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MECHANISM OF MUSCLE CONTRACTION
Sliding filament theory
In 1954, two researchers, Jean Hanson and Hugh Huxley from the Massachusetts Institute of
Technology, made a model for muscle tissue contraction which is known as the sliding filament
theory. This theory describes the way a muscle cell contracts or shortens as a whole by the sliding
of thin filaments over thick filaments and pulling the Z discs behind them closer.
Six different proteins and molecules participate in the contraction of a sarcomere, namely:

Myosin

Actin

Tropomyosin

Troponin

ATP

Ca2+ ions
Sliding filament theory, a proposed mechanism of muscle contraction in which the actin and
myosin filaments of striated muscle slide over each other to shorten the length of the muscle fibers.
Myosin-binding sites on the actin filaments are exposed when calcium ions bind to troponin
molecules in these filaments. This allows bridges to form between actin and myosin, which
requires ATP as an energy source. Hydrolysis of ATP in the heads of the myosin molecules causes
the heads to change shape and bind to the actin filaments. The release of ADP from the myosin
heads causes a further change in shape and generates mechanical energy that causes the actin and
myosin filaments to slide over one another.

Thick Filaments
Myosin molecules are bundled together to form thick filaments in skeletal muscles. A myosin
molecule has two heads which can move forward and backward and binds to ATP molecule and
an actin binding site. This flexible movement of the head provides power stroke for muscle
contraction.

Thin Filaments
The thin filaments are composed of three molecules - actin, tropomyosin, and troponin. Actin is
composed of actin subunits, joined together and twisted in a double helical chain. Each actin
subunit has a specific binding site to which myosin head binds. Tropomyosin entwines around the
actin. This cover the binding sites of actin subunits, preventing myosin heads from binding to them
in an unstimulated muscle. Troponin molecules are attached to tropomyosin strands and facilitate
tropomyosin movement so that myosin heads can bind to the exposed actin binding sites. The
sarcomeres can hence shorten. This, however, can only occur with the binding of Ca2+ ions to
troponins first.
Mechanism of contraction of the sliding filament

Excitation
Once an action potential arrives at the axon terminal, acetylcholine is released, resulting in the
depolarization of motor end plate. This action potential propagates along the sarcolemma and
down the T-tubules causing the release of Ca2+ ions from the terminal cisternae into the
cytosol. Ca2+ ions then bind to troponin causing a conformational change in the troponintropomyosin complex, which exposes the binding sites on actin. As illustrated in Figure 3, the
myosin head is already energized, as an ATP molecule binds to a myosin head where an
enzyme called myosin ATPase hydrolyzes the ATP. This releases the energy resulting in an
extension of myosin head, carrying high energy, while holding ADP and a phosphate group
temporarily.

Contraction
This energized and cocked myosin head binds to an active site on the exposed actin binding
site as shown in Figure 3. With a power stroke, the thin actin filaments slide along the myosin.
The myosin hears changes from a high energy extended position to a low energy flexed
position. ADP and a phosphate group are released. The myosin head still remains bound to
actin filament until it binds to a new ATP molecule. Once a new ATP binds to myosin head, it
releases actin and changes back to a high energy extended position, ready for a next cycle of
causing power stroke. Such alternative power stroke occurs concurrently in thousands of
myosin heads with actin filaments, resulting in an overall contraction of a muscle fiber. These
contractions occurring in millions of muscle fibers, in turn, cause an entire skeletal muscle to
contract.

Relaxation
After a brief time, the acetylcholine diffuses away from their receptor sites causing the
acetylcholine receptors to close back as shown in Figure 4. The acetylcholine is then broken
down by an enzyme acetylcholinesterase present at the synaptic cleft. Soon after contraction,
Ca2+ ions are actively transported from cytosol back to sarcoplasmic reticulum via specialized
Ca2+ pumps. ATP is expended in this process of active transport. After the Ca2+ ions are
removed from the cytosol, the troponin-tropomyosin complex covers the active binding sites
of actin subunits once again, so that myosin heads cannot bind to actin. This results in the
relaxation of a muscle cell.