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Movement often requires the
contraction of a skeletal muscle, as can be observed when the bicep muscle in
the arm contracts, drawing the forearm up towards the trunk. The sliding
filament model describes the process used by muscles to contract. It is a
cycle of repetitive events that causes actin and myosin myofilaments to slide
over each other, contracting the sarcomere and generating tension in the
To understand the sliding
filament model requires an understanding of sarcomere structure. A
sarcomere is defined as the segment between two neighbouring, parallel Z-lines.
Z lines are composed of a mixture of actin myofilaments and molecules of the
highly elastic protein titin crosslinked by alpha-actinin. Actin myofilaments
attach directly to the Z-lines, whereas myosin myofilaments attach via titin
Surrounding the Z-line is the
I-band, the region where actin myofilaments are not superimposed by myosin
myofilaments. The I-band is spanned by the titin
molecule connecting the Z-line with a myosin filament.
The region between two
neighboring, parallel I-bands is known as the A-band and contains the entire
length of single myosin myofilaments. Within the A-band is a region known as
the H-band, which is the region not superimposed by actin myofilaments. Within the H-band is the M-line, which is composed of myosin myofilaments and
titin molecules crosslinked by myomesin.
Titin molecules connect the
Z-line with the M-line and provide a scaffold for myosin myofilaments. Their elasticity
provides the underpinning of muscle contraction. Titin molecules are thought to
play a key role as a molecular ruler maintaining parallel alignment within the
sarcomere. Another protein, nebulin, is thought to perform a similar role for
Model of Contraction
The molecular mechanism whereby
myosin and acting myofilaments slide over each other is termed the cross-bridge
cycle. During muscle contraction, the heads of myosin myofilaments quickly bind
and release in a ratcheting fashion, pulling themselves along the actin
At the level of the sliding
filament model, expansion and contraction only occurs within the I and H-bands. The myofilaments themselves do not contract or expand and so the A-band remains
The amount of force and movement generated generated by an individual sarcomere is small. However, when multiplied by the number of sarcomeres in a myofibril, myofibrils in a myocyte and myocytes in a muscle, the amount of force and movement generated is significant.