T he Body Contains Three Different Types
of Muscular Tissue 154
• There are three types of muscular tissue: skeletal, cardiac,
and smooth, as shown.
Types of muscular tissues • Figure 6.1
Skeletal muscle fiber
Long cylindrical fiber with many peripherally
located nuclei; striated; unbranched
Muscles Are Built to Move 156
• As shown, skeletal muscles consist of bundles of fibers (cells),
wrapped in connective tissue. The fibers are large, cylindrical,
and multi-nucleated. The cell membrane has tunnel-like T-tu-
bules extending into the center of the cell; there are numerous
mitochondria; and the endoplasmic reticulum (sarcoplasmic
reticulum) is well developed and specialized for storing cal-
cium. The cells are filled with arrays of myofibrils that consist
of thick and thin filaments that are arranged into sarcomeres.
Thick filaments are made of the protein myosin, and thin fila-
ments are made of actin, tropomyosin, and troponin.
Cardiac muscle fibers
Branched cylindrical fiber, usually with one centrally
located nucleus; intercalated discs join neighboring
fibers; striated
Smooth muscle fiber
Fiber is thickest in the middle, tapered at each end;
has one centrally located nucleus; not striated
• Skeletal muscle is attached to bones; it is striated and can
be voluntarily controlled.
• Smooth muscle is located in the walls of hollow organs; it is
non-striated and involuntary.
• Cardiac muscle is found in the heart; it is striated and invol-
• Muscular tissue contracts and relaxes, thereby producing body
movements, stabilizing body positions, regulating organ vol-
ume, moving substances within the body, and producing heat.
The structure of a skeletal muscle and its connective
tissue coverings • Figure 6.2
Sarcoplasmic reticulum
- Sarcoplasm
— Sarcolemma
__- Nucleus
Thick filament
/ Thin filam
Mitochondrion -
• Muscles shorten when the myosin in thick filaments pulls
on the actin of thin filaments, thereby causing them to slide
past one another, toward the center of the sarcomere. This
process, called a contraction cycle, or cross-bridge cycle,
is triggered by a rise in the concentration of calcium ions in
the cytoplasm.
• In a process called excitation-contraction coupling, a
nerve impulse sets up an electrochemical impulse that gets
conducted along the muscle fiber. This impulse, or action
potential, is conducted down the T-tubules and causes the
release of calcium ions from the sarcoplasmic reticulum.
The calcium ions trigger the contraction cycle. When the cell
repolarizes, calcium gets pumped back into the sarcoplas-
mic reticulum, the contraction cycle stops, and the muscle
• The ATP that provides the energy for muscle contraction
comes from several sources, depending on the amount of
time the muscle remains contracted: ATP and creatine phos-
phate reserves (0-15 s), anaerobic respiration or glycolysis
(15-30 s), and aerobic respiration (less than 30 s). Working
muscles need good supplies of oxygen and blood for sus-
tained activity. When the muscle cannot contract forcefully,
it is said to be fatigued.
Summary 185
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