How water controls the speed of muscle contraction

The flow of water within a muscle fiber can dictate how fast the muscle can contract, according to a University of Michigan study.

Almost all animals use muscles to move, and it has been known for a long time that muscles, like all other cells, are made up of about 70% water. But researchers don’t know what determines the range and upper limits of muscle performance. Previous research into how muscles work only focused on how they worked on a molecular level and not how muscle fibers form, that they are three-dimensional and full of fluid.

UM physicist Suraj Shankar along with L. Mahadevan, a professor of physics at Harvard University, created a theoretical model of the role of water in muscle contraction and found that the way fluid moves through a muscle fiber determines how fast it can to contract a muscle fiber.

They also discovered that muscles exhibit a new type of elasticity called random elasticity that allows muscles to generate force using three-dimensional deformations, shown in the common observation that when a muscle fiber contracts lengthwise, it also swells perpendicularly. Their results are published in the journal Nature Physics.

“Our results suggest that even such basic questions as how fast muscles can contract or how many ways muscles can generate force have new and unexpected answers when one takes a more integrated and holistic view of muscle as a complex material. and hierarchically organized rather than just a bag of molecules,” Shankar said. “Muscle is more than the sum of its parts.”

Researchers envision each muscle fiber as an active self-squeezing sponge, a water-filled, sponge-like material that can contract and squeeze itself through the action of molecular motors, he says.

“Muscle fibers are made up of many components, such as various proteins, cell nuclei, organelles such as mitochondria, and molecular motors such as myosin that convert chemical fuel into movement and drive muscle contraction,” Shankar said. “All these components form a porous network that washes away in water. So a convenient, coarse-grained description of muscle is that of an active sponge.”

But the squeezing process takes time to move the water, so the researchers suspected that this movement of water through the muscle fiber sets an upper limit on how fast a muscle fiber can twitch.

To test their theory, they modeled muscle movements in multiple organisms across mammals, insects, birds, fish and reptiles, focusing on animals that use muscles for very fast movements. They found that muscles that produce sounds, such as the rattle in a rattlesnake’s tail, which can contract ten to hundreds of times per second, usually do not rely on fluid flows. Instead, these contractions are controlled by the nervous system and dictated more strongly by molecular properties, or the time it takes for molecular motors within cells to connect and generate forces.

But in smaller organisms, such as flying insects, which beat their wings several hundred to a thousand times per second, these contractions are too fast for neurons to directly control. Here fluid flows are most important.

“In these cases, we found that fluid flows within the muscle fiber are important, and our active hydraulics mechanism likely limits the faster contraction rates,” Shankar said. “Some insects such as mosquitoes appear to be close to our theoretically predicted limit, but direct experimental testing is needed to check and challenge our predictions.”

The researchers also found that when the muscle fibers act as an active sponge, the process also causes the muscle to act as an active elastic motor. When something is elastic, such as a rubber band, it stores energy as it tries to resist deformation. Imagine holding a rubber band between two fingers and pulling on it.

When you release the rubber band, the band also releases the energy stored while it was being stretched. In this case, energy is conserved—a basic law of physics that dictates that the amount of energy within a closed system must remain the same over time.

But when muscle converts chemical fuel into mechanical work, it can produce energy like an engine, violating the law of conservation of energy. In this case, the muscle shows a new property called “random elasticity,” where its response when pressed in one direction versus another is not reciprocal.

Unlike a rubber band, when muscles contract and relax along its length, it also inflates perpendicularly and its energy does not stay the same. This allows the muscle fibers to generate power from repeated deformations, behaving like a smooth motor.

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

“All together, our results suggest a revised view of how muscle works is essential for understanding its physiology. This is also essential for understanding the origins, extent and limits underlying different forms of animal locomotion.” .”