Quantum Mechanics Explains How Muscle Produces Force

It wasn’t so long ago that biologists would swear blind that their discipline would never be tainted by the strange effects of quantum mechanics. Today, quantum biology is an emerging discipline in many labs around the world and only the brave (or stupid) now argue against the idea that quantum effects play an important role in the workings of biological molecules, entire cells and even the brain.

Today, add muscle to this list. Tieyan Si at the Max Planck Institute for Complex Systems in Dresden, Germany, has created a quantum model of muscle behaviour. His idea is that myosin, the molecular motor responsible for muscle contraction, is essentially a quantum object and that its behaviour is best described by quantum mechanics.

The business part of muscle fibre consists of actin, which can be thought of as a rope, and myosin, which is a molecular motor which works rather like a tug-of-war team. Electrical stimulation sets the tug of war teams into action, frantically pulling their ropes and causing the muscle to contract. The actual force a muscle produces is the result of many myosin motors pulling and relaxing, although not necessarily in concert.

The challenge for theorists is to work out how these molecular motors generate the force and relaxation curves that occur in real muscle. These are well studied in systems as diverse as mammalian heart muscle and insect wings and biomechanicists have long known that different types of muscle and muscle action produce different force curves. For example, contractions that are quickly released have a different force signature to slowly release ones. Explaining this with a single classical theory is not easy.

Si’s approach is simply to assume that each myosin motor is a quantum object that can form two shapes and that the switch between these shapes causes a contraction. In other words, it has two states. (He also looks at a system in which myosin has three states.) Myosin switches to one state by absorbing energy and relaxes by emitting it and the combined effect of all the switchings determine the behaviour of the fibre.

A muscle fibre, then, is simply a chain of these quantum objects, for which it is possible to derive a mathematical object known as a Hamiltonian that describes the behaviour. The question that Si addresses is that what kind of force-relaxation curves does this Hamiltonian lead to.

His answer is that “this quantum Hamiltonian system gives us the classical force-velocity relation not only for a quick release but also for a slow release and unstable states.”

He shows that the two-level system accurately models the behaviour of heart muscle while the three level state explains the behaviour of insect flight muscle.

What Si does not do is clearly explain the failings of the conventional models of muscle behaviour and why the quantum approach is better. Neither does Si make any predictions about the behaviour of muscle that classical models cannot account for.

Nevertheless, this is an impressive first step in the quantum description of muscle behaviour. And as Si points out, there is plenty more work to do in understanding the interface between the quantum chain and the signals that trigger them, such as the electrical signals along nerves and the flow of ions across membranes that this triggers.

Ref: arxiv.org/abs/1004.3120: One Dimensional Chain Of Quantum Coherent Molecule Motors As A Model For Muscle Fibre