Sliding Filament Theory

1146 words 5 pages
How Do We Move?

Many of us go through our daily lives and activities without much thought on how

or why we move our bodies. Walking, jogging, lifting weights or even getting ourselves

out of bed in the morning requires an intricate pattern of processes that allow us to

move and access our enviroment. Our bodies move through a lever and pulley system

made up of our muscles bones and tendons acting on each other through muscle

contraction and relaxation. (1,3) To understand how a muscle contracts you must first

look at the anatomy of skeletal muscles.

Anatomy of Skeletal Muscle

Figure 1 shows the components of a cross section of muscle. Each muscle belly

is made up of thousands to tens of
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At

the junction between the nerve end

and the muscle, chemical signals

are released from the nerve

endings. This binds to a key on the

surface of the muscle. The binding

of this chemical to the receptor

causes calcium ions stored in the

sarcoplasmic reticulum to enter the muscle cells. (5) Tropomyosin has a control

function so without calcium ions, the tropomyosin prevents muscle contraction by

blocking the binding sites on actin so it can not interact with the myosin heads. When
calcium enters the cell it binds to troponin and exposes these sites. Myosin hydrolyzes

ATP into a high energy state and the head of the myosin attaches to the actin. This

forms a cross bridge between the actin and myosin filaments. The energy stored by

myosin is then released and turned to ADP and inorganic phosphate. This results in a

power stroke causing a rotation of the globular head that causes the filaments to slide

past each other. ATP will then trigger the release of the cross bridge and the head will

be in a low energy state. As ATP is split into ADP and inorganic phosphate the myosin

head cocks in preparation to attach again. As long as calcium is present the process

continues moving the filaments along each other. When the action potential or electrical

impulse ends the calcium ions return to the sarcoplasmic reticulum where it remains

until needed. The importance of

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