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Muscle
∙ Although bones provide leverage and form the framework of the body they cannot move body parts by themselves. Motion results from the alternating contraction and relaxation of muscles, which make up 40–50% of total adult body weight.
∙ Muscle is a specialised tissue of mesodermal origin. The scientific study of muscles is known as myology (myo-muscle, logy-study of). They have special properties like excitability, contractility, extensibility and elasticity.
∙ Muscles have been classified using different criteria, namely location, appearance and nature of regulation of their activities. Based on their location, three types of muscles are identified:
(i) Skeletal muscles
(ii) Visceral muscles
(iii) Cardiac muscles
1.0Cardiac Muscles
∙ As the name suggests cardiac muscles are the muscles of heart. Many cardiac muscle cells assemble in a branching pattern to form a cardiac muscle.
∙ Cardiac muscle cells (Cardiomyocytes) are striated, branched, contain many mitochondria. Each myocyte contains a single, centrally located nucleus.
∙ Cardiac muscle cells contain branched fibres connected via intercalated discs that contain gap junctions.
∙ Oblique bridges are present. They have numerous glycogen granules. Abundant supply with deep penetrating capillaries. They contract rapidly and never get fatigued.
∙ They are involuntary in nature as the nervous system does not control their activities directly. The alternating contraction and relaxation of the heart is not consciously controlled. Rather the heart, beats because it has a pacemaker that initiates each contraction. This built-in rhythm is termed autorhythmicity. Several hormones and neurotransmitters can adjust heart rate by speeding or slowing the pacemaker.
2.0Visceral Muscles
∙ Visceral muscles are located in the inner walls of hollow visceral organs of the body like the alimentary canal, reproductive tract, etc.
∙ They do not exhibit any striation and are smooth in appearance. Hence, they are called smooth muscles (nonstriated muscle). Their activities are not under the voluntary control of the nervous system and are therefore known as involuntary muscles.
∙ For example, they assist, in the transportation of food through the digestive tract and gametes through the genital tract.
Visceral Muscle
3.0Skeletal Muscles
∙ Skeletal muscle tissue is so named because most skeletal muscles move bones of the skeleton. (A few skeletal muscles attach to and move the skin or other skeletal muscles.)
∙ Skeletal muscle tissue is striated. Alternating light and dark protein bands (striations) are seen when the tissue is examined with a microscope.
∙ Skeletal muscle tissue works mainly in a voluntary manner. Its activity can be consciously controlled by neurons (nerve cells) that are part of the somatic (voluntary) division of the nervous system.
∙ They are primarily involved in locomotory actions and changes of body postures.
Structure of Skeletal Muscle
∙ Each of our skeletal muscle is composed of hundreds to thousands of cells, which are called muscle fibres because of their elongated shapes. Thus, muscle cell and muscle fibre are two terms for the same structure.
∙ Muscle fibres are organized into individual bundles called a fascicle held together by a, common collagenous connective tissue layer called fascia.
∙ Each muscle fibre is lined by the plasma membrane called sarcolemma enclosing the sarcoplasm. Muscle fibre is multinucleated as the sarcoplasm contains many nuclei. The endoplasmic reticulum, i.e., sarcoplasmic reticulum of the muscle fibres is the store house of calcium ions.
∙ Sarcoplasm includes a substantial amount of glycogen. Glycogen can be used for synthesis of ATP. In addition sarcoplasm contains a red colored oxygen storing pigment myoglobin. Myoglobin releases oxygen when it is needed by mitochondria for ATP production. The mitochondria lie in rows throughout the muscle fibre.
∙ A characteristic feature of the muscle fibre is the presence of a large number of parallelly arranged filaments in the sarcoplasm called myofilaments or myofibrils (Myo = muscle, fibrilla = little fibre).
∙ Each myofibril has alternate dark and light bands on it. A detailed study of the myofibril has established that the striated appearance is due to the distribution pattern of two important proteins – Actin and Myosin.
∙ The light bands contain actin and is called I-band or Isotropic band, whereas the dark band called ‘A’ or Anisotropic band contains myosin. Both the proteins are arranged as rod-like structures, parallel to each other and also to the longitudinal axis of the myofibrils.
∙ Actin filaments are thinner as compared to the myosin filaments, hence are commonly called thin and thick filaments respectively. In the centre of each ‘I’ band is an elastic fibre called ‘Z’ line which bisects it. The thin filaments are firmly attached to the ‘Z’ line. The thick filaments in the ‘A’ band are also held together in the middle of this band by a thin fibrous membrane called ‘M’ line.
∙ The ‘A’ and ‘I’ bands are arranged alternately throughout the length of the myofibrils. The portion of the myofibril between two successive ‘Z’ lines is considered as the functional unit of contraction and is called a sarcomere.
Diagrammatic representation of anatomy of a muscle fibre showing a sarcomere
∙ In a resting state, the edges of thin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments leaving the central part of the thick filaments. This central part of thick filament, not overlapped by thin filaments, is called the ‘H’ zone.