 |
| Introduction |
|
| The act of changing place or position by the entire body or by one or more of its parts is called movement. Movement is one of the characteristic features of living organisms. Study of movements is called kinesiology. |
| |
| In animals, movement is of two main types namely, muscular and non muscular. |
| |
| Muscular movements are further of two types namely, locomotion and movements of body parts. |
| |
| Locomotion is the movement of an animal as a whole from one place to another. |
| |
| Movements of body parts is that where an animal can move parts of its body. |
| |
|
| |
| Movement involves 3 basic mechanisms. They are amoeboid, ciliary and muscular. |
| |
| Amoeboid movement is typically found in amoeba, a unicellular animal. Amoeba moves by producing pseudopodia, which are cytoplasmic projections. This involves change in the shape of the cell body and streaming movement of cytoplasm into the pseudopodium. The movement due to pseudopodia in amoeba is termed as amoeboid movement. Amoeboid movement is characteristic of certain cells in other organisms. For example, the movement of white blood cells or leucocytes, in human blood. |
| |
| Amoeba moves about to obtain food or to avoid dangers or to escape from energy. Leucocytes like phagocytes or macrophages of the lymph, show amoeboid movements to engulf antigen or microbes and to immigrate in the circulatory fluid. |
| |
| Ciliary movement is the method by which ciliated protozoans like paramoecium, move from plate to place in water medium. Paramoecium uses cilia not only for moving from one plate to another (locomotion) also to drive water and food into their gullet. |
| |
| Cilia can perform a variety of functions: |
| |
| 1. In certain molluscs, cilia help to pass water currents over the gills |
| |
| 2. In echinoderms cilia helps to drive water through the water vascular system, (locomotion) |
| |
| 3. Cilia of the cells lining the respiratory tract of humans help to drive away the microbes and dust particles towards the nose or mouth |
| |
| 4. Cilia in the oviduct or fallopian tubes of human female transport ova |
| |
| Flagellum also helps in the movement in certain protozoans like euglena. Flagellum is a long, thread like cytoplasmic projection. Flagellum in sperms also helps in the swimming movement. |
| |
| Muscular movement is the method used in most of the vertebrates, including man. Muscular movement is based on the use of muscle fibres. Muscle fibres have the unique property of ability to contract and relax, which exerts a force. This force is responsible for movement of body parts and locomotion. |
| |
| Human movement and locomotion also results from co-operation between muscles and bones. Hence study of both skeletal and muscular system in the human body becomes essential. |
| |
| Types of Muscles |
| |
| Muscles constitute nearly 40 - 50% of the total human body weight. |
| |
| Muscles have the unique properties of: |
| |
| (a) Excitability (b) Contractibility (c) Extensibility and (d) Elasticity |
| |
| In man, muscles are classified into 3 categories: |
| |
| 1) Skeletal Muscles |
| |
| These muscles are under the control of the will of an individual. Therefore called as Voluntary muscles. These muscles are found in arms, legs, body wall, face, neck etc. They are attached to the bones by tendons and help in the movement of the parts of skeleton. Under the microscope these muscle fibres show transverse stripes and hence also called as striated muscles. These muscles are responsible for movement and locomotion. |
| |
| 2) Smooth muscles |
| |
| These are not under the direct control of one's will. Hence called as Involuntary muscles and are innervated by autonomic nervous system. These muscles are composed of stender, tapering muscle fibres which are non-striated. These are found in the wall of internal organs like alimentary canal, reproductive tract, blood vessels etc. Smooth muscles help in the movement of materials through tubular internal organs. For example movements of passage of food in the intestine. |
| |
| 3) Cardiac muscles |
| |
| These muscle fibres are striated and comparatively short. Unlike other muscles, cardiac muscle fibres are branched. It is found only in the heart. It is involuntary. |
| |
| Structure of Skeletal Muscle |
| |
| When a typical skeletal muscle is viewed under a microscope, it consists of many elongated, cylindrical cells called muscle fibres or muscle cells. Each muscle fibre is enveloped by a plasma membrane called the sarcolemma. The sarcolemma surrounds a quantity of cytoplasm called sarcoplasm. Within the sarcoplasm are many nuclei and a number of mitochondria. Skeletal muscle fibres are thus multi-nucleate. Also within the muscle fibre is the sarcoplasmic reticulum. The mitochondria are called sarcosomes. |
| |
 |
| |
| Myofibrils of a Muscle Fibre |
| |
| Bundles of muscle fibres are grouped in a fascicule, which are held together and enclosed by collagen fibres and by connective tissue. A tough outer layer called fascia, lying below the skin, surrounds the bundles of fascilli. |
| |
| A highly magnified view of skeletal muscle fibre reveals thread-like structures (about 1-2 mm in diameter) called Myofibrils. These bear the characteristic cross -striations. The myofibrils are stacked in compartments called sarcomeres. Narrow zones of dense material called Z lines separate sarcomeres from one another. A dark anisotropic band (A-band) is present in the centre of the sarcomere. Adjacent to this lies a light isotropic band (I-band). Alternate arrangement of dark and light bands produces the striated appearance to the skeletal muscle. At the centre of the A-band, a less dark zone called H-zone is present. In the centre of H-zone, M-line is present, made up of fine threads that connect the myofilaments. The Z-line is present at the centre of the I-band. |
| |
| The sarcoplasm has many thick and thin filaments. In each sarcomere, the thin filaments are present at the two ends and the thick one at the centre. |
| |
| The H-zone contains thick myofilaments only. The I-band is composed of thin myofilaments only. The rest of A-band has both thick and thin filaments. |
| |
 |
| |
| Components of Thin and Thick Filaments |
| |
| The thin myofilaments are composed mostly of the protein actin. The thick myo-filaments are composed mostly of the protein myosin. Both are contractile proteins. Actin has low molecular weight and it occurs as G-actin and F-actin. The thin filaments also contain two other protein molecules - tropomyosin and troponin. |
| |
| The myosin molecule is shaped like a rod with a round head. Myosin rods form the long axis of the thick myofilaments and the heads form projections called cross-bridges, which connect to the active site present on the actin. |
| |
