Biotechnology Letters 22: 521–529, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 521 Minireview Micromechanical measurements on biological materials: muscle fibres Michael A. Ferenczi National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK (Fax: +0208 906 4419; E-mail: michael.ferenczi@nimr.mrc.ac.uk) Received 15 February 2000; Accepted 18 February 2000 Key words: ATPase, caged-ATP, calcium, fluorescence, muscle concentration, muscle fibre, myosin Abstract Movement often distinguishes living from inanimate matter. Much of current understanding of the molecular mech- anism underlying biological movement has developed through the study of the characteristics of isolated muscle fibres. Techniques for handling single muscle cells, and salient results from such investigations are described. Introduction Muscle – the biological engine Muscle is an organ specialised in the transformation of chemical energy into mechanical work. This process takes place in all living cells, but muscle cells are packed with the proteins required to optimise this function. Muscle remains of great interest to study the functioning of a living machine. Muscle contraction is energetically efficient and its performance adapts to the demands made upon it (He et al. 1999). When active muscle is not shortening, it produces a force per unit cross-sectional area of 200 kPa, and also shortens at a velocity corresponding to four times its length per second. Many muscle types evolved to optimise performance to suit their function or environ- ment. Dissection in the light microscope revealed that muscles can be teased into small fibres: single muscle cells (Fishl & Kahn 1928). These cells can be several centimetres long with a diameter of ∼50 μm for mam- malian cells. At each end, the skeletal muscle cells are attached to tendon, a strong and elastic material and each is controlled by a motor nerve. Depolari- sation of the nerve ending releases neurotransmitter which causes a travelling wave of electrical depo- larisation along the muscle cell membrane. Calcium is released into the cytoplasm from the specialised stores, the sarcoplasmic reticulum, so that its concen- tration increases from <0.1 μM to nearly 10 μM. In the laboratory, direct electrical stimulation also causes depolarisation and calcium release. Calcium binds to troponin C, one of a number of proteins associated with the thin- or actin-filaments. Calcium binding enhances the interaction of actin with myosin mol- ecules, the other major protein arranged into filaments. Myosin is an ATPase: energy for muscle activity is provided by the breakdown of ATP to release ADP and inorganic phosphate. Actomyosin interaction causes stiffening of the muscle cell, a hundred-fold increase in myosin ATPase activity, force generation and short- ening. After stimulation, calcium is pumped back into the sarcoplasmic reticulum by an ion-pumping ATPase and muscle relaxes: force decreases and the cell becomes compliant again (for a broad review, see Woledge et al. 1985). Sarcomeres In skeletal muscle, both actin and myosin form fila- ments which inter-digitate in units called sarcomeres which repeat every 2–2.5 μm along the length of the muscle cells. In the light microscope, sarcom- eres appear as transverse alternating dark and pale regions: skeletal muscle is also called striated mus- cle. Longitudinal arrays of filaments, the myofibrils, are approximately 1 μm in diameter, separated by layers of cytoplasmic material which also contain the sarcoplasmic reticulum. The biochemistry of these muscle proteins is studied after extraction and solubil-