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9.7 The Biochemistry of Movement: 5. Skeletal muscle cell contraction
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5. Muscle cells cause movement by contraction along
their length
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Students learn to:
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Students: |
Extract from Chemistry Stage 6 Syllabus (Amended
October 2002) © Board of Studies, NSW.
[Edit:27 Jun 08]
Background: Muscles are the tissues of our body which cause movement. We have heart muscle which pumps blood and smooth muscle which lines arteries and the digestive tract. However, the greatest amount of muscle in our bodies is skeletal muscle which is attached to our skeleton and which causes movement of our bones. Muscles work by contracting and relaxing.
analyse information from secondary sources to describe the appearance of type 1 and type 2 skeletal muscle cells
- Obtain diagrams or photos (both light and electron microscope) of magnified
type 1 and type 2 skeletal muscle fibres (cells) and analyse
the structures to identify the difference between the type 1 and type 2 cells.
If you are having difficulty finding clear diagrams or photos that enable
you to distinguish them, use the diagram below. In your answer explain: the
colour of the fibres when they are stained and the reason for these colours;
whether they are generally found in postural muscles and needed for aerobic
activities such as long distance running or muscles that bring about fast
movement like swimming and short distance running.
- Explain why the types of muscles appear to form a checkerboard appearance. What is the connection with the neurone that connects to the fibres with their axons and the type of skeletal mucle fibres?
describe the generalised structure of a skeletal muscle cell
- Skeletal muscle is made up of thousands of cylindrical muscle cells (fibers)
often running all the way from origin, where the muscle attaches to a usually
stationary bone, across a joint to insertion, where the muscle attaches to
the bone that moves on contraction of the muscle. The fibers are bound together
by connective tissue through which run blood vessels and nerves. Skeletal
muscle cells are often called striated muscle cells due to the hundreds of
parallel, striated myofibrils in each cell.
- Each repeating unit of dark and light bands in a myofibril is called a sarcomere.
identify actin and myosin as the long parallel bundles of protein fibres which form the contractile filaments in skeletal muscle
- The dark bands are caused by a thick filament made of a protein called myosin. In between the myosin filaments, are thin filaments composed mainly of a protein called actin.
Some interesting information
The areas of just actin filaments appear lighter under the microscope. These filaments are much thinner than the myosin filaments. The areas of overlapping actin and myosin appear darkest.
You should be able to recognise the dark parallel arrays of myosin with lighter actin lying between them and be able to identify these with the banded appearance of the myofibrils. Since the myofibrils lie parallel and work in unison, the same banded appearance is seen right across the diameter of the whole cell.
Site showing
structure of muscle fibre, myofibril, actin and myosin 
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University of Guelph, Ontario, Canada.
identify the cause of muscle cell contraction as the release of calcium ions after a nerve impulse activates the muscle cell membrane
Background information
Messages are transmitted along nerves by an electrical impulse.
Motor nerve endings transmit their electrical impulse to the cell membrane of the muscle fibre.
Muscle fibres have a special cell membrane which in turn carries the impulse along the length of the muscle fibre and also throughout a membrane complex inside the muscle fibre.
- The electrical impulse carried throughout the muscle
cell along a membrane network rapidly releases calcium ions
(Ca2+) as the impulse passes.
- The Ca2+ concentration increases about ten fold and the Ca2+ ions interact with the surface of the thin actin filaments to set off the contraction.
You could do a role play of the arrival of the impulse and the release of calcium ions.
identify that the cause of the contraction movement is the formation of temporary bonds between the actin and myosin fibres and explain why ATP is consumed in this process
- Myosin is both a structural protein and an enzyme that
hydrolyses ATP to ADP and Pi.
- When Ca2+ sets off a contraction, actin and
myosin bind together by forming cross bridges. These cross
bridges form in such a way that the actin filaments are
pulled along past the myosin filaments. The actin and
myosin filaments slide past each other and overlap each
other more.
- The chemical energy of ATP is released as mechanical
energy to actively move the actin along. The myosin
catalyses the breakdown of ATP as it binds to actin.
- The bonds between the actin and myosin are temporary
and are released as the Ca2+ ion concentration
decreases and the muscle relaxes again.
- On contraction the sarcomere decreases in size. The
lighter bands within the sarcomere decrease in size. Actin
and myosin filaments do NOT change size. They just slide
past one another.
Myosin and actin model Access Excellence, National health
museum, USA.
The cycle of muscle action:
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