How water controls the speed of muscle contraction

A study from the University of Michigan suggests that the flow of water within muscle fibers may affect how fast muscles contract.

Nearly all animals use muscles for movement, and it has long been known that muscles, like other cells, are composed of about 70% water. However, researchers do not know what determines the range and upper limits of muscle performance. Previous studies of muscle function have focused only on how muscles function under specific conditions. Molecular Level It’s not about how muscle fibers are shaped, but that muscle fibers are three-dimensional and filled with fluid.

University of Michigan physicist Suraj Shankar, in collaboration with Harvard University physics professor L. Mahadevan, developed a theoretical model of the role of water in muscle contraction and discovered how fluid moves through muscles. Muscle fibers It determines how fast muscle fibers can contract.

They also found that muscles exhibit a new kind of elasticity called paradoxical elasticity, which allows muscles to use deformations in three dimensions to generate force, as seen in the common observation that muscle fibers bulge vertically when they contract longitudinally. Published In the journal Natural Physics.

“Our results suggest that a more integrated and holistic view of muscle, not just as a mass of molecules, but as a complex, hierarchically organized substance, may provide new and unexpected answers to fundamental questions like how fast a muscle can contract and how many ways a muscle can exert force,” Shankar said. “Muscles are much more than the sum of their parts.”

The researchers envision each muscle fiber as a self-compressing active sponge, a water-filled sponge-like substance that can contract and compress on its own through the action of molecular motors, he says.

“Muscle fibres are made up of many components, including various proteins, cell nuclei, organelles such as mitochondria, and molecular motors such as myosin that convert chemical fuels into movement. Muscle contraction“All these components form a porous network that is saturated with water. Hence, a rough description of muscle can be best described as an active sponge,” Shankar said.

But because the compression process takes time to move water, the researchers hypothesized that the movement of water through muscle fibers upper limit About how fast muscle fibers can twitch.

To test their theory, the researchers modeled muscle movement in a wide variety of organisms, including mammals, insects, birds, fish, and reptiles, focusing in particular on animals that use their muscles to move very quickly. They found that the muscles that make sounds like the rattle in a rattlesnake’s tail contract from 10 to hundreds of times per second, but don’t typically rely on fluid flow. Instead, these contractions are Nervous system And it depends more on the properties of the molecules, i.e. the time it takes for the molecular motors inside the cell to bind and generate force.

But in small organisms such as flying insects, which beat their wings hundreds to thousands of times per second, these contractions are too fast for neurons to control directly, and fluid flow becomes more important.

“In these cases, we find that fluid flow within the muscle fibre is important, and that an active hydraulic mechanism likely limits the maximum speed of contraction,” Shankar says. “Some insects, such as mosquitoes, appear to be close to the theoretically predicted limit, but direct experimental tests are needed to confirm or refute our predictions.”

The researchers also discovered that when muscle fibers act as active sponges, in the process, muscles also act as active elastic engines: Elastic objects like rubber bands store energy in an effort to resist deformation. Think of a rubber band you hold between two fingers and try to pull it.

When you release the rubber band, it also releases the energy it stored while it was stretched. In this case, energy is conserved, which is a fundamental law of physics that states that the amount of energy in a closed system remains constant over time.

But muscles use chemical fuel. Machine operationMuscles violate the law of conservation of energy because they can generate energy like an engine. In this case, muscles exhibit a new property called “odd elasticity”: their reaction when squeezed in one direction is not reciprocal to that when squeezed in another direction.

Unlike a rubber band, muscles contract and relax along their length while also bulging vertically, so their energy is not constant. This allows muscle fibers to generate power through repeated deformations, acting as a soft engine.

“These results contrast with the prevailing view, which focuses on molecular details and ignores the fact that muscles are long, filamentous, contain water and have processes at multiple scales,” Shankar said.

“Overall, our findings suggest that a new view of muscle function is essential for understanding its physiology, which is also crucial for understanding the origins, scope and limitations underlying diverse animal movements.”