Classification of Muscle and Structure of SKeletal Muscle .
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| Muscle fiber . |
Muscles Introduction .
[1]. Human body has more than 600 muscles. Muscles
perform many useful functions and help us in doing
everything in day-to-day life.
perform many useful functions and help us in doing
everything in day-to-day life.
[2]. Muscles are classified by
three different methods, based on different factors:
three different methods, based on different factors:
1. Depending upon the presence or absence of
striations .
striations .
2. Depending upon the control .
3. Depending upon the situation.
Depending upon Striations .
Depending upon the presence or absence of cross
striations, the muscles are divided into two groups:
striations, the muscles are divided into two groups:
1. Striated muscle .
2. Non-striated muscle.
1. Striated Muscle .
Striated muscle is the muscle which has a large number
of cross-striations (transverse lines). Skeletal muscle
and cardiac muscle belong to this category.
of cross-striations (transverse lines). Skeletal muscle
and cardiac muscle belong to this category.
2. Non-striated Muscle .
[1]. Muscle which does not have cross-striations is called
non-striated muscle.
non-striated muscle.
[2]. It is also called plain muscle or
smooth muscle.
smooth muscle.
[3]. It is found in the wall of the visceral
organs.
organs.
Depending upon Control .
Depending upon control, the muscles are classified into
two types:
two types:
1. Voluntary muscle
2. Involuntary muscle.
1. Voluntary Muscle
[1]. Voluntary muscle is the muscle that is controlled by the
will.
will.
[2]. Skeletal muscles are the voluntary muscles.
[3]. These
muscles are innervated by somatic nerves.
muscles are innervated by somatic nerves.
2. Involuntary Muscle .
[1]. Muscle that cannot be controlled by the will is called
involuntary muscle.
involuntary muscle.
[2]. Cardiac muscle and smooth muscle
are involuntary muscles.
are involuntary muscles.
[3]. These muscles are innervated
by autonomic nerves.
by autonomic nerves.
Depending upon situation .
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| skeletal, cardiac and smooth muscle fibers . |
Depending upon situation, the muscles are classified
into three types:
into three types:
1. Skeletal muscle .
2. Cardiac muscle .
3. Smooth muscle.
1.Skeletal Muscle .
[1]. Skeletal muscle is situated in association with bones
forming the skeletal system.
forming the skeletal system.
[2]. The skeletal muscles
form 40% to 50% of body mass and are voluntary
and striated.
form 40% to 50% of body mass and are voluntary
and striated.
[3]. These muscles are supplied by somatic
nerves .
nerves .
[4]. Fibers of the skeletal muscles are arranged in
parallel.
parallel.
[5]. In most of the skeletal muscles, muscle fibers
are attached to tendons on either end.
are attached to tendons on either end.
[6]. Skeletal muscles
are anchored to the bones by the tendons.
are anchored to the bones by the tendons.
2. Cardiac Muscle .
[1]. Cardiac muscle forms the musculature of the heart.
[2]. These muscles are striated and involuntary.
[3]. Cardiac
muscles are supplied by autonomic nerve fibers.
muscles are supplied by autonomic nerve fibers.
3. Smooth Muscle .
[1]. Smooth muscle is situated in association with viscera. It
is also called visceral muscle.
is also called visceral muscle.
[2]. It is different from skeletal
and cardiac muscles because of the absence of cross striations, hence the name smooth muscle.
and cardiac muscles because of the absence of cross striations, hence the name smooth muscle.
[3]. Smooth
muscle is supplied by autonomic nerve fibers.
muscle is supplied by autonomic nerve fibers.
[4]. Smooth
muscles form the main contractile units of wall of the
various visceral organs.
muscles form the main contractile units of wall of the
various visceral organs.
Muscle mass .
 |
| A. Skeletal muscle mass; B. Cross-section of muscle; C. One muscle fasciculus. |
[1]. Muscle mass or muscle tissue is made up of a large number of individual muscle cells or myocytes. The muscle cells are commonly called muscle fibers because these cells are long and slender in appearance.
[2]. Skeletal muscle fibers are multinucleated and are arranged parallel to one another with some connective tissue in between .
[3]. Muscle mass is separated from the neighboring tissues by a thick fibrous tissue layer known as fascia. Beneath the fascia, muscle is covered by a connective tissue sheath called epimysium.
[4]. In the muscle, the muscle fibers are arranged in various groups called bundles or fasciculi.
[5]. Connective tissue sheath that covers each fasciculus is called perimysium.
[6]. Each muscle fiber is covered by a connective tissue layer called the endomysium .
Muscle Fiber .
[1]. Each muscle cell or muscle fiber is cylindrical in shape. Average length of the fiber is 3 cm.
[2]. It varies between 1 cm and 4 cm, depending upon the length of the muscle.
[3]. The diameter of the muscle fiber varies from 10 µ to 100 µ. The diameter varies in a single muscle.
[4]. Muscle fibers are attached to a tough cord of connective tissue called tendon.
[5]. Tendon is in turn attached to the bone.
[6]. Tendon of some muscles is thin, flat and stretched but tough. Such type of tendon is called aponeurosis.
[7]. Each muscle fiber is enclosed by a cell membrane called plasma membrane, that lies beneath the endomysium. It is also called sarcolemma .
[8]. Cytoplasm of the muscle is known as sarcoplasm .
Structures embedded within the sarcoplasm are:
1. Nuclei
2. Myofibril
3. Golgi apparatus
4. Mitochondria
5. Sarcoplasmic reticulum
6. Ribosomes
7. Glycogen droplets
8. Occasional lipid droplets.
[9]. Each muscle fiber has got one or more nuclei. In long muscle fibers, many nuclei are seen.
[10]. Nuclei are oval or elongated and situated just beneath the sarcolemma. Usually in other cells, the nucleus is in the interior of the cell.
[11]. All the organelles of muscle fiber have the same functions as those of other cells.
Myofibril .
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| Myofibril . |
[1]. Myofibrils or myofibrillae are the fine parallel filaments present in sarcoplasm of the muscle cell.
[2]. Myofibrils run through the entire length of the muscle fiber.
[3]. In the cross-section of a muscle fiber, the myofibrils appear like small distinct dots within the sarcoplasm. Diameter of the myofibril is 0.2 to 2 µ.
[4]. The length of a myofibril varies between 1 cm and 4 cm, depending upon the length of the muscle fiber .
[5]. In some muscle fibers, some of the myofibrils are arranged in groups called Cohnheim’s areas or fields.
Microscopic Structure of a Myofibril .
Light microscopic studies show that, each myofibril consists of a number of two alternating bands which are also called the sections, segments or disks. These bands are formed by muscle proteins. The two bands are:
1. Light band or ‘I’ band.
2. Dark band or ‘A’ band.
Light Band or ‘I’ Band .
[1]. Light band is called ‘I’ (isotropic) band because it is isotropic to polarized light.
[2]. When polarized light is passed through the muscle fiber at this area, light rays are refracted at the same angle.
Dark Band or ‘A’ Band .
[1]. Dark band is called ‘A’ (anisotropic) band because it is anisotropic to polarized light.
[2]. When polarized light is passed through the muscle fiber at this area, the light rays are refracted at different directions (An = not; iso = it; trops = turning).
[3]. Dark band is also called ‘Q’ disk (Querscheibe = cross disk). In an intact muscle fiber, ‘I’ band and ‘A’ band of the adjacent myofibrils are placed side-by-side.
[4]. It gives the appearance of characteristic cross striations in the muscle fiber.
[5]. I band is divided into two portions, by means of a narrow and dark line called ‘Z’ line or ‘Z’ disk (in German, zwischenscheibe = between disks).
[6]. The ‘Z’ line is formed by a protein disk, which does not permit passage of light.
[7]. The portion of myofibril in between two ‘Z’ lines is called sarcomere.
Sarcomere .
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| Sarcomere. A = A band, I = I band |
[1]. Sarcomere is defined as the structural and functional unit of a skeletal muscle. It is also called the basic contractile unit of the muscle.
[2]. Each sarcomere extends between two ‘Z’ lines of myofibril. Thus, each myofibril contains many sarcomeres arranged in series throughout its length.
[3]. When the muscle is in relaxed state, the average length of each sarcomere is 2 to 3 µ.
Components of Sarcomere .
[1]. Each myofibril consists of an alternate dark ‘A’ band and light ‘I’ band .
[2]. In the middle of ‘A’ band, there is a light area called ‘H’ zone (H = hell = light – in German, H = Henson – discoverer).
[3]. In the middle of ‘H’ zone lies the middle part of myosin filament. This is called ‘M’ line (in German-mittel = middle). ‘
[4]. M’ line is formed by myosin binding proteins.
Electron Microscopic Study Of Sarcomere .
Electron microscopic studies reveal that the sarcomere consists of many threadlike structures called myofilaments. Myofilaments are of two types:
1. Actin filaments .
2. Myosin filaments.
Actin Filaments .
[1]. Actin filaments are the thin filaments with a diameter of 20 Å and a length of 1 µ.
[2]. These filaments extend from either side of the ‘Z’ lines, run across ‘I’ band and enter into ‘A’ band up to ‘H’ zone.
Myosin Filaments .
[1]. Myosin filaments are thick filaments with a diameter of 115 Å and a length of 1.5 µ.
[2]. These filaments are situated in ‘A’ band.
Cross-bridges .
[1]. Some lateral processes (projections) called cross bridges arise from each myosin filament. These bridges have enlarged structures called myosin heads at their tips.
[2]. Myosin heads attach themselves to actin filaments. These heads pull the actin filaments during contraction of the muscle, by means of a mechanism called sliding mechanism or ratchet mechanism .
[3]. During the contraction of the muscle, the actin filaments glide down between the myosin filaments towards the center of ‘H’ zone and approach the corresponding actin filaments from the next ‘Z’ line . [4]. The ‘Z’ lines also approach the ends of myosin filaments, so that the ‘H’ zone and ‘I’ bands are shortened during contraction of the muscle.
[5]. During the relaxation of the muscle, the actin filaments and ‘Z’ lines come back to the original position.
Contractile Elements (Proteins ) of Muscle .
[1]. Myosin filaments are formed by myosin molecules.
[2]. Actin filaments are formed by three types of proteins called actin, tropomyosin and troponin.
[3]. These four proteins together constitute the contractile proteins or the contractile elements of the muscle.
Myosin Molecule .
[1]. Each myosin filament consists of about 200 myosin molecules.
[2]. Though about 18 classes of myosin are identified, only myosin II is present in the sarcomere.
[3]. Myosin II is a globulin with a molecular weight of 480,000.
[4]. Each myosin molecule is made up of 6 polypeptide chains, of which two are heavy chains and four are light chains .
[5]. Molecular weight of each heavy chain is 200,000 (2 × 200,000 = 400,000).
[6]. Molecular weight of each light chain is 20,000 (4 × 20,000 = 80,000). Thus, total molecular weight of each myosin molecule is 480,000 (400,000 + 80,000).
Portions of Myosin Molecule .
Each myosin molecule has two portions:
1. Tail portion .
2. Head portion.
Tail portion of myosin molecule .
It is made up of two heavy chains, which twist around each other in the form of a double helix .
Head portion of myosin molecule .
[1]. At one end of the double helix, both the heavy chains turn away in opposite directions and form the globular head portion. Thus the head portion has two parts.
[2]. Two light chains are attached to each part of the head portion of myosin molecule .
[3]. Each myosin head has two attachment sites.
[4]. One site is for actin filament and the other one is for one ATP molecule .
[5]. Myosin head is absent in the central part of myosin filament, i.e. in the ‘H’ zone.
Actin Molecule .
[1]. Actin molecules are the major constituents of the thin actin filaments.
[2]. Each actin molecule is called F-actin and it is the polymer of a small protein known as G-actin.
[3]. There are about 300 to 400 actin molecules in each actin filament. The molecular weight of each molecule is 42,000.
[4]. The actin molecules in the actin filament are also arranged in the form of a double helix.
[5]. Each F-actin molecule has an active site to which the myosin head is attached .
Tropomyosin .
[1]. About 40 to 60 tropomyosin molecules are situated along the double helix strand of actin filament. [2]. Each tropomyosin molecule has the molecular weight of 70,000.
[3]. In relaxed condition of the muscle, the tropomyosin molecules cover all the active sites of F-actin molecules.
Troponin .
It is formed by three subunits:
1. Troponin I, which is attached to F-actin .
2. Troponin T, which is attached to tropomyosin .
3. Troponin C, which is attached to calcium ions.
Other Proteins of the Muscle .
In addition to the contractile proteins, the sarcomere contains several other proteins such as:
[1] . Actinin, which attaches actin filament to ‘Z’ line.
[2]. Desmin, which binds ‘Z’ line with sarcolemma.
[3] . Nebulin, which runs in close association with and parallel to actin filaments.
[4]. Titin, a large protein connecting ‘M’ line and ‘Z’ line.
[4a]. Each titin molecule forms scaffolding (framework) for sarcomere and provides elasticity to the muscle.
[4b]. When the muscle is stretched, the titin unfolds itself. However, if the stretching is more, it offers resistance and protects the sarcomere from overstretching.
[5] . Dystrophin, a rod-shaped large protein that connects actin filament to dystroglycan.
[5a]. Dystroglycan is a transmembrane protein, present in the sarcolemma.
[5b]. Dystrophin and dystroglycan form dystrophin dystroglycan or dystrophin-glycoprotein complex.
Sarcotubular System .
[1]. Sarcotubular system is a system of membranous structures in the form of vesicles and tubules in the sarcoplasm of the muscle fiber.
[2]. It surrounds the myofibrils embedded in the sarcoplasm .
Structures Consisting the Sarcotubular System .
Sarcotubular system is formed mainly by two types of structures:
1. T-tubules
2. L-tubules or sarcoplasmic reticulum.
T-Tubules .
[1]. T-tubules or transverse tubules are narrow tubules formed by the invagination of the sarcolemma. [2]. These tubules penetrate all the way from one side of the muscle fiber to an another side.
[3]. These tubules penetrate the muscle cell through and through. Because of their origin from sarcolemma, the T-tubules open to the exterior of the muscle cell. Therefore, the ECF runs through their lumen.
L-Tubules or Sarcoplasmic Reticulum .
[1]. L-tubules or longitudinal tubules are the closed tubules that run in long axis of the muscle fiber, forming sarcoplasmic reticulum. These tubules form a closed tubular system around each myofibril and do not open to exterior like T-tubules.
[2]. L-tubules correspond to the endoplasmic reticulum of other cells. At regular intervals, throughout the length of the myofibrils, the L-tubules dilate to form a pair of lateral sacs called terminal cisternae .
[3]. Each pair of terminal cisternae is in close contact with T-tubule.
[4]. The T-tubule along with the cisternae on either side is called the triad of skeletal muscle.
[5]. In human skeletal muscle, the triads are situated at the junction between ‘A’ band and ‘I’ band.
[6]. Calcium ions are stored in L-tubule and the amount of calcium ions is more in cisternae.
Functions of the Sarcotubular System .
Function of T-Tubules .
[1]. T-tubules are responsible for rapid transmission of impulse in the form of action potential from sarcolemma to the myofibrils.
[2]. When muscle is stimulated, the action potential develops in sarcolemma and spreads through it.
[3]. Since T-tubules are the continuation of sarcolemma, the action potential passes through them and reaches the interior of the muscle fiber rapidly .
Function of L-Tubules .
[1]. L-tubules store a large quantity of calcium ions.
[2]. When action potential reaches the cisternae of L-tubule, the calcium ions are released into the sarcoplasm.
[3]. Calcium ions trigger the processes involved in contraction of the muscle.
[4]. The process by which the calcium ions cause contraction of muscle is called excitation-contraction coupling .
Composition of Muscle .
[1]. Skeletal muscle is formed by 75% of water and 25% of solids. Solids are 20% of proteins and 5% of organic substances other than proteins and inorganic substances .
[2]. Myoglobin is present in sarcoplasm. It is also called myohemoglobin.
[3]. Its function is similar to that of hemoglobin, that is, to carry oxygen.
[4]. It is a conjugated protein with a molecular weight of 17,000.
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