Support and movement systems: Muscles
Within the human body there are more than 600 muscles which make
up about 35% of our body mass. Some of these muscles we can see and
feel while others we can't, but each plays a specialised role in helping
specific parts of our body to move.
Voluntary and involuntary muscles
Muscles can be classified on the basis of our conscious
control:
- Voluntary muscles can be controlled
by our conscious thought, e.g. the muscles in our hands as we catch
a ball.
- Involuntary muscles, like those that
control the beating of the heart, do not require conscious thought
to make them move even though their action is still controlled by
the nervous system.
Types of muscles
Muscles can also be classified into three groups, based on their structure.
- Skeletal Muscle: All muscles
attached to the bones of the skeleton are called skeletal muscles.
They are all voluntary muscles because we can make them work
by our conscious thinking. We can either lift our arm by moving
our muscles or allow them to relax, which means that our arm
will fall to our side.
There are four main types of skeletal muscle, named according
to their shape - spindle, triangular, flat and circular (ring).
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- Smooth Muscle: All smooth muscle is
involuntary muscle. Its action is not controlled by our thoughts.
Smooth muscle makes up tissues that are vital for continuous, everyday
life. In the digestive system, it is found in the stomach walls to
mechanically digest our food and in the intestine it passes the food
along by wave-like contractions (peristalsis). In the circulatory
system, the smooth muscle of the artery walls maintains the blood
pressure and helps regulate flow. In the iris of the eye smooth muscle
regulates the opening of the pupil. Thankfully all these actions are
involuntary. Imagine having to think about all these things all the
time!
- Cardiac Muscle: Cardiac muscle, another
important involuntary muscle, is found only in the walls of the heart.
Without the regular contractions of the heart (about seventy/minute)
to pump blood all over our body, life could not be sustained.
Muscle action
Voluntary muscles receive an electrical signal from the central nervous
system via motor nerve cells that connect with individual muscle fibres
to make them contract or relax. As the muscle contracts, it becomes
thicker and shorter, thus pulling on the bone. As it relaxes, it lengthens
again.
Muscles
that work with the skeleton are usually found in pairs with opposing
actions. A muscle that bends a joint when it contracts is called
a flexor (e.g. biceps). The opposing muscle on the other side
of the bone that straightens the joint when it contracts is called
an extensor (e.g. triceps).
Skeletal muscles are attached to the bones with tendons which
are very tough tissue that do not stretch much. An example of
this is the Achilles tendon, which you can feel behind your ankle.
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