TYPES OF MUSCLES
Each muscle is made up of a
bundle of fibers, each of which is about the size of a hair and capable of
supporting 1,000 times its own weight. All told, there is something more than 6
trillion fibers in the muscles throughout the body. There are three types of
muscle tissue. One, when viewed under a microscope, has dark and light bands
across the fibers, and is known as striped muscle. It is also called skeletal
muscle because it is attached to some part of the skeleton. And it is classed
as voluntary muscle because it is under the control of the conscious part of
the brain. This is the type of muscle used to walk, write, lift, and throw to
perform any movement we actually will to be done.
A second type of muscle, called smooth or
involuntary, lacks the dark and light bands. This type of muscle handles the
functions of all internal organs except the heart. It is involved in the vital
movements of the stomach and intestine, for example. It is called involuntary
since we do not have direct control over its action. Its workings are
automatic, freeing us from concern over it, allowing us opportunity to
concentrate on other matters. The third type of muscle that of the heart is
called cardiac. It is striped like the voluntary type but has no sheaths as the
voluntary does. It, too, is involuntary.
Muscle fibers vary considerably in length,
from as little as 0.04 inch or even less to 1.5 inch or more. The diameter, of
course, is very small, as little as 0.004 inch or even less. When a muscle
contracts, it becomes shorter by as much as one third to one half, and as it
shortens, it thickens. Most skeletal muscles are linked to a bone either at one
end or at both ends. The tendons, or sinews, which do the linking, vary
considerably in size, ranging up to more than a foot in length. Ligaments, like
the tendons, are made up of strong fibrous tissue, but their function is to
bind bones together. Tendons join muscle to bone; ligaments join bone to bone.
Ligament fibers stretch. Tendons are so strong that a bone may break before the
tendon attached to it gives way. When muscles join bones, one of the bones
usually functions as an anchor to help move the other bone.
The point where the muscle attaches to the
anchor bone is known as the point of origin. The attachment to the bone that
does the moving is called the insertion. Before a muscle can contract, it must
receive a contract signal. In the case of voluntary muscles, the signal comes
from the brain via the central nervous system and is relayed' instantly through
tiny nerves that reach each of the fibers involved. Involuntary muscles get
their signals from the autonomic nervous sys- tem, which is concerned with the
regulation of body functions that do not have to be under conscious control. To
be sure, even in the case of voluntary muscles, you are not required to give a
direct order-to take the time to stop and think and issue a command for a
particular muscle to contract. You simply decide to bend an elbow, move a
finger, throw a ball-and the brain and nervous system translate the decision
into orders which go to the proper muscle fibers.
