Muscle System

If you enjoy action, then this is the system for you. No matter how still you
try to be, there's always a movement taking place in your muscles. Did you
know there are over 650 muscles in the human body, and that muscles make up
1/3 of a grown human body? Muscles cover the entire skeleton, and bones can't
move without muscles. Flex your muscles and tour this exciting system!

Three types of muscles are skeletal muscles, smooth muscles, and cardiac
muscles. Skeletal muscle tissue contains many long, cylindrical cells.
Usually, a number of skeletal muscle cells are bundled together, and then
several bundles are enclosed in a tough connective tissue sheath to form "a
muscle" such as biceps in the arms. This muscle type functions in contraction
for voluntary movements. Cardiac muscle tissue is made up of branching,
striated cells that are fused together at their plasma membranes. This allows
the cells to contract as a unit. This type of muscle is found in the walls of
the heart and functions in the pumping of blood through circulatory system.
Smooth muscle consists of long, spindle-shaped cells, each with a single
nucleus, and connective tissue holds the cells together. Smooth muscle tissue
can be found in the stomach and the walls of blood vessels. This type of
muscle functions in propulsion of substances along internal passageways. The
fleshy part of all muscles is called the belly.

Sarcomeres are the basic units of muscle contraction. They are made up of
actin and myosin filaments which are a part of myofibrils. Myofibrils are
threadlike structures within each muscle cell. Actin is a contractile protein
that is a thin filament, and myosin is another contractile protein but is a
thick filament. According to the sliding-filament model, myosin filaments
slide along and pull the actin filaments toward the center of a sarcomere
during contraction. First, acetylcholine is produced in motor neurons. This
is a transmitter substance that stimulates contraction in adjacent muscle
cells. Cholinesterase is an enzyme present in all nerve tissue that breaks up
acytylcholine after contraction and prevents build-up. Once acetylcholine
stimulates muscle cells, the head of a myosin molecule attaches to a binding
site on actin, forming a cross-bridge. The myosin head is bent because an ATP
molecule is associated with it. When energy is released, the head bends back
to its normal position, bringing with it the actin filament. Another energy
input from ATP causes the myosin head to detach, and then the process begins
again until the muscle is fully contracted. Contraction of muscles is an all
or nothing response because it is based on the action potential in motor
neurons. In these neurons, a threshold must be reached before the action
potential is stimulated and carried to the muscle cells.

Muscles, bones, and tendons are all related. Tendons are the tissues that
connect muscles and bones. They are what ultimately allow movement. When
muscles contract, the tendons move because of the contraction, causing the
movement of the bones to which they are attached. Extensors are muscles that
when contracted, extend or straighten out a limb or part of the body. Flexors,
however, are muscles that bend the angle of a limb or other body part when
contracted. Bones are connected to one another by ligaments. Muscle fatigue is
a protective mechanism that protects muscles against deleterious energy
depletion and irreversible impairment of muscle function. As potassium
channels are activated as a result of fatigue, potassium increases and action
potential duration decreases. The shorter action potential and the increased
potassium then contribute to the decrease in membrane excitability, allowing a
muscle to preserve energy.

With all its action, this system provides for an exciting experience. Enjoy
your tour through the world of myology, as you will be able see first-hand the
structure, function and diseases of muscles.