Muscle
powers the movements of multicellular animals and maintains
posture. Its gross appearance is familiar as meat or as
the flesh of fish. Muscle is the most plentiful tissue in
many animals; for example, it comprises 50 to 60 percent
of the body mass in many fishes, and 40 to 50 percent in
antelopes. Some muscles are under conscious control and
are called voluntary muscles. Other muscles, called involuntary
muscles, are not consciously controlled by the organism;
for example, in vertebrates, muscles in the walls of the
heart contract rhythmically, pumping blood around the body;
muscles in the walls of the intestines move food along by
peristalsis; and muscles in the walls of small blood vessels
constrict or relax, controlling the flow of blood to different
parts of the body. (The effects of muscle changes in the
blood vessels are apparent in blushing and paling due to
increased or decreased blood flow, respectively, to the
skin.)
Muscles
are not the only means of movement in animals. Many protists
(unicellular organisms)move instead by using cilia or flagella
(actively beating processes of the cell surface that propel
the organism through water). Some unicellular organisms
are capable of amoeboid movement, in which the cell contents
flow into extensions (pseudopodia) from the cell body. Some
of the ciliated protozoans move by means of rods called
myonemes, which are capable of shortening rapidly.
Nonmuscular
methods of movement are important for multicellular animals
as well. Many microscopic animals swim by means of beating
cilia. Some small mollusks and flatworms crawl using cilia
on the underside of the body. Some invertebrates that feed
by filtering particles fromwater use cilia to create the
necessary water currents. In higher animals, white cells
use amoeboid movements, and cilia from cells lining the
respiratory tract remove foreign particles from the delicate
membranes.
Muscles consist of long, slender cells (fibres) each of
which is a bundle of finer fi
s (Figure 1). Within each
fi
are relatively thick filaments of the protein myosin
and thin ones of actin and other proteins. When a muscle
fibre lengthens or shortens, the filaments remain essentially
constant in length but slide past each other as shown in
Figure 2. Tension in active muscles is produced by cross
bridges (i.e., projections from the thick filaments that
attach to the thin ones and exert forces on them). As the
activemuscle lengthens or shortens and the filaments slide
past each other, the cross bridges repeatedly detach and
reattach in new positions. Their action is similar to pulling
a rope in hand over hand. Some muscle fibres are several
centimetres long, but most other cells are only a fraction
of a millimetre long. Because these long fibres cannot be
served adequately by a single nucleus, numerous nuclei are
distributed along their length.
The
work done by muscle requires chemical energy derived from
the metabolism of food. When muscles shorten while exerting
tension and performing mechanical work, some of the chemical
energy is converted to work and some is lost as heat. When
muscles lengthen while exerting tension (such as in slowly
lowering a weight), the chemical energy that is used along
with the mechanical energy absorbed by the action is converted
to heat. Generation of heat is an important function of
muscle in warm-blooded animals. Shivering is muscle activity
that generates heat and warms the body. Similarly, some
insects vibrate their wings for a while beforeflight, heating
the muscles to the temperature at which they work best.
