The British Biophysical Society

Muscle Biophysics

Michael A. Ferenczi

 

Movement is essential to life, and takes many forms, from cytoplasmic streaming in plant and animal cells and the growth of neurones, to the long distance flight of the albatross or the explosive performance of a sprinter. The mechanism by which energy is transformed into movement has bewildered Man since antiquity. Using a wide range of biophysical techniques, a considerable level of understanding has been reached, but many new and puzzling questions remain unanswered.

The transformation of chemical energy into movement is carried out by a few families of proteins, across all forms of life. The importance of movement to life is examplified by the fact that muscle, an organ specializing in movement, is by far the largest organ in the body. It is packed with the proteins specialised in transforming energy into mechanical work, myosin and actin. Actin provides a set of cleverly arranged filaments along which myosin filaments can move. The coupling of the sets of filaments all along the muscle cells results in shortening of the muscle. Other proteins are responsible for muscle assembly and regularity (titin, nebulin) and for the control of contraction (caldesmon, troponin, tropomyosin). Other proteins responsible for movement in non-muscle systems,for example for the movement of organelles in cells, are the microtubules along which the molecular motors kinesin, ncd and dynein are moving.

There are many types of muscles, but they fall into three categories: skeletal muscle (or striated muscle), responsible for locomotion, flight etc; cardiac muscle, which is able to function for a century or more without ever taking a break, and smooth muscle (or involuntary muscle) which lines the walls of the arteries to control blood pressure, or controls the digestion of food by causing movement of the intestine. Skeletal muscle as seen under the light microscope has a striated appearance, as seen in the picture of a myofibril:

This micrograph was taken by Ronnie Burns in March 1997 with a 100x objective in brightfield/phase. The myofibrils are about 1 micrometer thick, and are the result of dissection of single muscle cells. The striated appearance is due to the arrangement of two sets of interdigitating arrays of filaments. The thick filaments (orange below) consist of myosin, and the thin filaments (green below) consist predominantly of actin. The striations result from the fact that the thick and thin filaments have different refractive indices in the light microscope. The myofibrils are also birefringent. The dark bands in the micrograph represent regions of overlap between the thin and thick filaments. The Z-line which ties the thin filaments together can also be seen.

Two sarcomeres, or sets of filament arrays, shortening according to the filament sliding hypothesis

Scientists have a tendency to break down organized systems into ever smaller parts, in the hope of better understanding how they work. Muscle is no exception. Sophisticated electron microscopy is used extensively, as well as x-ray diffraction.

Low angle x-ray diffraction makes use of the regular packing of the proteins in muscle to diffract electrons in a regular pattern, thus revealing the molecular organization of the proteins, and more importantly, providing the opportunity to see the proteins move. The advantage of X-ray diffraction is that live, contracting muscles or muscle fibres can be used, rather than fixed and sectioned tissue. It is a method which provides information at the molecular level without the need to break down the organ to minute fractions. With synchrotron radiation, it is possible to obtain information with a time resolution in the tens of microsecond range. An example of such a diffraction pattern is shown below:

The position and intensity of spots or lines in the pattern are characteristic of the organisation of the proteins. Unfortunately a recognisable picture of the muscle proteins can be difficult to reconstruct from patterns in reciprocal space. Nevertheless considerable work and progress has been made in this area.

The arrangement of the proteins in the sarcomeres is revealed by protein biochemistry and protein crystallography. The detailed structure of the myosin cross-bridge and of the actin is now known, but further progress is needed to understand how actin and myosin bind to each other, and how the binding changes during muscle contraction.

ATP hydrolysis provides the energy for movement. The mechanism and kinetics of ATP hydrolysis, and the link between hydrolysis and muscle shortening or force production are important areas of research, where rapid reaction techniques are used, both for the study of isolated proteins in solution, and for the study of the ATPase in muscle fibres.

The reductionist quest has resulted in the development of a new area of study called in-vitro motility. Here muscle proteins are isolated, and their movement studied on a microscope slide. New techniques such as optical tweezers (laser trap), and force measurements using ultra fine needles to hold the ends of single protein filaments are now used. Single force-production events can be monitored, suggesting that molecular size motors could be constructed using biological building blocks.

Many aspects of muscle biophysics have not been touched here. Regulation of muscle contraction, and the biophysics of all the other types of molecular motors are of great interest.

Michael A. Ferenczi is Chair in Physiological Sciences and Head of the Biological Nanosciences Section at Imperial College. He is currently the Vice-Pesident of the European Biophysical Societies Association (EBSA).