intermediate fibers. Red muscle fibers have a high content of cytochrome
and myoglobin and, beneath the plasmalemma, contain many mitochon-
dria required for the high metabolism of these cells. Mitochondria are also
found in a longitudinal array surrounding the myofibrils. The presence of
numerous mitochondria provides a strong staining reaction with the use of
cytochemical stains such as that for succinic dehydrogenase. Physiologi-
cally, red fibers are capable of continuous contraction (high concentrations
of myosin ATPase), but are incapable of rapid contraction (answer b).The
term “red (type I) fibers” is due to the presence of large concentrations of
myoglobin, the colored oxygen-binding protein. White (type IIB) muscle
fibers(answer a)are fast-twitch in function, stain very lightly for succinic
dehydrogenase and myosin ATPase, and few mitochondria would be visi-
ble at the ultrastructural level. White fibers are capable of rapid contraction
but are unable to sustain continuous heavy work. They are larger than red
fibers and have more prominent innervation. The white fibers contain rel-
atively little myoglobin. Human skeletal muscle fibers are composed of red,
white, and intermediate-type fibers. The intermediate (type IIA) fibers pos-
sess characteristics including a size and innervation pattern intermediate
between red and white muscle fibers. The intermediate fibers contain a
concentration of myoglobin between white and red muscle fibers.
128.The answer is b.(Alberts, pp 961–965. Junqueira, pp 186–191.)The
“powerstroke” is initiated by the release of Pi from the myosin heads, lead-
ing to the tight binding of actin and myosin. The tight binding induces a
conformational change in the myosin head. The myosin head subsequently
pulls against the actin filament to cause the “powerstroke” of the myosin
head walking along the actin filament. This walking process is unidirec-
tional and is based on the polarity of the actin filament (i.e., walking occurs
from the minus to the plus end of the actin filament). The cycle of ATP-
actin-myosin interactions during contraction begins with the resting state.
In the quiescent period, ATP binds to myosin heads (answer a);however,
hydrolysis occurs slowly and only allows the weak binding of myosin
heads to the actin filaments. Tight binding occurs only when Pi is released
from myosin heads, leading to the “powerstroke.” Recycling occurs
through the release of ADP and the subsequent addition of an ATP molecule
(answer c)and detachment of the myosin head from actin (answer d).
Rigor results from the lack of ATP because one ATP molecule is required for
each myosin molecule present in the muscle. Rigor mortisoccurs from the
224 Anatomy, Histology, and Cell Biology