosin binding site on the actin molecule
Figure 3.9: Thin Filament
Adapted from Seeley’s, Vanputte, Regan & Russo (2017)
The thin filament extends from the Z lines at the ends of each sarcomere towards the centre, where
they partly overlap the myosin (thick filaments) (Figure 3.10).
Figure 3.10: Myofilament Arrangement
Adapted from Seeley’s, Vanputte, Regan & Russo (2017)
Actin and myosin interaction
Actin filaments have binding sites designed to interact with 'heads' that extend from the sides of the
myosin filaments. Once bonded together, the myosin heads will bend, or try to bend, to draw the
actin inwards, towards the centre of the sarcomere and shorten it. This is how tension is developed
in the muscle fibre.
The myosin heads will repeatedly make contact with actin binding sites, then bend, or try to bend,
then break free and snap back to position to bond with another site. If a muscle is able to shorten, it's
as if the myosin 'walks' along the actin.
The need for ATP
ATP is required for the bending of the myosin, and the release of myosin from actin, so that it is ready
for more bonding. You can remember this by thinking of rigor mortis - lack of ATP following death
makes muscles rigid for some hours, till the cells start to break down and the actin-myosin bonds are
released.
The role of calcium (Excitation-Contraction Coupling)
An action potential (AP) in the motor neuron is required to generate a corresponding AP in the
muscle fibre, and the AP rapidly spreads over the sarcolemma and deep into the cell via the T-
tubules. This triggers the sarcoplasmic reticulum to release its stored Ca2+, which quickly diffuses
throughout the sarcomeres and binds to troponin. Troponin in turn makes the tropomyosin move
out of the way, exposing the myosin binding site and allowing contraction to take place. This
sequence of events, from an action potential on the surface of the muscle cell to the interaction of
the myofilaments is called Excitation-Contraction Coupling (Figure: 3.11).
EXT ISAP 20 Sem 2, 2018
