A whole skeletal muscle is considered an organ of the muscular
system. Each organ or muscle consists of skeletal muscle tissue,
connective tissue, nerve tissue, and blood or vascular tissue.
Skeletal muscles vary considerably in size, shape, and arrangement
of fibers. They range from extremely tiny strands such as
the stapedium muscle of the middle ear to large masses such
as the muscles of the thigh. Some skeletal muscles are broad
in shape and some narrow. In some muscles the fibers are parallel
to the long axis of the muscle, in some they converge to a
narrow attachment, and in some they are oblique.
Each skeletal muscle fiber is a single cylindrical muscle
cell. An individual skeletal muscle may be made up of hundreds,
or even thousands, of muscle fibers bundled together and wrapped
in a connective tissue covering. Each muscle is surrounded
by a connective tissue sheath called the epimysium. Fascia,
connective tissue outside the epimysium, surrounds and separates
the muscles. Portions of the epimysium project inward to divide
the muscle into compartments. Each compartment contains a
bundle of muscle fibers. Each bundle of muscle fiber is called
a fasciculus and is surrounded by a layer of connective tissue
called the perimysium. Within the fasciculus, each individual
muscle cell, called a muscle fiber, is surrounded by connective
tissue called the endomysium.
Skeletal muscle cells (fibers), like other body cells, are
soft and fragile. The connective tissue covering furnish support
and protection for the delicate cells and allow them to withstand
the forces of contraction. The coverings also provide pathways
for the passage of blood vessels and nerves.
Commonly, the epimysium, perimysium, and endomysium extend
beyond the fleshy part of the muscle, the belly or gaster,
to form a thick ropelike tendon or a broad, flat sheet-like
aponeurosis. The tendon and aponeurosis form indirect attachments
from muscles to the periosteum of bones or to the connective
tissue of other muscles. Typically a muscle spans a joint
and is attached to bones by tendons at both ends. One of the
bones remains relatively fixed or stable while the other end
moves as a result of muscle contraction.
Skeletal muscles have an abundant supply of blood vessels
and nerves. This is directly related to the primary function
of skeletal muscle, contraction. Before a skeletal muscle
fiber can contract, it has to receive an impulse from a nerve
cell. Generally, an artery and at least one vein accompany
each nerve that penetrates the epimysium of a skeletal muscle.
Branches of the nerve and blood vessels follow the connective
tissue components of the muscle of a nerve cell and with one
or more minute blood vessels called capillaries.
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