Biomedical Engineering Reference
In-Depth Information
Deep fascia
Tendons
Tendons
Epimysium
Perimysium
Sarcolemma
Humerus
Skeletal muscle
Muscle
fiber(cell)
Endomysium
Fascicle
Humerus
Motor
neuron
Blood
vessels
FIGURE A.1
Typical skeletal muscle showing epimysium, perimysium, and endomysium
layers.
Nerves and blood vessels supply muscle in abundance. Motor neurons are
responsible for stimulating muscle fibers. In order for muscle tissue to contract, it
must use a lot of ATP (adenosine triphosphate, an energy-rich molecule; see section
A.6) and also must produce ATP by inflow of nutrients and oxygen brought about
by the blood capillaries (microscopic blood vessels) that are found in abundance in
muscle tissue. Each muscle fiber (cell) is in close contact with one or more capillaries.
Connective tissue surrounds and protects muscle tissue. Fascia is a sheet of
broadband fibrous connective tissue beneath the skin, around the muscles and other
organs in the body. Deep fascia—a dense irregular connective tissue—lines the body
wall and extremities, holds muscles together, and separates them into functional
groups. Deep fascia allows free movement of muscles; carries nerves, blood, and
lymphatic vessels; and fills spaces between muscles. Beyond deep fascia are three
layers of dense, irregular connective tissues that further protect and strengthen
skeletal muscle. The outermost layer encircling the whole muscle is the epimysium.
Perimysium then surrounds bundles (faciculi or fascicles) of 10 to 100 or more
individual muscle fibers. Further penetrating the interior of each individual fascicle
and separating muscle fibers from one another is endomysium (fig. A.1). Epimysium,
perimysium, and endomysium are all continuous with and contribute collagen fibers
to the connective tissue that attaches the muscle to other structures such as bone or
other muscle. All three may extend beyond the muscle fibers as tendons, which are
cords of dense connective tissue that attach muscles to the periosteum of a bone.
Aponeurosis is an extension of connective tissue elements in a broad and flat
layer. It also attaches to the coverings of a bone, another muscle, or the skin. Certain
tendons, especially those of the ankle and wrist, are enclosed by tubes of fibrous
connective tissue called the tendon sheath (synovial sheath). They are similar in
structure to bursae and contain a film of synovial fluid. Tendon sheaths permit
tendons to slide back and forth more easily.
A motor neuron delivers the stimulus that ultimately causes a muscle fiber to
contract. A motor neuron plus all the muscle fibers it stimulates is called a motor
unit (fig A.2). A single motor neuron makes contact with an average of 150 muscle
fibers; that is, activation of one neuron causes simultaneous contraction of about 150
muscle fibers. All the muscle fibers of a motor unit contract and relax together. Total
