A brief introduction to muscle
Muscle is an organ specializing in the transformation of chemical energy into movement. Movement is essential to life, and takes many forms, from cytoplasmic streaming and the growth of neurones at the cellular level, to the long distance flight of the albatross or the explosive performance of a sprinter. Although only a few families of proteins are responsible for movement in the biological world, muscle has developed to optimize this function, and is packed with movement-related proteins. 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 has a vital role and 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 muscles contain thousands of muscle cells, or muscle fibres, which run from one tendon to the other. The fibres can be several centimetres long, and are 50-150 micrometer (μm) thick, roughly cylindrical in cross-section. Single fibres or small bundles are readily dissected. In the dark-field dissecting microscope, the bundles have an iridescent appearance revealing the fact that the fibres contain transverse bands of isotropic and anisotropic material.
This is seen in the compound microscope where in a single fibre the bands appear as vertical dark and bright lines. The striations are called the sarcomeres which form a repeating transverse structure every 2.4 μm. The longitudinally-repeating structures are called the sarcomeres.
In a fluorescence microscope too skeletal muscle has a striated appearance:
This micrograph was taken in the laboratory of Muscle Biophysics by Valentina Caorsi in 2009. It shows a segment of a skeletal muscle fibre in a confocal fluorescence microscope. The green bands show fluorescence labelling on the essential light chain of myosin. The red bands show fluorescence labelling of the actin molecules. Thus the striations are formed by the alternating bands of actin and myosin filaments. The myosin filaments known as thick filaments, and the actin filaments (thin filaments) interdigitate. A muscle cell is composed of thousands of longitudinal myofibrils, which are arrays of thick and thin filaments. The myofibrils are surrounded by membranes and other components of the muscle machinery: mitochondria, sarcoplasmic reticulum and transverse tubules. A myofibril shows the thick and thin filaments as alternating dark and light bands (picture below taken by Ronnie Burns at NIMR, 1997.)
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. Stretching to scale two of the sarcomeres above shows the relationship between the bands and the filament arrays:
Click on the above sarcomeres to see a more detailed picture. The birefringent properties of muscle has led to the names I- and A-bands, where the I-band is isotropic and the A-band is anisotropic.
A sarcomere from a mammalian muscle is about 2.4μm long at rest. It can be extended reversibly to more than 3 μm (as in the micrograph above), and it can shorten to less than 2 μm. The appearance of the striations change during shortening. The filaments do not change length during shortening (recent experiments have shown that the filaments are slightly elastic, but most of the shortening is caused by sliding; Sosa et al., 1994). The sliding movement of the sarcomere is shown below (Huxley A.F. and Niedergerke, 1954; Huxley H.E. and Hanson, 1954):
The sliding of the filaments is the result of interactions between myosin molecules in the thick filaments and binding sites on the actin filaments. The myosin molecules have a region which extends away from the backbone of the filaments and binds to actin filaments. This region is called the myosin cross-bridge. This region of the myosin molecule binds, and hydrolyses adenosine triphosphate (ATP), the cellular fuel for muscle contraction. Hydrolysis of ATP results in the release of adenosine diphosphate (ADP) and inorganic phosphate (Pi). When ATP is abundant, and in the presence of calcium, the cross-bridges reversibly bind to actin and produce a mechanical impulse which results in force transmission along the filaments, which either results in force production at the tendons, or results in shortening (or a combination of both). The energy for this process is derived from the hydrolysis of ATP and release ADP and Pi.
During contraction, skeletal muscles generate force, and may shorten. But what brings the filaments back to their original length, once contraction has ended? This is the work of the antagonistic muscles on the other side of the limb or body, which, by shortening, re-stretch the muscles. In other tissues, the inherent elasticity of the structures restores the initial state, such as in the heart.
The link between movement or force and the utilization of ATP is the fundamental aspect of muscle contraction, sometimes referred to as the energy transduction process. The areas of current research relate to understanding the link between ATP binding, hydrolysis and product release, and the production of a mechanical impulse. The nature of cross-bridge binding to the actin is crucial. Is there a single binding site, or are there several? How does ATP binding or hydrolysis affect the nature of actin binding? Which part of the cross-bridges change shape? Each myosin molecule is a double helical coil and ends in two globular heads or cross-bridge. Does each head behave independently of the other?
The study of muscle contraction involves the use of a large variety of biophysical techniques, in many laboratories. Such techniques range from physiological studies of muscle contraction to biochemical studies of muscle proteins and nucleotide hydrolysis and include protein crystallography, low angle X-ray diffraction, the design of mutant proteins, NMR, electron microscopy, photolysis of caged compounds, in vitro motility assays, laser-trap studies of isolated myosins interacting with actin filaments, fluorescence lifetime imaging microscopy (FLIM), Förster resonance energy transfer (FRET) and others.
