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Movement often requires the
contraction of a skeletal muscle, as can be observed when the biceps muscle in
the arm contracts drawing the forearm upwards towards the trunk. The sliding
filament theory describes the process used by muscles to contract. It is a
cycle of repetitive events that cause actin and myosin myofilaments to slide
over each other, contracting the sarcomere and generating tension in the
To understand the sliding
filament model and understanding of sarcomere structure is first required. 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 which is the region where actin myofilaments are not superimposed by myosin
myofilaments. An alternative description is the region spanned by the titin
molecule connecting the Z-line with a myosin filament.
The region between two
neighbouring, parallel I-bands is known as the A-band which contains the entire
length of a single myosin myofilaments. Within the A-band is a region known as
the H-band which is the region not superimposed by actin myofilaments; and
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 but they are also 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. Briefly, during muscle contraction the heads of myosin myofilaments 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
At the level of an individual sarcomere the amount of force and movement generated is small. However, when this is multiplied by the numbers of sarcomeres in a myofibril, myofibrils in a myocyte and myocytes in a muscle the amount of force and movement generated is significant.