Shreechakradhar U Mungal et al. New hypothesis for skeletal muscle contraction
National Journal of Physiology, Pharmacy & Pharmacology | 2015 | Vol 5 | Issue 1 | 72 – 75
Figure 4: Movement of Z discs according to the alternative
hypothesis
When we slided actin filaments present on both sides
of a myosin filament toward the center of A band in a
single sarcomere as it occurs during contraction, two
Z discs on either side of A band moved toward the
center of the sarcomere. This observation was
completely in accordance with the sliding filament
theory of muscle contraction under consideration.
But when we tried sliding actin filaments in two
adjacent sarcomeres as if contraction is occurring in
both the sarcomeres simultaneously, the Z disc that
was common to both the sarcomeres was unable to
move toward the center of either sarcomere, resulting
in no change in length of sarcomeres. This finding was
not in accordance with the sliding filament theory of
muscle contraction. Considering that the Z disc
common to adjacent sarcomeres under contraction
was not moving toward the center of either
sarcomere, we found the following reasons: (1) Each Z
disc is attached to actin filaments on both sides. (2)
During contraction, actin filaments on either side of Z
disc will move toward the center of the sarcomere in
which they are present. (3) Eventually both sets of
actin filaments on either side of Z disc will try to pull
the disc in opposite direction. (4) It is analogous to tug
of war having same power on both sides. (5) It is not
mentioned anywhere that actin filaments in different
sarcomeres are having different strength. (6) As the Z
disc is being pulled in two opposite directions by actin
filaments of either sides, it will not move.
Therefore, we tried to find out another possible
mechanism in context of the sliding filament theory of
muscle contraction. We proposed a new theory that Z
discs and actin filaments in the half part of each
sarcomere, which is toward origin end of muscle fiber,
are not pulled toward the center of myosin filament
because if they are pulled, the ultimate pull will be
applied on the origin end of the muscle that is not
movable. Instead myosin filament slide toward origin
end. In the other half of each sarcomere (which is
toward insertion end), actin filaments slide over
myosin filament toward origin end of the muscle. This
assumption can be supported by Figure 4.2 of Best &
Taylor’s Physiological Basis of Clinical Practice, 13th
edition. This study adds a new insight into the sliding
filament theory of muscle contraction and needs
further confirmation on electron microscopic studies.
To support the above hypothesis, we can consider
analogy of a sportsperson doing pull-ups on a single
bar, as shown in Figure 3. Here single bar is analogous
to the first Z disc toward origin end that is not
movable, and hands of the sportsperson are analogous
to myosin heads. The rest of the body of the
sportsperson is analogous to the body of myosin
filament. When myosin heads try to pull actin
filaments, which are attached to the first Z disc of
origin end side, power generated by myosin heads
may be insufficient to pull that Z disc along with actin
filaments. Thus, myosin filaments move toward origin
end instead of actin filaments along with Z disc moving
toward insertion end, this is analogous to body of
sportsperson going upward when he tries to pull the
single bar downward, as shown in Figure 3. When we
consider simultaneous contraction in all sarcomeres,
all myosin filaments move toward the origin end, and
all the Z discs of insertion end side move toward the
center of myosin filaments, which are toward origin
end side. Ultimately, Z discs on both sides of A band
seem to be moved toward the center of A band but
actually myosin filament slide toward origin end, and
in the other half of each sarcomere (which is toward
insertion end) actin filaments slide over myosin
filament toward origin end of the muscle. This is
shown in Figure 4.
The sliding filament theory of skeletal muscle
contraction can explain the shortening of a single
sarcomere, but when we consider multiple
sarcomeres in a myofibril arranged in series, it fails to
justify the movement of Z discs as each Z disc is being
pulled in the opposite direction by myosin heads of
adjacent sarcomeres. Therefore, we proposed a new
theory that Z discs and actin filaments in the half part
of each sarcomere, which is toward origin end of
muscle fiber, are not pulled toward the center of
myosin filament, instead myosin filament slide toward
origin end. And in the other half of each sarcomere
(which is toward insertion end), actin filaments slide