A
combination of random protein movements and the elasticity inside muscles helps
to maintain a steady force during skeletal muscle contraction
The process of skeletal muscle contraction is based
around protein filaments sliding inside sarcomeres — the structural units of
muscle fiber. Inside each sarcomere is a set of filament motors, which appear
in different densities in different areas. Scientists previously thought that
the motor force would change according to the filament load in the muscle, but
recent studies have shown that the motor force actually maintains a constant
level during the muscle contraction. Despite such breakthroughs, however, it
remains unclear exactly how this constant force is maintained in an otherwise
chaotic system.
Bin Chen of the A*STAR Institute of High Performance
Computing and Huajian Gao at Brown University, US, have now built a model to
illustrate the process of skeletal muscle contraction and show how a constant
force can be sustained by the protein motors1.
The two key proteins in muscle contraction are actin
and myosin. Myosin drives the system, forming a thick filament made up of
numerous motors which ‘grab’ onto, bind to and slide past the thinner actin
filaments during contraction. This ‘grabbing’ and sliding motion has been shown
to be fairly chaotic in nature, with attachment and release happening at
random. When the weight of an object exerts a load on the filaments — for
example, when you try to lift something up — the muscles must contract,
requiring the protein motors to generate a force opposite to the load.
Chen and Gao have created a new skeletal muscle
fiber model to demonstrate how contraction forces work. “Our model is designed
for the sarcomere,” Chen explains. “We consider the thin filament as an elastic
rod under a filament force, which is driven by multiple stochastic myosin
motors that convert the chemical energy of adenosine-5'-triphosphate (ATP)
hydrolysis into stored elastic energy and then function like swinging arms.”
The results show that the unique way in which the
myosin motors randomly attach and release from actin, coupled with the elastic
properties of the motors, generate a consistent force across the whole
sarcomere. When there is a higher filament load, more myosin motors are
attached to the actin, but the overall motor force remains constant.
“This regulation mechanism may exist in various
biological processes and dramatically induces order within a chaotic system,”
explains Chen. “Our modeling framework can also be further adapted to study the
behaviors of other actomyosin complex structures, which is part of our plan for
future work in this area.”
The A*STAR-affiliated researchers contributing to
this research are from the Institute
of High Performance Computing
References
- Chen, B. & Gao, H. Motor force homeostasis in skeletal muscle
contraction. Biophysical Journal 101, 396–403
(2011). | article
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