Video Transcript
Myofibrils are largely composed of
two types of protein myofilaments. What are these two proteins
called? Is it (A) actin and myosin, (B)
actin and sarcolemma, (C) myosin and T tubules, or (D) A bands and I bands?
To answer this question, let’s
first look at the structure of myofibrils in more detail.
Muscles are made up of bundles of
muscle fibers, and these fibers are in turn composed of myofibrils, which are long
filaments that run parallel to each other. Each muscle fiber contains between
1,000 and 2,000 myofibrils, which together provide the muscle with their combined
strength. Each myofibril is a long
cylindrical organelle made of protein filaments, and it has a specialized role in
contraction. So, how are they specialized for
their role?
Myofibrils are composed of smaller
units called sarcomeres, as seen in this diagram. The myofibrils have alternating
bands that appear lighter and darker, which gives them a striped appearance. These bands are due to the two
protein myofilaments called actin and myosin. As actin is a thin filament, made
up of two strands twisted around each other, it has a lighter appearance. Myosin, on the other hand, is a
thick filament that appears darker in color than actin. It is a long rod-shaped filament
that has globular-shaped heads that project outwards; these bind to actin and move
it during contraction.
The thick myosin and thin actin
filaments form partially overlapping layers, as you can see here. Where there are only actin
filaments, it forms the I band, which is the light band. Down the middle of the I band is
the Z line, where the actin filaments join together. A sarcomere runs from one Z line to
the next. The thicker myosin filaments are
found in the darker A band. Some actin is found at the edges of
the A band, and this overlapping of filaments causes an even darker appearance.
Contraction of the muscle fiber
requires an electrical impulse from the nervous system. This impulse passes along the
sarcolemma, the cell membrane of the muscle cell, and across the fibers through the
transverse, or T tubules. These help the rapid transmission
of the impulse to coordinate the contraction.
Now that we have reviewed the key
facts about the structure of myofibrils, we can revisit the question which asks
about the name of the proteins found in a myofibril. We now know the sarcolemma is the
cell membrane of the muscle cell and that T tubules carry the nervous impulse across
the myofibril. So, we can rule out choices (B) and
(C). A bands are the dark striations
seen in the myofibrils and I bands are the light striations, so we can also rule out
(D). This leaves us with the correct
answer (A). The names of the protein
myofilaments found in myofibrils are actin and myosin.
