Introduction to Muscle Contraction, Part 1
Why did it take so long to work out?
To understand the chemical events which drive muscle one needs to know the protein structures involved at atomic resolution. Muscle is made of massive arrays of macromolecules. How does one get data from such systems? A great deal of technology has been invested in this problem, which has also driven technology. Some early insight was provided by light microscopy. However, the first radical new insight came from electron microscopy. More recently, the structures of the component molecules have been determined x-ray crystallography at atomic resolution. These results now allow us to describe in some detail how the hydrolysis of ATP by the component proteins actin and myosin leads to movement.
An understanding of muscle contraction is an important example of the success of protein crystallography, in particular when used in conjunction with high resolution electron microscopy.
Time-resolved X-ray Diffraction from Frog Muscle
Excised living frog muscles will continue to contract for may hours if bathed in Ringers solution and stimulated electrically. HE Huxley showed that living frog muscles give a detailed low-angle x-ray fibre diagrams with a series of layer-lines arising from the helical arrangement of cross-bridges. One of these has a strong meridional peak with a Bragg spacing of 143.5Å arising from the distance between cross bridges along the myosin filament helix. If the cross bridges tilt during contraction, then this reflexion should get weaker. Muscles contract fast, so millisecond time resolution is necessary. Initial attempts to observe this effect with conventional x-ray sources were unable to get enough signal in the short times available. Therefore, the first beam lines for synchrotron radiation as an x-ray source were set up at DESY Hamburg (Rosenbaum et al. 1971). Synchrotron radiation provided the requisite power.
The key experiments were actually carried out in 1980 (Huxley et al. 1981). These finally showed the anticipated changes in intensity of the meridional reflexions. If a contracting muscle is released the intensity of the 14.3.5Å meridional reflexion drops within a few ms to a fraction of its initial value. If the muscle is extended quickly, the intensity is recovered. If one waits at the new length the intensity recovers. These experiments have recently been repeated with very high time resolution using sophisticated mechanics and the excellent two dimensional detectors at Daresbury (Irving et al. 1992) These observations are fully consistent with the swinging cross bridge hypothesis and represent the most important time resolved experiments supporting this class of hypothesis.