100
UNIT 2
Basic Tissues
Ca
++
-mediated interaction between
actin
and
myosin f
laments
,
but in contrast to skeletal and cardiac muscles, the f
laments
are not organized into sarcomeres (Fig. 6-13B). Furthermore,
the Ca
++
enters the cell ±rom the extracellular space rather than
the sarcoplasmic reticulum (which is poorly developed in
smooth muscle). There are small, cup-shaped indentations in
the sarcolemma called
caveolae
that may play a role in seques-
tering calcium (Fig. 6-12). Smooth muscle is diverse in its char-
acteristics and is ±ound in many di±±erent places in the body,
including the gastrointestinal system, the vascular system, the
respiratory system, the reproductive system, the urinary system,
and the ciliary muscle o± the eye. For a given volume o± muscle
tissue, some types o± smooth muscles are capable o± generating
more ±orce and maintaining that ±orce ±or a longer time than
skeletal muscle. In some locations, including the ciliary muscle
o± the eye, some arteries, and the vas de±erens, synapses occur
directly between autonomic nerve f
bers and individual mus-
cle f
bers and contraction is under direct neural control. This
type o± muscle is termed
multiunit muscle
. In contrast,
unitary
(or
visceral
) smooth muscle has ±ewer motor nerve endings,
the transmitter is released into the intercellular space at mul-
tiple
varicosities
along the terminal portion o± the axon, and the
muscle f
bers tend to have spontaneous, rhythmic contractions,
modulated but not directed by the autonomic nervous system.
Hormones in the bloodstream and stretch o± the muscle itsel±
can also infl
uence muscle contractions, and excitation o± the
muscle f
bers can move directly ±rom f
ber to f
ber via gap junc-
tions that link the membranes o± adjacent muscle f
bers.
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