• A single muscle contraction is called a twitch, while a sus-
tained contraction is called tetanus. The tension generated
by a muscle can be increased by increasing the frequency of
nerve impulses to the motor unit (via temporal summation
or wave summation) or by increasing the number of motor
units that are contracting at one time (recruitment).
• Smooth muscles can be stretched considerably and still
maintain the ability to contract, as shown. The contraction
and relaxation is slower, and this helps maintain muscle
tone. Smooth muscles contract in response to nerve im-
pulses, hormones, stretching, and local factors.
Smooth Muscle Tissue Is in the Walls of
Most Organs 165
• Smooth muscle tissue is either visceral (single-unit) or
multi-unit, as shown. Smooth muscle cells are much smaller,
thinner, and more spindle shaped than skeletal muscle. In
addition to thin and thick filaments, smooth muscle cells
have intermediate filaments attached to dense bodies, form-
ing a net-like network in the cell. Smooth muscles cells do
not have a well-developed sarcoplasmic reticulum. Visceral
smooth muscle cells communicate with each other through
Cardiac Muscle Tissue Forms the Muscular
Wall of the Heart 167
• As shown, the structure of cardiac muscle has some char-
acteristics of smooth muscle and some of skeletal muscle.
It is intermediate in size between the other muscle types
and has cylindrical, branched fibers that are interconnected
by intercalated discs that contain gap junctions. Cardiac
muscle cells have large mitochondria, one nucleus, and
bundles of myofibrils arranged into sarcomeres. They have
well-developed sarcoplasmic reticulum and T-tubule sys-
tems, like skeletal muscle.
Types and structure of smooth muscle cells • Figure 6.10
. _ - Autonomic —^
Structure of cardiac muscle cells • Figure 6.11
Intercalated discs are
thickenings of plasma
membrane where two cardiac
cells come together. They
hold cells together and aid in
cell-cell communications. •
• Cardiac cells respond to signals from specialized heart
muscle cells called
p a c e m a k e r c e lls .
Via pacemaker cells,
the autonomic nervous system can speed up or slow down
the contraction of cardiac cells as needed.
186 CHAPTER 6
The Muscular System