Fig. 1.1
Organisational levels and connective tissue sheaths in skeletal muscle. Courtesy of CIC Edizioni Internazionali—From “Il tendine e il muscolo” (Eds.: Giuseppe M. Peretti, Gian Luigi Canata), CIC Edizioni Internazionali, Rome 2014
Muscle cells range in size from 10 to100 μm in diameter and from 1 mm to 20 cm in length. The sarcoplasm is rich in mitochondria needed to provide ATP and hosts a highly developed sarcoplasmic reticulum (SR; smooth endoplasmic reticulum of myocytes) which contains an elevated concentration of calcium ions (Ca++) which is necessary for muscle contraction. The multiple nuclei are located peripherally adjacent to the sarcolemma, the plasma membrane that surrounds the muscle fibre and regulates the movement of chemical substances in and out of the cell. The sarcoplasmic reticulum is composed of tubules and cisternae which wrap around the myofibril. The tubules run parallel to the myofibril and anastomose with one another within the H-zone, while between the A- and I-bands, they open into terminal cisternae which run perpendicularly to the axis of the muscle fibre (Fig. 1.2). The terminal cisternae are connected by transverse tubules (or T-tubules). The sarcoplasm also contains numerous myofibrils, composed of bundles of multiple myofilaments, which in turn are composed of specialised contractile proteins. Both myofilaments and myofibrils are distributed parallel to one another and to the longitudinal axis of the muscle fibre.
Fig. 1.2
Sarcoplasmic reticulum and T-tubules in the skeletal muscle fibres. The figure shows a cross-sectional view of a location within the sarcomere. Courtesy of CIC Edizioni Internazionali—From “Il tendine e il muscolo” (Eds.: Giuseppe M. Peretti, Gian Luigi Canata), CIC Edizioni Internazionali, Rome 2014
The contractile proteins within the myofibrils are distributed in a partially overlapping manner which gives the myofibrils the distinct banding pattern (striation) of dense and less dense areas seen in striated muscle. The contractile proteins that form the myofilaments are myosin, a filamentous protein with a globular head domain that forms the dense bands (A, anisotropic when viewed with polarised light), and actin which constitutes the thin filaments and, together with troponin and tropomyosin, makes up the less dense (I, isotropic) bands (Fig. 1.3). The Z-disc, a thin dark band, corresponds to either end of the sarcomere, while the M-line, a dark band in the centre of the lighter H-zone, marks the centre of the sarcomere where the thick filaments are stabilised by crosslinking (Agarkova and Perriard 2005). The thin filaments (actin) are present in the A-band as well as myosin; however, the H-zone is occupied solely by myosin. Thus, the dark appearance of the A-band is due to the overlap of thin and thick filaments. Within the A-band each filament of myosin is surrounded by six actin filaments (Fig. 1.2). The sarcomere represents the functional unit of the muscle cell as it is the sliding movement of the overlapping actin and myosin strands that cause the contraction of the myofibril and thus muscle contraction (Huxley 2004).
Fig. 1.3
Microscopic anatomy of the skeletal muscle fibre: basic structure of the sarcomere. Courtesy of CIC Edizioni Internazionali—From “Il tendine e il muscolo” (Eds.: Giuseppe M. Peretti, Gian Luigi Canata), CIC Edizioni Internazionali, Rome 2014
The contractile proteins are therefore filamentous molecules that constitute the myofilaments present in the sarcomere of muscle fibres. The thick filaments are composed of myosin molecules; the globular head moieties project out from the filament and make contact with actin. The thin filaments are composed predominantly of actin and make transverse connections to one another at the Z-disc. Actin filaments are formed from three molecules of globular actin and assume the form of a polarised helical chain, with a plus and minus end, and a number of additional accessory proteins including tropomyosin and troponin. Tropomyosin forms two filaments that insert into the groove of actin and binds three molecules of troponin. When cellular Ca++ levels are low, the interaction of tropomyosin and troponin blocks interaction with myosin. Upon release from the SR, calcium ions bind troponin C and cause a conformational change in the troponin/tropomyosin complex which uncovers the myosin binding site on the actin molecule. Myosin binding causes the thin and thick filaments to slide past one another, due to the motor activity of myosin which is powered by the hydrolysis of ATP.
The calcium ions are rapidly removed from the sarcoplasm by a large number of ATP-dependent Ca++ pumps; tropomyosin, in the absence of calcium ions, returns to its original conformation blocking the actin active site, and the muscle returns to its resting length.
1.1.2 Classification of Skeletal Muscles
Skeletal muscles are located under the integumentary system, covered by a sheet of connective tissue which, in some points, may be attached to the periosteum that covers the bone surface. Their function is to produce movement, by pulling on the skeletal frame of the body, to maintain posture and to protect the underlying structures, particularly in areas lacking skeletal protection. In addition, muscular contraction causes thermogenesis and acts as a vascular pump.
The physical form differs greatly between different muscles, and some are flat and thin, while others are short and thick or long and spindly. The length of the muscle, excluding the tendon component, correlates with the contractile length, while the contractile force depends on the mass of the muscle in question, which may be used only partially depending on the level of force desired. The contractile mass is termed muscle belly; it is the most developed form of skeletal striated muscle and is both highly vascularised and innervated. The extremities of the muscle belly develop into either tendons or aponeuroses which attach the contractile part of the muscle to bones or, in the case of the facial muscles, to the skin.
Single muscle fibres may be orientated in one of two ways; parallel or oblique to the direction of muscle action. On this basis, the muscle can be classified as muscles in which the fascicle arrangement is parallel or muscles in which the fascicle arrangement is oblique to the body midline (Fig. 1.4). In the former case (Fig. 1.4a–c), the fibres run directly parallel to the long axis of the muscle, and the tendons are arranged in the same orientation as the muscle. Contraction of the muscle occurs in the same direction as the fibres are oriented, increasing the amplitude of the movement. Muscle of this type may assume various shapes, such as the straplike muscles (Fig. 1.4a) and fusiform muscles, with an expanded central belly and more thinned extremities where the muscle gives way to tendon (Fig. 1.4b) as well as muscles with circular pattern of fascicles, usually called sphincters, in which fibres are arranged in concentric rings (Fig. 1.4d). In triangular muscles, the fibres fan out over a wide area (Fig. 1.4e), converging on a thick central tendon. The triangular arrangement of the fibre allows the muscle to function in various planes of movement depending on the subset of fibres utilised; the trapezoid and pectoral muscles exemplify this type of muscle.
Fig. 1.4
Patterns of fascicle arrangement in muscles. In (a–c) the fibres are parallel to the long axis of the muscle. In (d) the fibres are arranged in concentric rings. In e, the origin of the muscle is broad, and the fibres converge towards a thick central tendon. In (f) and (g) muscle fibres are short and attach obliquely to a tendon. Courtesy of CIC Edizioni Internazionali—From “Il tendine e il muscolo” (Eds.: Giuseppe M. Peretti, Gian Luigi Canata), CIC Edizioni Internazionali, Rome 2014
In the muscles in which the fascicle arrangement is oblique, the muscle fibres are shorter and attach obliquely to the tendon with respect to the direction of muscle action. These muscles are referred to as pennate muscles, due to the muscle structure which resembles the fibres of a feather (Fig. 1.4f–g). This arrangement maximises the number of fibres incorporated in a given area, and because a greater number of fibres result in a greater cross-sectional area, the force produced by these muscles will be greater. In reality, the force produced by a muscle is influenced by several morphological factors, including the cross-sectional area and the fascia angle (the angle formed by the longitudinal axis of the muscle and the tendon).
1.1.3 Number of Points of Origin
A muscle group may be composed of several independent muscles, called “muscle heads”. These are named according to the number of muscles involved; examples include the biceps (“two heads”), triceps (“three heads”) and quadriceps (“four heads”) which are composed, respectively, of 2, 3 and 4 muscles with different origins that converge on a common tendon.
1.1.4 Mode of Action
On the base of functional criteria, muscles may be classified as flexors or extensors, abductors or adductors, pronators or supinators and internal or external rotators, depending on the movement generated by contraction. The names of many forearm and leg muscles begin with extensor or flexor, indicating how they move the hand, foot and digits. To transmit their action, a muscle must bridge a point of articulation and attach to two bones.
On the basis of the insertion point of the muscle to the skeleton, and how many articulations are involved, a muscle may be classified as mono-, bi- or polyarticular. Monoarticular muscles bridge one joint and join two bones; thus, movement is confined to one joint, e.g. the coracobrachialis muscle. Biarticular muscles span two joints; often these muscles have two or more tendons at one extremity and one tendon at the other end; one tendon is monoarticular, while the other is biarticular. Polyarticular muscles span more than two joints, with tendon insertions on several bone sites; these include very long muscles, usually located along the spinal column, that make contact with each vertebra.
1.1.5 Classification of Skeletal Muscle Fibres
The body is capable of a wide range of movements, from simple maintenance of posture to the capacity to engage in activities such as the high jump to running the marathon. The skeletal muscle is able to generate a finely tuneable output; the power generated represents the product of the force produced by the shortening of the muscle fibres multiplied by the velocity at which the muscles contract. The velocity of contraction is, to a large degree, determined by the expression of different isoforms of the myosin heavy chain (MHC), which have greatly differing ATPase activity. Type I, or slow fibres, contain MHC-1B; intermediate fibres, type IIa, contain MHC-IIA; and the fast fibres contain MHC-IIx. MHC-1B, in slow fibres, are seven to nine times slower than type IIx fibres which contain MHC-IIx. Therefore the power can be altered by regulating either of the two input variables: force and velocity; both of these variables depend on the properties of the muscle fibres, the characteristics of the motor units and by their recruitment.
Skeletal muscle fibres have different characteristics depending on their different molecular, metabolic, structural and contractile properties which permit their classification into three groups.
- 1.
Type I fibres (also known as red fibres or slow oxidative (SO) fibres). These are small fibres, containing a high concentration of myoglobin, a red-pigmented molecule similar to haemoglobin which similarly stores oxygen. Thus, even at rest, the fibres contain a reserve of oxygen which is made available the instant of contraction. Tissues composed of these fibres are rich in capillaries, ensuring the supply of blood to the muscle. The sarcoplasm of type I fibres contains few myofibrils but many mitochondria. Type I fibrils are also known as slow fibres due to their slow speed of contraction. The large number of mitochondria produces more ATP than myosin can hydrolyse combined with the large reserve of myoglobin help to avoid fatigue. Therefore, this type of muscle fibre is prevalent in postural muscles, which must remain contracted for long periods of time.
- 2.
Type II fibres, or white fibres, are fast-twitch glycolytic (FTG) fibres. Fibres of this group are much more voluminous that type I fibres due to the presence of increased numbers of myofibrils and which consequently produce more force. They contain fewer mitochondria and less myoglobin than type I fibres and contain fewer capillaries. Type II fibres are also known as fast fibres, as they contract at a greater velocity than type I, due to a faster form of myosin, as well as a system of T-tubules and a SR that is more efficient at releasing calcium ions. The cost of faster contraction is more rapid exhaustion of ATP, so type II fibres are more susceptible to fatigue with respect to type I. With fewer mitochondria, type II fibres rely on the cleavage of glycogen to produce ATP. Since anaerobic respiration is far less efficient at producing ATP, these muscles cannot maintain prolonged contraction. However, since these fibres contract and relax rapidly, they are well suited to activities that require short bursts of intense power, such as sprinting.
- 3.
Intermediate fibres are fast-twitch oxidative-glycolytic (FTOG) fibres. Recently discovered, these fibres are structurally, architecturally and biochemically a mix of the previous two types in that they consist of rapid contracting fibres that are resistant to fatigue. Histologically they are similar to fast fibres but contain a greater number of mitochondria and capillaries, and as a result they are more resistant to fatigue with respect to type II fibres while generating more force more rapidly than type I fibres. These fibres have been found in muscles which perform both rapid, high power tasks, such as jumping, but also work to maintain posture (e.g. in the leg).
The proportion of the diverse groups of fibres depends on the type of activities for which an individual uses their muscles, for example, athletes that practise endurance sports tend to have prevalently slow fibres. Training represents a stimulus for the recruitment of fibres or rather the motor units, which are activated in a precise order, depending on the intensity of the exercise. The type I fibres are recruited first during light exercise, before the intermediate type and finally the white fibres, which are most susceptible to fatigue (Table 1.1).
Table 1.1
Characteristics of the muscle fibres found in skeletal muscle and their innervation
Fibre | Characteristics | Motor unit | Nerve axon diameter | Conduction velocity | Order of activation | Order of deactivation |
|---|---|---|---|---|---|---|
Type I, slow oxidation | Small cells, high in myoglobin and mitochondria. Found in postural and low-power, high resistance muscles | FR | Thin | Slow | 1 | 3 |
Intermediate, FTOG fibres | Recently described, fast contracting cells with intermediate resistance to fatigue | S | Intermediate | Intermediate | 2 | 2 |
Type II, FTG, white fibres | Voluminous cells with lots of myofibrils but few mitochondria. Rapid and powerful contraction but susceptible to fatigue | FF | Large | Fast | 3
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