Structure and function

A skeletal muscle is composed of a group of fibers that are roughly cylindrical, with diameters ranging between 10 and 100 µm and up to a few centimeters long (Figure 1.2).

Skeletal fibers are formed by the fusion of myoblasts and can therefore have hundreds of nuclei that are localized near the plasma membrane (or sarcolemma). Between fibers and the extracellular matrix (or basal lamina) are large numbers of satellite cells that are important in the growth and repair of fibers. In adult muscle, these satellite cells are quiescent and differentiate only in case of muscle damage.

Fibers are formed by a network of myofibrils that follow the muscle. Each myofibril is composed of proteins that are involved in force production. These proteins are actin which composes the thin filament and myosin, which composes the thick filament. Myosin is the protein which is responsible for force generation. It is composed of a globular head with both adenosine triphosphate (ATP) and actin binding sites. Actin, when polymerized into filaments, forms the "ladder" along which the myosin filaments attach to generate motion.

The thick and thin filaments are regularly arranged and form the skeletal muscle typical pattern of light and dark bands visible with the light microscope. It is also because of these fibrils that skeletal muscle demonstrates its characteristic striated pattern. The functional unit of the muscle, called sarcomere, is composed by a light and a dark band. During contraction, actin and myosin slide one on each other.

Other proteins are closely linked to actin and myosin and stabilize these proteins before and after contraction. Troponin, which is one of the major regulators of force production, blocks the myosin binding site of actin in the absence of Ca²⁺. Troponin itself is regulated by tropomyosin. Titin, an enormous protein involved in maintaining the striation pattern by close association with the myosin molecule, probably anchors the myosin network to the actin network. Nebulin, which is also associated with actin, acts by regulating the length of actin filaments.

When muscle contracts, there is a close association between actin and myosin. During contraction the length of thin filaments and the length of thick filaments remain constant.

Thus, during contraction the length of the sarcomere and band I decreases; the overlap between thick and thin filaments increases; and the length of the thick and thin filaments remains unchanged. Consequently, the 2 filaments slide on each other. This theory has been proposed in 1954 by A.F. Huxley and is called the sliding-filament theory (Huxley & Nieder-

^Actin Troponin Myosin Thick filament

Thin filament

Z disc Z disc

Z disc dark band light band

Sarcomere myofibrill

muscle fiber nucleus muscle fibers bundled in sheath

myofibrill myofibrill

Troponin complex

Figure 1.2: Structure of skeletal muscle fiber.

A muscle is composed of muscle fibers bundled in sheaths. Fibers are multinucleated cells and each is constituted by myofibrils. These are made of thick filaments (myosin) and thin filaments (actin). Thin and thick filaments form the contractile unit called sarcomere which appears as alternating light and dark bands when observed with a microscope.

The thin filaments form the band I and the band between 2 thin filaments is called Z disc. Sarcomere is surrounded by Z discs. The region where thin and thick filaments overlap each other forms the band A. The central zone where there are only thick filaments is called H zone.

gerke, 1954).

When ATP binds to the myosin head in the thick filaments, ATP is hydrolyzed by myosin and disconnects actin from myosin. The energy released by the splitting of ATP is stored in the myosin molecule and represents a high-energy state; this is the predominant state at rest. Upon muscle stimulation, the inhibition of actin-myosin interaction, imposed by the regulatory proteins, is lifted and consequently the myosin in high-energy state binds to actin.

The actin-myosin interaction triggers the release of ADP from the myosin head, resulting in movement. The energy stored in the myosin molecule triggers a conformational change in the cross-bridge between actin and myosin modifying the angle from 90^o to 45^o. This tilting pulls the actin filament about 10 nm toward the center of the sarcomere, while the energy stored in myosin is utilized. Contraction occurs. With a new ATP a new cycle may begin and the cycling may continue until the regulatory mechanism stops the interaction of actin and myosin.

The energy used by skeletal muscle is ATP. As normal metabolism cannot produce energy as quickly as a muscle cell can use it, an extra storage source is needed: the main energy suppliers are glucose and glycogen that can generate ATP through aerobic or anaerobic reactions. Glucose is brought by food and is taken up from the blood for a direct use. Glycogen is synthesized from glucose and stocked mainly in the liver but also in skeletal muscle. Thus, through glycogenolysis, glycogen is metabolized in glucose.

The aerobic reaction takes place in mitochondria and is based on oxidative phosphorylation reactions: Nicotinamide Adenine Dinucleotide (NADH), which is generated during tricarboxylic acid (TCA) cycle from the degradation of glucose, plays a central role in oxidative metabolism. Through the mitochondrial electron transport chain, NADH can transfer two electrons and this is enough energy to synthesize approximately 7 ATPs from ADP. Flavin Adenine Dinucleotide (FADH2) is another important part of oxidative metabolism. Oxidation to FAD releases enough energy to generate nearly 6 ATPs. Muscle cells also use the creatine phosphate as energy supplier. The phosphate group of the creatine can be quickly transferred to ADP to regenerate the ATP necessary for muscle contraction.

The reaction is catalyzed by creatine kinase which is found to be elevated in blood serum in DMD patients (see 1.2.3).

Muscle can also use anaerobic metabolism to generate ATP. In this case, ATP is produced by degradation of glycogen into pyruvate and lactic acid. However, this process is not very efficient as only 3 ATP molecules are produced per cycle. It is also limited by the

accumulation of lactic acid which causes a pH decrease and extreme fatigue that leads to cramps. When combined, both anaerobic and aerobic pathway could produce 36 ATP molecules for 1 glucose molecule that is consumed.

Skeletal muscles contain 2 different fiber types, the slow-twitch fibers (type I) and the fast-twitch fibers (type IIA and IIB). Their main characteristics are summarized in Table 1.1:

(from Ruegg, 1992) Type I (red)

SLOW-TWITCH Ex.: soleus

Type II A (red) FAST-TWITCH

Ex.: quadriceps

Type II B (white) FAST-TWITCH

Ex.: EDL

Myoglobin content high high low

Contraction speed low high high

Myosin ATPase activity low high high

Myosin isoenzyme slow fast fast

SR Ca²⁺ pumping capacity moderate high high

Parvalbumin* content absence moderate high

Resistance to fatigue high moderate low

Number of mitochondria high high low

Metabolism oxidative glycolytic and oxidative glycolytic

Myofibre diameter moderate small large

*Parvalbumin is a Ca²⁺-binding protein that transports Ca²⁺ from troponin to the SERCA during muscle relaxation.

The relative proportion of type I and type II composing muscle fiber determines its characteristics. Type II fibers are fatigable and are used for fast and strong contraction while type I fibers are less fatigable and are used for long and continuous contraction. In human, the soleus muscle is a slow muscle as it is composed by around 90 % of type I fibers, the quadriceps is considered as an intermediate muscle as it is made of equal amount of type I and type II fibers. Extensor Digitorum Longus muscle is considered as a fast muscle as it is composed by around 70 % of type II fibers.