Over 600 skeletal
muscles function for body movement through contraction and relaxation
of voluntary, striated muscle fibers. These muscles are attached
to bones, and are typically under conscious control for locomotion,
facial expressions, posture, and other body movements. Muscles account
for approximately 40 percent of body weight. The metabolism that
occurs in this large mass-produces heat essential for the maintenance
of body temperature.
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is only in the heart and makes up the atria and ventricles (heart
walls). Like skeletal muscle, cardiac muscle contains striated fibers.
Cardiac muscle is called involuntary muscle because conscious thought
does not control its contractions. Specialized cardiac muscle cells
maintain a consistent heart rate.
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is throughout the body, including in visceral (internal) organs,
blood vessels, and glands. Like cardiac muscle, smooth muscle is
involuntary. Unlike skeletal and cardiac muscle, smooth muscle is
nonstriated (not banded). Smooth muscle, which is extensively within
the walls of digestive tract organs, causes peristalsis (wave-like
contractions) that aids in food digestion and transport.
Except the heart,
any action that the body performs without conscious thought is done
by smooth muscle contractions. This includes diverse activities
such as constricting (closing) the bronchioles (air passages) of
the lungs or pupils of the eye or causing goosebumps in cold conditions.
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A skeletal muscle
has regular, ordered groups of fascicles, muscle fibers, myofibrils,
and myofilaments. Epimysium (thick connective tissue) binds groups
of fascicles together. A fascicle has muscle fibers; perimysium
(connective tissue) envelops the fascicle. Endomysium (connective
tissue) surrounds the muscle fibers.
A muscle fiber
divides into even smaller parts. Within each fiber are strands of
myofibrils. These long cylindrical structures appear striped due
to strands of tiny myofilaments. Myofilaments have two types of
protein: actin (thin myofilaments) and myosin (thick myofilaments).
The actin and
myosin myofilaments align evenly, producing dark and light bands
on the myofibril. Each dark band depicts an area where the myofilaments
overlap, causing the striated appearance of skeletal muscle.
All dark and
light bands of the myofilaments have names. At the Z-line, actin
strands interweave. The region between two Z-lines is a sarcomere,
the functional unit of skeletal muscle. Muscle contraction occurs
when overlapping actin and myosin myofilaments overlap further and
shorten the muscle cell. The myofilaments keep their length. The
overlapping of myofilaments is the basis for the sliding filament
theory of contraction.
is a system of pairs that relax and contract to move a joint. For
example, when front leg muscles contract, the knee extends (straightens)
while back leg muscles relax. Conversely, to flex (bend) the knee,
back leg muscles contract while front leg muscles relax. Some muscles
are named for their ability to extend or flex a joint; for example,
extensor carpiradialis longus muscle and flexor digitorum brevis
most skeletal muscles to bones. Tendons are strong sheets of connective
tissue that are identical with ligaments. Tendons and ligaments
differ in function only: tendons attach muscle to bone and ligaments
attach bone to bone. Physical exercise strengthens the attachment
of tendons to bones.
have muscle tone (remain partly contracted), which helps maintain
body posture. Ongoing signals from the nervous system to the muscle
cells help maintain tone and readiness for physical activity.
aids in heat generation. During muscle contractions, muscle cells
expend much energy, most of which is converted to heat. To prevent
overheating, glands in the skin produce sweat to cool the skin;
and, the body radiates heat from the blood and tissues through the
skin. When the body is chilly, shivering causes quick muscle contractions
that generate heat.
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fibers and exercise
have two types of muscle fibers: fast-twitch and slow-twitch. Anaerobic
exercise uses fast-twitch fibers. Such exercise includes activities
that are fleeting and require brief high-energy expenditure. Weightlifting,
sprinting, and push-ups are examples of anaerobic exercise. Because
all cells require oxygen to produce energy, anaerobic exercise depletes
oxygen reserves in the muscle cells quickly. The result is an oxygen
debt. To repay the debt, humans breathe deeply and rapidly, which
restores the oxygen level. Anaerobic exercise creates excess lactic
acid (a waste product). By increasing oxygen intake, the liver cells
can convert the excess lactic acid into glucose, the primary food
molecule used in cellular metabolism.
uses slow-twitch muscle fibers. Such exercise includes activities
that are prolonged and require constant energy. Long distance running
and cycling are examples of aerobic exercise. In aerobic exercise,
the muscle cell requires the same amount of oxygen that the body
supplies. The oxygen debt is slashed and lactic acid is not formed.
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