Define sliding filament theory of muscle contraction.

1. Movement in animals:

(i) Movement is one of the basic features of living beings.

(ii) Different types of locomotory movements in animals are wiggling, crawling, walking, running, climbing, jumping, flying, swimming, and so on.

(iii) The movements of an animal resulting in a change of location or place is called locomotion.

(iv) Animals generally move in search of food, shelter, mate, breeding ground, better climates, or to protect themselves.

(v) Three main types of movements are amoeboid, ciliary, and muscular.

(vi) Some specialized cells in our body, like macrophages and leucocytes in blood, exhibit amoeboid movement. 

(vii) Ciliary movements of cells lining the trachea of humans help to remove the invading microbes and dust particles.

(viii) Muscular movement is the basic mechanism used in the majority of vertebrates, including humans.

(ix) Locomotion and many other movements require coordinated muscular activities.

2. Muscles:

(i) Muscle has four major functional characteristics: contractility, excitability, extensibility, and elasticity.

(ii) Based on their location, in humans, muscles are broadly classified into three categories: skeletal muscles, smooth muscles, and cardiac muscles.

(iii) Skeletal muscles are attached to the bones by tendons and help in the movement of the parts of the skeleton.

(iv) Skeletal muscles are striated and voluntary in nature.

(v) Smooth muscles are involuntary and non-striated muscles.

(vi) Smooth muscles are present in the inner walls of visceral organs.

(vii) Cardiac muscles are the muscles of the heart.

(viii) Cardiac muscles are also striated, but these muscles are branched and involuntary.

(ix) Muscle fibre is the anatomical unit of muscle.

(x) Skeletal muscle consists of numerous physiological units, called myofibrils.

(xi) Each myofibril contains many serially arranged functional units called sarcomeres.

(xii) Each sarcomere has a central ‘A’ band of thick myosin filaments and two half ‘I’ bands of thin actin filaments on either side of it marked by ‘Z’ lines.

3. Structure of contractile proteins:

(i) Myosin head contains ATPase and has ATP binding sites and active sites for actin.

(ii) The myofilaments consist mainly of four types of proteins viz., myosin, actin, tropomyosin and troponin.

(iii) Myosin and actin are called the contractile proteins as they shorten muscle fibre.

(iv) Tropomyosin and troponin are called the regulatory proteins.

4. Mechanisms of muscle contraction:

(i) The sliding filament model of muscle contraction was first proposed in 1954, mainly by Huxley and Jean Hanson.

(ii) Sliding of the thin filaments over thick filaments is produced by numerous cross bridges extending out from the myosin towards the actin.

(iii) A motor neuron carries the signal to the muscle fibre which generates an action potential in it.

(iv) This causes the release of $C{a}^{2+}$ from sarcoplasmic reticulum.

(v) $C{a}^{2+}$ activates actin which binds to the myosin head to form a cross bridge.

(vi) These cross bridges pull the actin filaments causing them to slide over the myosin filaments and thereby causing contraction.

(vii) ATP is the immediate source of energy for muscle contractions.

(viii) Repeated stimulation of muscles leads to fatigue.

(ix) The mammalian and avian skeletal muscles contain two major types of striated muscle fibres: red and white.

(x) This classification is based primarily on the amount of red-coloured myoglobin pigment in them.

(xi) Myoglobin is an important haem-containing dark globular protein.

(xii) Myoglobin is an oxygen-storing protein in red muscle fibres of vertebrate muscles.

5. Skeletal system:

(i) The skeletal system has four components: bones, cartilage, tendons and ligaments.

(ii) The average adult human skeleton has $206$ bones.

(iii) The skeletal system is divisible into axial and appendicular.

(iv) Skull, vertebral column, ribs and sternum constitute the axial skeleton.

(v) Limb bones and girdles form the appendicular skeleton.

(vi) Three types of joints are formed between bones or between bone and cartilage -fibrous, cartilaginous and synovial.

(vii) Synovial joints allow considerable movements and therefore, play a significant role in locomotion.

6. Joints:

(i) Joints are essential for all movements involving the body's bony parts. 

(ii) Joints are points of contact between bones or between bones and cartilage. 

(iii) Force generated by the muscles is used to carry out movement through joints, where the joint acts as a fulcrum. 

(iv) Fibrous Joints/Sutures: Fibrous joints do not allow any movement. This type of joint is shown by the flat skull bones which fuse end-to-end with the help of dense fibrous connective tissues in the form of sutures, to form the cranium.

(v) Cartilaginous Joints: In cartilaginous joints, the bones involved are joined together with the help of cartilages. The joint between the adjacent vertebrae in the vertebral column is of this pattern and it permits limited movements.

(vi) Synovial Joints: Synovial joints are characterized by the presence of a fluid-filled synovial cavity between the articulating surfaces of the two bones. Such an arrangement allows considerable movement. These joints help in locomotion and many other movements. ball and socket joint, saddle joint, hinge joint are some examples

7. Disorders Of Muscular And Skeletal System:

(i) Myasthenia Gravis: Autoimmune disorder affecting neuromuscular junction leading to fatigue, weakening, and paralysis of skeletal muscle.

(ii) Muscular Dystrophy: The progressive degeneration of skeletal muscle is mostly due to genetic disorder.

(iii) Tetany: Rapid spasms (wild contractions) in muscle due to low Ca⁺² in body fluid.

(iv) Arthritis: Inflammation of joints. It is of three types: Rheumatoid arthritis, osteoarthritis, and Gout.

(v) Osteoporosis: Age-related disorder characterized by decreased bone mass and increased chances of fractures. Decreased levels of estrogen is a common cause.

(vi) Gout: Inflammation of joints due to accumulation of uric acid crystals.