MUSCLE CONTRACTION
Muscle tissue has a highly developed ability to
contract. Contractibility enables a muscle to become
shorter or thicker, and this ability, along
with interaction with other muscles,
produces movement of internal and
external body parts. Muscle contraction in
a tissue or organ produces motion and provides power
and speed for body activity. A contracting
muscle is referred to as a prime
mover. Amuscle that is relaxing
while a prime mover is contracting is called the
antagonist.
STIMULUS FOR CONTRACTION
All muscles respond to stimulus. This property is
called excitability or irritability. The
mechanical muscular action of
shortening or thickening (also called
contraction) is activated by a stimulus sent
through a motor nerve. All muscles are linked to nerve
fibers that carry messages from the central
nervous system.
CONTRACTION AND RECOVERY
The chemical action of muscle fibers consists of
two stages, contraction and recovery. In the
contraction stage, two protein substances
(actin and myosin) react to provide
energy through the breakdown of
glycogen into lactic acid. In the recovery
stage, oxygen reacts with lactic acid to release carbon
dioxide and water.
MUSCLE FATIGUE
When a muscle contracts, it produces chemical
waste products (carbon dioxide, lactic acid, and acid
phosphate) which make the muscle more
irritable. If contraction is continued,
the muscle will cramp and refuse to
move. This condition is known as fatigue. If it
is carried too far, the muscle cells will
not recover and permanent damage will
result. Muscles, therefore, need rest
to allow the blood to carry away the waste
materials and bring in fresh glucose, oxygen, and
protein to restore the muscle protoplasm and
the energy that was used.
TONICITY
Tonicity, or muscular tone, is a continual state of
partial contraction that gives muscles a
certain firmness. Isometric muscle
contraction occurs when the muscle is
stimulated and shortens, but no
movement occurs, as when a person tenses his or her
muscles against an immovable object. Isotonic
muscle contraction occurs when the
muscle is stimulated. The muscle
shortens and movement occurs. An example
would be lifting an object.
EXTENSIBILITY AND ELASTICITY
Muscles are also capable of stretching when force
is applied (extensibility) and regaining their original
form when that force is removed
(elasticity).
MAINTENANCE OF MUSCLE TISSUE
During exercise, massage, or ordinary activities,
style="mso-spacerun: yes"> the blood supply of muscles is increased.
This additional blood brings in fresh
nutritional material, carries away
waste products more rapidly, and enables
the muscles to build up and restore their efficiency and
tone.
The importance of exercise for normal muscle
activity is clear, but excessive muscle strain is
damaging. For example, if a gasoline motor
stands idle, it eventually becomes
rusty and useless. Similarly, a muscle
cell that does not work atrophies,
becoming weak and decreasing in size. On the other
hand, a motor that is never allowed to stop
and is forced to run too fast or to do
too much heavy work soon wears out so
that it cannot be repaired. In the same way, a
muscle cell that is forced to work too hard without
proper rest will be damaged beyond repair.
When a muscle dies, it becomes solid and rigid and
no longer reacts. This stiffening, which occurs from 10
minutes to several hours after death, is
called rigor mortis.
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