Revision: Biology Zoology >> Locomotion and Movement Biology (Botany and Zoology) HSC Science Class 11 Tamil Nadu Board of Secondary Education

Key Points: Types of Movement

Human body cells show three types of movements: Amoeboid, Ciliary, and Muscular.
Amoeboid movement - caused by pseudopodia; involves microfilaments; seen in leucocytes and macrophages.
Ciliary movement - occurs in organs lined by ciliated epithelium; removes dust in the trachea; moves ova in the fallopian tube.
Muscular movement - due to the contractile property of muscles; seen in jaws, limbs, eyelids, alimentary canal, and ureters.
Muscular movement is used for locomotion and other movements in humans and most multicellular organisms.
Locomotion requires coordinated activity of muscular, skeletal, and neural systems.

Key Points: Skeletal Muscle (Voluntary Muscle)

Skeletal muscle is made up of muscle bundles (fascicles) held together by fascia (collagenous connective tissue). Each bundle contains many muscle fibres.
Each muscle fibre is lined by sarcolemma, contains sarcoplasm, is a syncytium (many nuclei), and the sarcoplasmic reticulum stores calcium ions.
Muscle fibres contain myofibrils with alternate light and dark bands due to actin and myosin proteins.
I-band - light, contains thin actin filaments. A-band - dark, contains thick myosin filaments.
'Z' line - bisects each I-band; thin filaments are attached to it. 'M' line - in the middle of the A-band; holds thick filaments together.
Sarcomere - region between two successive 'Z' lines; functional unit of contraction.
In the resting state, thin filaments partially overlap thick filaments; the non-overlapped part of the thick filament is the 'H' zone.

Key Points: Structure of Contractile Proteins

Each thin filament (actin) is made of two 'F' (Filamentous) actins helically wound together. Each 'F' actin is a polymer of monomeric 'G' (Globular) actins.
Two tropomyosin filaments run alongside 'F' actins. Troponin proteins are distributed at regular intervals on tropomyosin.
In the resting state, a troponin subunit masks the active binding sites for myosin on actin filaments.
Each thick filament (myosin) is made of many monomeric proteins called meromyosins. Each meromyosin has a globular head, short arm, and tail.
Meromyosin is of two types - HMM (Heavy Meromyosin) - the globular head + short arm, and LMM (Light Meromyosin) - the tail.
The globular head (HMM) projects outward, forming a cross arm. It acts as an active ATPase enzyme and has binding sites for both ATP and actin.

Key Points: Mechanism of Muscle Contraction

Muscle contraction follows the sliding filament theory, where thin (actin) filaments slide over thick (myosin) filaments.
Contraction begins with a signal from the CNS through a motor neuron; the neuron and muscle fibres together form a motor unit.
The neuromuscular junction (motor-end plate) is the point where the motor neuron connects to the muscle fibre.
Acetylcholine is released, which generates an action potential and causes the release of Ca²⁺ from the sarcoplasmic reticulum.
Calcium ions activate actin, allowing myosin heads to bind and form cross-bridges.
Cross bridges pull actin filaments, causing sliding, shortening of the sarcomere, and muscle contraction (I-band shortens, A-band remains the same).
ATP helps in breaking and reforming cross-bridges; when Ca²⁺ is pumped back, the muscle relaxes.

Key Points: Disorders of Muscular and Skeletal System

Myasthenia gravis - an autoimmune disorder affecting the neuromuscular junction; causes fatigue, weakness, and paralysis of skeletal muscles.
Muscular dystrophy - progressive degeneration of skeletal muscles; mainly caused by a genetic disorder.
Tetany - rapid spasms/wild contractions in muscles due to low Ca²⁺ (calcium) levels in body fluids.
Arthritis - inflammation of joints causing pain and stiffness.
Osteoporosis - an age-related disorder; decreased bone mass and increased risk of fractures; caused by a decrease in oestrogen levels.
Gout - inflammation of joints due to the accumulation of uric acid crystals; causes intense pain and swelling.