For example, you cannot make your heart muscle stop beating. Likewise, you cannot make
food stop moving through your intestines. You can however control the movement of your
skeletal muscle. When you want to move your foot, electrical messages calledimpulsesmove
along nerve cells from your brain to the muscles of your foot. At the point at which the
nerve cell and muscle cells meet, the electrical message is converted to a chemical message.
The muscle cells receive the chemical message, which causes tiny protein fibers inside the
muscle cells to get shorter. The muscles contract, pulling on the bones, and your foot moves.
Contraction
Amuscle contraction occurs when amuscle fiber, which is a muscle cell, generates
tension through the movement ofactinandmyosin, two of the proteins involved in this
process (see below).
Each muscle fiber contains cellular proteins and hundreds or thousands of myofibrils. Each
myofibrilis a long, cylindrical organelle that is made up of two types of protein filaments:
actin and myosin. The actin filament is thin and threadlike, while the myosin filament is
thicker. Myosin has a “head” region that uses energy from ATP to “walk” along the actin
thin filament (Figure16.24). The overlapping arrangement of actin and myosin filaments
gives skeletal muscle its striated appearance. The actin and myosin filaments are organized
into repeating units called sarcomeres, which can be seen inFigure16.24. The sarcomeres
stretch from one Z-line to the next, with thin actin filaments anchored to these Z lines. When
each end of the myosin thick filament moves along the actin filament, the two actin filaments
at opposite sides of the sacromere are drawn closer together and the sarcomere shortens, as
shown inFigure16.25. When a muscle fiber contracts, all sarcomeres contract at the same
time, which pulls on the fiber ends.
The Sliding Filament Theory
The widely accepted theory of how muscles contract is called thesliding-filament model
(alsoknownasthesliding filament theory), whichisshowninFigure16.26. Thepresence
of calcium ions (Ca2+) allows for the interaction of actin and myosin. In the resting state,
these two proteins are prevented from coming into contact. Two other proteins, troponin
and tropomyosin, act as a barrier between the actin and myosin, preventing contact between
them. When Ca2+binds to the actin filament, the shape of the troponin-tropomyosin com-
plex changes, allowing actin and myosin to contact with each other. Below is an outline of
the sliding filament theory.
- Once an action potential (see theControlling the Bodychapter) reaches a muscle fiber,
the action potential spreads through the muscle fiber’s network, activating specialized
storage sites throughout the muscle, called thesarcoplasmic reticulum, to release