131.The answer is a.(Junqueira, pp 198–202.)Smooth muscle is the least
specialized type of muscle and contains no troponin. The contractile process
is similar to the actin-myosin interactions that occur in motility of nonmus-
cle cells. In the smooth-muscle cell, actin and myosin are attached to inter-
mediate filaments at dense bodies in the sarcolemma and cytoplasm. Dense
bodies contain α-actinin and, therefore, resemble the Z-lines of skeletal
muscle. Contraction causes cell shortening and a change in shape from
elongate to globular. Contraction occurs by a sliding filament action analo-
gous to the mechanism used by thick and thin filaments in striated muscle.
The connections to the plasma membrane allow all the smooth-muscle cells
in the same region to act as a functional unit. The sarcoplasmic reticulum is
not as well developed as that in the striated muscles. There are no T tubules
present; however, endocytic vesicles called caveolae are believed to function
in a fashion similar to the T tubule system of skeletal muscle.
When intracellular calcium levels increase, the calcium is bound to the
Ca^2 +-binding protein, calmodulin. Ca^2 +-calmodulin(answers b and c)is
required and is bound to myosin light-chain kinase (answer d)to form a
Ca^2 +-calmodulin-kinase complex. This complex catlayzes the phosphoryla-
tion of one of the two myosin light chains on the myosin heads. That phos-
phorylation allows the binding of actin to myosin. A specific phosphatase
dephosphorylates the myosin light chain, which returns the actin and
myosin to the inactive, resting state. The actin-tropomyosin interactions
(answer e)are similar in smooth and skeletal muscle.
Smooth-muscle cells (e.g., vascular smooth-muscle cells) also differ
from skeletal muscle cells in that like fibroblasts, they are capable of colla-
gen, elastin, and proteoglycan synthesis.
132.The answer is d.(High-Yield Facts, p 18. Junqueira, pp 188–191.)
Myofibrils are composed of sarcomeres, which are repeating units that
extend from Z disk (Aon the transmission electron micrograph) to Zdisk
in the TEM. With the use of polarizing microscopy, the A(anisotropic)
bands (Eon the TEM) are visible as dark, birefringent structures, and the I
(isotropic) bands are visible as light-staining bands (Con the TEM). The I
band consists of thin filaments without overlap of thick filaments. At the
center of the myofibril and consisting of thick filaments is the A band,
which interdigitates with the I band. Each I band is bisected by the Z disk.
The Z disk is composed mostly of the intermediate filament protein desmin
and other proteins such as α-actinin, filamin, and amorphin, as well as Z
226 Anatomy, Histology, and Cell Biology