skeletal muscle

skeletal muscle moves the skeleton and is responsible for all our voluntary movements, as well as for the automatic movements required, for example, to stand, to hold up our head, and to breathe. (Other involuntary functions involve smooth muscle and cardiac muscle.)

As well as being the ‘motors’ of the body, muscles are also the brakes and shock absorbers. They can be used as heaters (when shivering) and also function as a store of protein if we should face malnutrition.

Individual muscles, such as the biceps in the arm, are made up of large numbers (about 100 000 in biceps) of giant cells, known as muscle fibres. Each fibre is formed from fusion of many precursor cells and therefore has many nuclei. The fibres are each as thick as a fine hair (50 μm in diameter) and 10–100 mm long. They are arranged in bundles, separated by sheets of connective tissue containing collagen. These bundles rarely run straight along the axis of the muscle, more usually at an angle, called the angle of pennation because many muscles show a pennate (featherlike) pattern of fibre bundles.

Each muscle fibre is surrounded by a cell membrane, which allows the contents of the fibres to be quite different from that of the body fluids outside them. Inside the fibre are the myofibrils, which constitute the contractile apparatus, and a system for controlling the myofibrils through changes in calcium concentration. This system, the sarcoplasmic reticulum (SR), is a closed set of tubes containing a high concentration of calcium. Each myofibril runs the whole length of the muscle fibre with a variable number of segments, the sarcomeres; it is only one or two micrometres in diameter, and is surrounded by the SR network. The myofibril consists of many much thinner and shorter protein rods, which are the myofilaments. These are of two kinds: thick filaments, which are made predominantly from a single protein, myosin, and thin filaments, which contain the protein actin. The actual contraction takes place by an interaction of the actin with projections on the myosin molecules (crossbridges). Each of the crossbridges can develop force (about 5 × 10^-12 Newtons) and can pull the thin filament along past the thick filament by about 10 × 10^-9 metres (10 millionths of a mm). The net effect of many of these small movements and small forces is to shorten the myofibrils, and thus the whole muscle; hence some part of the skeleton is moved, by means of the attachment of the muscle at each end to bone, directly or via tendons.

When a person initiates a movement, events in the brain and the spinal cord generate action potentials in the axons of the motor neurons. Each of these axons branches to send action potentials to many muscle fibres. (A motor unit is this collection of perhaps several hundred muscle fibres controlled by one axon.) At the nerve terminals of each axon branch (neuromuscular junction) acetylcholine is liberated by the arriving action potential, and this combines with receptors on the membrane of the muscle fibre, causing it, in turn, to generate an action potential. This action potential spreads over the whole surface of the fibre and also down an extensive network of fine tubes (T-tubules), which conduct it into the interior. Here a message, the nature of which is uncertain, passes from the T-tubule to the sarcoplasmic reticulum, causing it to allow some of the calcium it contains to leak out into the interior of the muscle fibre. The thin filaments in the myofibrils contain, as well as actin, two proteins, troponin and tropomyosin; the calcium which leaks from the SR is able, for a brief period, to interact with the troponin molecule of the thin filament; this, through movements of the tropomyosin molecules, alters the thin filament so that the actin molecules are available to be joined by the crossbridges, starting the process of contraction. As soon as calcium escapes from the SR the process starts of mopping it up again. There are calcium pumps in the membranes of the SR, which are able to move the calcium back inside, thus bringing to an end the short period of muscle activity (a muscle twitch). More sustained periods of activity are the norm in the movements we make; they require a sequence of action potentials to be sent to the muscle, at perhaps 30 per second. The contractions produced in this way are stronger than a twitch.

Muscle contraction requires energy to drive the crossbridges through their cyclic interactions with actin: in each cycle the myosin molecule does work in moving the thin filament. Also, energy is used for the process of calcium pumping by the SR. Energy consumption is highest when muscles are used to do external work — for example in climbing stairs, when the body weight has to be lifted. However energy is also used when a weight is held up without doing work on it (isometric contraction). Least energy is used when muscles are used to lower weight, as when descending stairs.

The energy for muscle contraction comes from the splitting of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and phosphate. The muscle contains enough ATP to power it at maximum output for only a couple of seconds. ATP can be regenerated in muscle rapidly from phosphocreatine (PCr), and there is enough of this substance in the muscle to last perhaps 10 to 20 seconds of maximum activity. The fact that we can sustain strenuous activity beyond 10 seconds is due to the utilization of carbohydrate in the muscles, where it is stored as glycogen. This can be used to regenerate the ATP supply in two ways. If oxygen is available, glucose can be oxidized to water and carbon dioxide, with two-thirds of the energy released used to rebuild the ATP supply. If oxygen is not available, the process stops with glucose converted to lactic acid and only about 6% of the energy used for building ATP. The lactic acid leaves the muscle cells and can accumulate in the blood. In addition to carbohydrate, muscles use fat, in the form of fatty acids taken up from the blood, as a substrate for oxidation; this is important for prolonged activity, since the body's energy stored as fat is much greater than that stored as carbohydrate. The availability of oxygen depends on its delivery by the blood; when muscle becomes active, the products of its metabolism cause the vessels to dilate, and this enables a rapid increase in the blood flow.

Muscle fatigue is the effect of a set of mechanisms which ensure that muscle is not made active when there is not enough energy available for the activity. If that were to happen, theoretically the muscle could go into rigor mortis, and could fail to retain the large amount of potassium it contains, with dire consequences for the body as a whole.

The body contains several different varieties of skeletal muscle fibre, which can be seen as specialized for different purposes. The ‘slower’ muscles are more economical at holding up loads, such as maintaining posture of the body itself, and probably also more efficient at producing external work. Related to their lower energy use they are less easily fatigued. Faster muscle fibres, however, can produce faster movements and higher power outputs, and are essential for such tasks as jumping or throwing. The way different muscles are constructed also allows for specialization of function: muscles with shorter fibres hold forces more economically, muscle with longer fibres can produce faster movements. A pennate arrangement allows muscles to be built with many short fibres, increasing the force they can exert, whereas long fibres, running almost parallel to the axis of the muscle, give the fastest movements.

Some people have more muscular strength than others; they can exert larger forces, do external work more rapidly, or move faster. To a large extent this is because the stronger individuals have larger muscles, but there seem to be other factors at work as well. Training can change the properties of muscle. Strength training consists in using the muscles to make just a few very strong contractions each day. Over months and years this leads to an increase in the force that can be exerted and in increase in the size of the muscles. Force increase often precedes size increase. Endurance training consists of using the muscles less intensely but for longer periods. Again, over months of training the ability of the muscles to get energy through the oxidation of carbohydrate and fat is raised. The supply of blood to the muscle is also increased through changes in the blood vessels and also in the heart. Training can also lead to changes in the fatigue resistance of muscle fibres, and perhaps cause them to change into a slower type of fibre.

Roger Woledge

See musculo-skeletal system.See also exercise; fatigue; glycogen; metabolism; movement, control of; muscle tone; sport; strength training.