126.The answer is a.(Alberts, pp 961–965. Junqueira, pp 186–191.)Ca2+is
responsible for the coupling of excitation and contraction in skeletal mus-
cle. The sarcoplasmic reticulum (SR) is a modified smooth endoplasmic
reticulum. Ca2+is concentrated in the lumen of the SR. Depolarization of
the muscle cell membrane during an action potential triggers the opening
of ryanidine receptor channels in the membrane of the SR. Ca2+passes
through the open ryanidine receptor channels, traveling from the lumen of
the SR into the muscle cell cytoplasm. When Ca2+in the muscle cell cyto-
plasm is high, the Ca2+binds to the troponin complex. The binding of Ca2+
to the troponin complex results in a conformational change in the com-
plex, thereby allowing myosin to interact with actin. The interaction of
myosin and actin produces force. Ca2+is constantly pumped back into the
lumen of the SR by Ca2+-ATPases. As the Ca2+inthe cytoplasm is pumped
back into the SR the concentration of Ca2+in the cytoplasm drops, and Ca2+
dissociates from the troponin complex. The troponin complex subse-
quently reverts to the conformational state in which the troponin complex
blocks the interaction of myosin with actin (answer b).Glycogen is stored
as particles or droplets in the cytoplasm, which contains the enzymes
required for the synthesis and breakdown of glycogen (answer c).The
transverse tubule system, or T system, is an extension of the plasma mem-
brane of the myofiber (sarcolemma). The T system allows for simultaneous
contraction of all myofibrils since it encircles the A-I bands in each sarcom-
ere of every myofibril. In combination with the paired terminal cisternae,
the transverse tubules form a triad. Two triads are found in each sarcomere
of skeletal muscle—one at each junction of dark (A) and light (I) bands.
Depolarization of the T system during contraction is transmitted to the sar-
coplasmic reticulum at the triad (answers d and e). It is important to note
that cardiac muscle also has a T system, although it is not as elaborate and
well organized as that found in skeletal muscle (e.g., diads are present rather
than the triads of skeletal muscle and there are fewer T tubules in the atrial
vs. ventricular muscle).
127.The answer is c.(Kumar, pp 1327–1328. Junqueira, p 196.)The
histologicsample contains red fibers. The deductive process is based on
the fact that the sample must be skeletal or cardiac muscle due to the pres-
ence of cross-striations (answer e).The presence of peripherally placed
nuclei eliminates cardiac muscle as a possibility (answer d).Skeletal mus-
cle may be subclassified into three muscle fiber types: red, white, and
Muscle and Cell Motility Answers 223