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4.13 Fine structure of a sarcomere in the non- 4.14 Fine structure of a contracted sarcomere with
contracted state in a skeletal muscle cell (x12,000). nearby triad in a skeletal muscle cell (x12,000).

Between the two Z lines lies the smallest functional unit Heavy meromyosin has ATPase activity. Light meromyo-
of striated muscle myofibrils, the sarcomere (length 2 sin is flexible, permitting coupling of myosin with actin
μm, width 1.5 μm) (Figure 4.7). Changes in the length of a (actin–myosin complex). Shortening of skeletal muscle
group of sarcomeres results in lengthening or shortening is brought about by the sliding of actin and myosin fila-
of the muscle. Sarcomeres thus represent the contractile ments over one another (sliding filament theory, refer to
units of the muscle. biochemistry texts for further detail).
The cylindrical myofibrils are composed of parallel
bundles of thin actin and thick myosin filaments (Figure Muscle contraction
4.7). Actin filaments traverse the length of the I band Contraction is initiated by release of Ca into the sarco-
2+
2+
and insert on the Z line. These filaments are 1 μm long plasm. Following depolarisation, Ca ions penetrate the
and have a diameter of 6 nm. Actin filaments are com- interior of myofibrils and activate the contractile mecha-
posed of a double helix of F actin, a polymer of globular nism. Calcium acts on the troponin–tropomyosin system.
2+
actin molecules. An elongated molecule, tropomyosin, is In the absence of free Ca ions, troponin inhibits the
wound around the actin double helix, its periodicity cor- interaction between actin, myosin and the magnesium–
2+
responding to that of the actin molecule. Attached to each ATP complex. Binding of Ca with troponin overcomes
tropomyosin molecule is a globular troponin complex, this blockage by causing tropomyosin to move away from
2+
comprising three subunits. During contraction, Ca binds the myosin binding site on the actin filament. The myosin
to troponin, which acts on tropomyosin to expose the heads, which act both as centres of enzymatic ATP hydro-
binding site for myosin. lysis and as actin-binding sites, contact the actin filaments.
Myosin filaments (myosin II), which constitute the Energy from ATP hydrolysis is used to reposition the
thick fibres of the A band, are approximately 15 nm myosin head, which then reattaches to the actin filament
wide and 1.6 μm long. They are composed of six protein and returns to its original position causing the thin filament
chains. The myosin molecule is divided into the following to move along the thick filament. This results in shortening
components: of the sarcomere.
During repolarisation, the release of Ca ceases and
2+
2+
2+
· a rod-like, double helical tail (light meromyosin) a Ca -activated ATPase pump returns Ca against a
and concentration gradient to the sarcoplasmic reticulum,
· two globular heads (heavy meromyosin). resulting in muscle relaxation.

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