4a

Question 1

Muscle contraction

Answer 1

Muscle can develop tension and shorten = Muscle contraction
Largest group of tissues in our body: About 50% of our body mass (Skeletal muscle ~35 to 40%)

Question 2

Skeletal muscle - striated

Answer 2

A myofibre is formed by the fusion of undifferentiated and mononucleated cells = Myoblasts
A single muscle cell = muscle fibre or myofibre.
=> One single cylindrical, relatively large and elongated, multinucleated cell.
In adults = 20-100µm diameter, up to 20 cm length (varies greatly)
Microscope => Alternating light bands perpendicular to the long axis.
Cardiac and skeletal muscle = striated muscles.

Question 3

Composition

Answer 3

Composition of muscle = all of the muscle fibres bound together by layers of connective tissue.
Layers of connective tissue epimysium binds entire muscle; perimysium binds groups of
fibres/cells or ‘fascicles‘; endomysium binds individual cells
Tiers:
Muscle
Fascicle
(group of cells)
Muscle fibre
(a single cell)
Myofibril
(inside cells

Question 4

What gives muscle its ‘striations’?

Answer 4

Arrangement of numerous thin and thick filaments, composed of the contractile proteins actin (thin
filaments) and myosin (thick filaments), respectively.

They are located in the cytoplasm in long links
called ‘myofibrils’ (80% of the volume of the muscle).
Filaments are arranged in a repeating pattern along the length of the myofibril. One unit of this repeating
pattern = Sarcomere (functional unit of the muscle).

Question 5

A and I Bands

Answer 5

Thick filaments are located in the center of the sarcomere => Wide dark band = A band.
For each sarcomere => Two sets of thin filaments, one at each end, are anchored to a network of connecting proteins = Z line. The other extremities overlap a portion of the thick filaments.

One sarcomere: Between two Z lines.

Lighter band = I band = Between the ends of the bands A where thin filaments do not overlap
the thick ones.

H Zone: Center of the band A where without any thin filament overlaping the thick ones.

The sarcomere is the functional unit of skeletal muscle (i.e the smallest component that can
contract in the myofibre

Question 6

titin

Answer 6

The thick filaments of myosin are anchored in place
by titin fibres; and thin filaments called actin are anchored to Z-lines (Z-lines have MANY types of anchoring proteins)
.
Titin: the largest protein in the body

Tropomyosin and troponin are important regulatory
molecules for contraction
= Bind to the thin actin
filaments
.

Question 7

Notion of cross-bridge

Answer 7

= Portion of myosin molecule (thick bands) extending to the thin actin filaments.
= This will enable the muscle contraction.
The sliding movement => Thin filaments move toward the center of the sarcomere.
=> Myosin’s cross-bridges bind to actin; the cross-bridges then flex to slide actin.
A cross-section through a sarcomere shows that:
* each myosin can interact with 6 actin filaments, and
* each actin can interact with 3 myosin filaments.