In this article we will discuss about:- 1. Distribution of Skeletal Muscle 2. Origin and Development of Skeletal Muscle 3. Features 4. Histological Structure 5. Blood Vessels, Lymphatics and Nerves.
Distribution of Skeletal Muscle:
The skeletal (voluntary and striated) muscle fibres are multinucleated cylindrical structures having a clear display of longitudinal and cross-striations. This muscular tissue is responsible for voluntary movement of the living system. It can perform works of rapid, powerful contraction as well as that of prolonged slow sustained tonic contraction.
Skeletal muscles mostly in all instances are attached to osseous tissues (bones), innervated with somatic nerves through which volitional control is performed. In the fresh state the human skeletal muscle is pink in colour due to the presence of muscle pigments and high vascularity. Due to variation in colour there are red and white (or pale) muscles.
Origin and Development of Skeletal Muscle:
The skeletal muscle is developed from the solid mass of the mesoderms (myotomes) except the muscle of the head which is developed from the loose mesenchyme.
The cells, which give rise to the muscular tissue, are named as the myoblasts. In the myotome they are regular cylindrical but gradually they become elongated, spindle-shaped, parallel bundles and ultimately the multinucleated myofibrils with characteristic cross-striation appearance.
There are three theories for the appearance of the multinucleated skeletal muscle cell:
1. The myoblasts fuse together during the process of development of the muscle fibre which was supported by many with electron micrography (observed in developing myotome of amphibian larvae).
2. In the course of the development of the skeletal muscle fibre (cell), the nucleus is multiplied by mitotic division, but the cytoplasm does not divide accordingly.
3. Both the processes as described under (1) & (2) may take place. During the early phase of development of the skeletal muscles of mammals the nucleus of the muscle fibre migrates towards the periphery from the centre of the cell to accommodate myofibrils at the central core.
Features of Skeletal Muscle:
The skeletal muscles, the name derived, are attached to a bone by means of the tendons nearly in all cases. A tendon is composed of densely packed white fibrous (non-elastic) connective tissue. At the junctional point, the fibre of the tendon is affixed to the sarcolemma of the muscle fibres which is again surrounded by areolar tissue to strengthen the junction (Fig. 1.51).
Like other muscles, the skeletal muscle is also supported by various connective tissues. Epimysium, the connective tissue coat, is the outer most covering for each whole skeletal muscle bulk (Fig. 1.62).
The whole muscle bulk is divided into smaller bundles, the fasciculi, bounded by the perimysium, the connective tissue septa (Fig. 1.62). Now again each fasciculus consists of muscle fibre (the structural unit) which is enclosed in a delicate areolar connective tissue jacket, the endomysium (Fig. 1.62).
Each muscle fibre (cell) consists of multiple muscle fibrils or myofibrils. A blood capillary is seen near each muscle fibre along with the endomysium. The connective tissue content of different muscle varies widely. The proportion of connective tissue is highest in the muscles responsible for the fine and precise movements.
Histological Structure of Muscle Fibres:
The skeletal muscle fibres are cylindrical, elongated cells with multiple nuclei.
The extent of the muscle fibres in the bulk may be from:
(i) One end to the other,
(ii) One end to somewhere at midway, or
(iii) Both the ends within the muscle having no attachments with either side. The length and breadth of the muscle fibre vary from 1.0 to 4.0 mm and 0.01 mm (10µ) to 0.1 mm (100µ) respectively.
Though the fibres of various sizes in the same muscle yet there is some correlation between the amount of work to be performed by the muscle and the thickness and size of the fibres. The thickness also varies with the degree of nourishment of the individual. The growth of the muscle on systematic exercise is due to increase in total fibre volume and improvement of blood supply and not due to increase of fibre number.
The transparent cell wall (Fig. 1.65) of the muscle fibre is named as Sarcolemma. Under light microscope it is visible when fresh muscle fibres are teased. Electron micrograph shows that it is made up not only of the plasmalemma but also of an extrinsic coat of amorphous material, similar to the basement membrane.
This amorphous layer is pierced and encircled by reticular fibres. The plasmalemma is of same structure as that of any other cell, i.e., two protein layers enclosing a lipid one with a thickness of 100 A. Inside the plasmalemma, elongated multiple nuclei and transversely striated myofibrils (bundles of myofilaments) are embedded in the sarcoplasm (in many respects the cytoplasm of any other cell).
The sarcoplasm contains other constituents as that of any other cell, e.g., numerous mitochondria (sometimes called sarcosomes), a small Golgi apparatus near each nucleus, myoglobin (protein pigment)., lipid, glycogen, sarcoplasmic reticulum (endoplasmic reticulum in case of other-tissue), etc. Fibres which are richer in sarcoplasm are darker in colour and vice versa.
Myofibrils (Fig. 1.62):
The characteristic features of skeletal muscle, the alternate light and dark shades (transverse striations) and thick longitudinal strands can be studied with light microscope. Electron micrography reveals that the longitudinal striation is due to the presence of myofibrils of different thickness whereas the transverse striation is due to the presence of alternate light and dark segments of longitudinally arranged elements.
In cross-section, myofibrils appear as fine dots either distributed uniformly or in a group of polygonal areas and are separated from adjacent bundles by clear sarcoplasm. The separated myofibrils by the sarcoplasmic areas are known as fields of Cohnheim (Fig. 1.59).
The dark band is doubly refractive (anisotropic) when studied under polarized light, hence the name A band (sometimes called Q-band). The light band is monorefractive (isotropic) under polarised microscope, from which the name is derived l-band (sometimes known as J-band). This I-band is bisected at the midpoint by a thin darkly stained line—the Z-line (sometimes called as Dobie’s line) which is also known as Krause’s membrane.
The Z-line is made of a membrane extending across the myofibrils. The myofibrils are tortuous at these Z-lines. The portion enclosed by two adjacent Z-lines of a myofibril is considered as the contractile unit and is named as sarcomere. It extends 2µ to 3µ in length. In certain exceptional preparations, central portion of the A-band is paler in colour and called the H-band (Hensen’s line).
At the midpoint of the H-band, i.e., also of the A-band, there is a narrow dark line, M-line or M-band, where the myosin filaments are thickened. The myosin and actin filaments are overlapped at the peripheral dark portion of the A-band, which is named as the O-band. On both sides of the Z-line, somewhere in the midregion of the I-band there is a comparatively darker thin transverse line-the N-line.
Fine Structure:
Electron micrography reveals that fine thread-like protein filaments are forming together the myofilaments, of which the thicker one (100Å in diameter)—myosin filament and thinner one (50Å in diameter) — actin filament. The primary structure of myosin and tropomyosin is characterised by a large amount of acidic and basic amino acid content, that confer a very high charge on the molecules.
Actin, troponin and a-actin in contrast have got low charge and are further distinguished by their high proline content. The myosin filaments are present as parallel strands throughout the whole length of the A-band. The length of myosin filament is about 1.5µ (15,000 Å) and is slightly thicker at the midline. On further dissociation of these filaments, the myosin molecules (1500 Å in length) appear as rod-shaped with a globular projection at one end (Fig. 1.63-E).
The thickness of the myosin filament is believed to be the cause of antiparallel arrangement of myosin molecules in such a fashion that the globular heads project outward near the ends of the filaments and the rod-like part takes its position in the smooth central portion of the filament. The peripheral bead like structures of myosin filaments are due to the presence of globular heads of the myosin molecule (Fig. 1.63-D). Rod-like portion is named as light (L) meromyosin and the head portion, i.e., the globular portion is as heavy (H) meromyosin (Fig. 1.63-B).
The heavy meromyosin has got two components. The head portion is known as heavy meromyosin sub fragment I and the neck is heavy meromyosin sub-fragment II (Fig. 1.63-C). The heavy meromyosin sub-fragment I possesses all the enzymatic and actin-combining properties of the parent (myosin) molecule. In each 400 Å segment of the myosin filament, there are six heavy meromyosin heads (cross-bridges). These heavy meromyosin heads are arranged in a helical manner with a radial pattern of 60° and each set of six bridges complete one revolution around the myosin filament.
Each heavy meromyosin head is pointed towards a separate actin filament and thus the cross-bridges with actin occur at approximately 400 Å intervals along the myosin filament (Fig. 1.63-E). The heavy meromyosin is responsible for the ability in formation of the cross- bridge with actin and also for the ATP-ase activity essential for muscular contraction. This heavy meromyosin is sometimes named as the active point of myosin molecules.
At the M-band region (midpoint of myosin filament) the union of myosin molecules occurs tail to tail, but at this region the myosin filaments appear thicker indicating that there is possibility of presence of some proteins of unknown nature which may help in strengthening the tail-to-tail union of the myosin molecules.
The detailed structure of the M-band region with high E.M. Photography has been deduced by Knappeis and Carlsen (Fig. 1.62-2). The M-band filaments, made up of proteins of unknown nature, are attached to the central portion of the myosin filament (Fig. 1.64A) by means of the M-band cross-bridges (transverse).
Functions of M-bands are:
(i) To maintain the parallel alignment of the myosin filaments, and
(ii) To guide the actin filaments during contraction.
Actin filaments are extended from each Z-line within a sarcomere towards the H-band and connected with the other actin filaments by the S-filament (Fig. 1.62) within the H-band. It extends 1µ in both sides of the Z-line. According to Knappeis and Carlsen the length of the actin filament is about 1µ.
With very high magnification, the F-actins—fibrous actins appear as beaded and seem to consist of globular subunits (55 Å) — G-actins (Fig. 1.63 A) forming two strands entwined in a helix. These strands are therefore a polymer of G-actin containing 13 G-actins at each turn of the helix (Fig. 1.63B). Low-angle x-ray studies and also chemical investigations suggest that the tropomyosin and newly known troponin are oriented in the grooves of the actin helices.
The actin filaments, approaching the Z-lines, appear to be continuous with four fine diverging filaments-Z-filaments (Fig. 1.64B). It is believed that these Z-filaments contain the muscle protein -tropomyosin. Recent low-angle x-ray studies have suggested that at the Z-disc along with tropomyosin, another protein a-actin (similar to actin in structure and proline content) is also present.
Transverse section of the myofibrils will give different representations at different levels of the A-and I-bands. Transection through the I-band will show the transverse section of thin actin (Fig. 1.62-4) filaments only, that at the O-band will present the transverse section of both thin actin and thick myosin (Fig. 1.62-1), that at the H-band only myosin (Fig. 1.62-3) and lastly at the M-band region, the transverse section of myosins along with M-band filaments (Fig. 1.62-2) will be present.
In cross-section the arrangement of the actin filaments appears in hexagonal shape with one myosin at the centre, but again when that of the thick filaments is considered then the myosin filaments are forming triangles with one central actin filament (Fig. 1.62-1). So in the longitudinal section, each myosin filament is followed by two (sometimes one) actin filaments. The thin filaments, actions are connected to each other longitudinally by means of the still far more thinner S-filaments (Fig. 1.62).
Sarcotubular System:
Under electron microscope, the myofibrils are seen to have surrounded by a canalicular network of membrane-limited tubules—sarcoplasmic reticulum. The sarcoplasmic reticulum is identical with the endoplasmic reticulum of other cell type but with the difference that its membrane does not possess ribosome.
The sarcoplasmic reticulum is extended longitudinally along the A-band with frequent anastomosis in the region of the H-band and also in the I-band. The sarcoplasmic reticulum is connected at its both longitudinal and terminal ends by another set of the transverse cisterns—terminal cisternae (Fig. 1.65).
The terminal cisternae have got larger caliber and are thus continuous and confluent with the longitudinal reticulum (sarcotubule). Pairs of parallel terminal cisternae (adjacent terminal cisternae) are separated from each other by a slender transverse tubule which is known as T-tubule. This T-tubule is not confluent with the terminal cisternae and is a tubular invagination of the sarcolemma but not a part of sarcoplasmic reticulum. It is continuous with the extracellular space.
These tubules are generally called as T-system. The pair of terminal (transverse) cisternae and the central T-tubules is collectively called as triads (Fig. 1.65). In amphibian muscle the triads encircle the I-band at the region of the Z-line but in mammalian muscle, the same is present at the junction of each A-band with the adjacent I-band. So in mammals there are two sets of triads in each sarcomere (Fig. 1.65). The T-system plays an important role in quick transmission of impulse from the cell surface to each myofibril.
Micro-anatomical organization of the muscle fibre has been presented schematically:
Mechanism of Contraction:
i. Theory or Folding of Myofilaments:
Theory of folding of myofilaments (contractile proteins) stated that certain muscle proteins are shortened or folded during the muscular contraction by forming actin-myosin complex. But morphological studies do not show any such evidence that during muscular contraction the myosin filaments are shortened. Now if the actin filaments attain any shortening or folding at all, still it is not possible during normal life.
ii. Interdigitation or Sliding of Myofilaments:
According to this theory, the shortening of the length of certain bands during muscular contraction was observed by many classical cytologists, but the cause or explanation for that was not satisfactory. In the past few years, the analysis of submicroscopic elements with electron microscopy and X-ray diffraction techniques has advanced a lot.
These new techniques also led to an entirely new concept to the mechanism of contraction. The theory of sliding filament mechanism is now mostly accepted. It is observed with phase contrast and interference microscopy that during contraction the I-band and H-band diminish in length but the A-band remains constant. The sliding filament hypothesis (Fig. 1.66) states the alteration of relative position of the myofilaments during the muscular contraction but neither the actin nor the myosin filaments are shortened themselves.
During contraction, the actin filaments slide past the myosin filaments and thereby the actin filaments are further extended into the A-band causing shortening of the length of the H-zone and narrowing the sarcomere (Fig. 1.67). In this process the myosin filament gradually approaches the Z-line and the actin filament, the M-line by altering the site of attachment of the cross-bridges—one point ahead in the manner of an animated cog-wheel (Fig. 1.67 B & C). At certain stages of contraction, the ends of the two adjacent actin filaments may touch each other and the I-band is of minimum length.
There are two theories regarding the position of two opposite actin filaments of the same sarcomere, during maximum contraction (Fig. 1.67 D & E):
(i) At the free end of the actin filaments in the M-band there is a slide over each other (Fig. 1.67 D).
(ii) The zigzag Z-line is straightened causing an increment in distance between the adjacent actin filaments. Due to this there will be a stretching of the actin filaments towards the Z-line causing further shortening of the sarcomere (Fig. 1.67E). Energy required for this sliding process is maintained from the break-down of the ATP by the myosin ATP-ase present at the local heavy meromyosin molecule.
The function of the T-system, as already mentioned, is to propagate the impulse from the sarcolemma to the myofilament within a short time. Following stimulation the impulse is transmitted to the myofibrils by the T-system and the depolarization causes the release of calcium from the sarcoplasmic reticulum which in turn activates the myosin ATP-ase. This activated ATP-ase breaks the ATP to ADP and the ADP to AMP with the release of certain amount of energy which is required for the process of contraction.
At the end of contraction, the Ca++ ions return to the sarcoplasmic reticulum and relaxation occurs. A relaxing factor has been isolated from muscle homogenates. From electron microscopic studies of the muscle homogenates, it has been claimed that membrane-limited vesicles are present which are derived from the fragmentation of sarcoplasmic reticulum. These vesicles have the capacity of binding Ca++ in presence of ATP.
Blood Vessels, Lymphatics and Nerves of Skeletal Muscle:
Skeletal muscles are richly supplied with networks of anastomosing capillaries which run longitudinally with intercommunicating transverse branches. No capillaries penetrate sarcoplasm. The great arteries and veins are seen in the perimysium. The largest and smallest veins possess valves.
The lymphatic supply communicates with blood vessels of epimysium and perimysium. But lymphatics are not found between these muscle fibres. Myelinated nerve fibres supply striated muscle. The motor nerve endings pierce the sarcoleumma, enter the muscle fibres and terminate in end plates. The sensory nerves end in groups of modified muscle fibres known muscle spindles. Functions of sympathetic nerve here are not known.
Ending of Muscle in Tendon:
At the musculotendinous junctions the endomysium, perimysium and epimysium of the muscle become continuous with the fibrous tissue of the tendon.
Red and White (or Pale) Muscles:
A muscle fibre, being composed of a number of delicate fibrils surrounded by a more fluid sarcoplasm and having mitochondria and sarcoplasmic reticulum, possess respiratory pigment, myoglobin (muscle haemoglobin) within the sarcoplasm. Red colour of the muscle fibre is due to the presence of myoglobin. This myoglobin acts in the transport of oxygen from blood vessels (capillaries) in the extracellular space to the sites of oxidation (mitochondria).
In most mammals, all the muscle fibres contain cytochrome and more myoglobin and look red. These red muscle fibres possess more nuclei frequently central in position, much granular sarcoplasm, well-marked longitudinal striation and irregular transverse striation. Red muscle fibres have a high capacity for oxidative metabolism with a strong activity of Krebs cycle and electron transport enzymes and slower in their contractile action. Red muscle fibres undergo fatigue less rapidly than white (or pale) muscle fibres and are well adapted for static or postural contractions.
White (or pale) muscle fibres are deficient in myoglobin and generally present in frogs. These white fibres are small, regular and have poor sarcoplasm, peripheral nuclei. These white fibres possess a high rate of anaerobic glycolysis with intense activity of glycolytic enzymes and phosphatase. White fibres being predominant in flexor muscles help phasic contractions by which changes in the position of the body or a limb are done.
In some mammals (e.g., rabbits) including the human and birds have both red and white (or pale) muscle fibres.