The Mechanism of Musculr Contracion
When a muscle contracts, one kind of ilament within it slides
past another kind. Electron microscopy and other techniques have
begun to disclose how the ilaments exert a force on each other
t outstanding characteristic of all
animals is their ability to move
voluntarily by contracting their
muscles. When I summarized our un
derstanding of muscle contraction sev
en years ago see "The Contraction of
Muscle," by H. E. Huxley; SCIENTIFIC
AMERICAN, November, 1958], it had al
ready been determined that during con
traction two kinds of ilament in volun-
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BAND�I--A
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by H. E. Huxley
tary muscle-thick ilaments and thin
ones-slide past each other so as to pro
duce changes in the length of the mus
cle. At that time one could ofer only
a hypothetical description of contrac
tion at a more detailed level; it was as
sume. that a relative force is somehow
exerted between the thick and thin ila
ments at sites where they are connected
by tiny cross-bridges. Now, thanks to
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BAND�I
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Z
LINE
PSEUDO
H ZONE
Z
LINE
STRIATED MUSCLE from the leg of a frog is shown in longitudinal section in an electron
micrograph (top) and the overlap of ilaments that gives rise to its band pattern is illus·
trated schematically (bottom). Parts of two myoibrils (long parallel strands organized
into muscle iber) are enlarged some 23,000 diameters in the micrograph. The myoibrils are
separated by a gap running horizontally across the micrograph. The major features of the
sarcomere (a functional unit enclosed by two membranes, the Z lines) are labeled. The I
band is light because it consists only of thin ilaments. The A band is dense (and thus
dark) where it consists of overlapping thick and thin ilaments; it is lighter in the H zone,
where it consists solely of thick ilaments. The 11 line is caused by a bulge in the center of
eacb tbick ilament, and tbe pseudo H zone by a bare region immediately surrounding the
bulge. The electron micrograph and others illustrating this article were made by the author.
18
advances in electron microscopy and
allied techniques, we have been able to
substantiate thathypothesis and to learn
considerably more about the nature of
the interaction of the thick ilaments
(composed mainly of the protein myo
sin) and the thin ones (composed of an
other protein, actin). It appear; that at
eachsitewhere the proteins of the two
kinds of ilament are in contact one of
them (probably myosin) acts as an en
zyme to split a phosphate group from
adenosine 'iphosphte (ATP) ad thus
provide the energy for contraction. The
basic problem is to understand how the
conversion of chemical into mechanical
energy takes place.
Let us briey review what is known
about thestructureandfunction ofmus
cle. Under the microscope voluntary
muscles-for example those that can
move the leg of a frog-appear regu
larly striated at right angles to their
length. The muscles responsible for the
slow and regular movements of organs
that work involuntarily, suchas the gut,
appearsmooth. For reasons o technical
convenience most investigations of mus
cle have dealt with striated muscle, and
so our discussion will refer speCiically
to muscle of that type. A good deal of
what has been learned about striated
muscle, however, may apply to smooth
muscle as well.
Sriated muscle can shorten at speeds
equal to several times its length per sec
ond; it can generate a tension of some
40 pounds per square inch of its cross
section; it can contract or relax in a
very small fraction of a second. A mus
cle consists of individual ibers with a
diameter of between 10 and 100 mi
crons (a micron is a thousandth of a
millimeter); the ibers run the length of
the muscle, or a good part of it. Each
iber issurroundedby an electricallypo-
© 1965 SCIENTIFIC AMERICAN, INC