The Z-line (Z-disc) is a dense protein disc bisecting each I band, anchoring the actin filaments. The sarcomere is defined as the distance between two successive Z-lines — the functional contractile unit of muscle.

The H-zone (Hensen's zone) is a lighter central region within each A band where only thick (myosin) filaments are present — actin filaments do not extend this far. The H-zone narrows or disappears during muscle contraction.

The A bands (anisotropic bands) are the dark bands in striated muscle, appearing dark because they contain overlapping thick (myosin) and thin (actin) filaments. They maintain their length during contraction unlike I bands.

Each skeletal muscle fibre (cell) contains hundreds of myofibrils — long cylindrical protein structures running the length of the cell. Each myofibril is composed of repeating sarcomeres containing actin and myosin filaments.

The myocardium (heart muscle) is composed of cardiac muscle — a specialised type of striated involuntary muscle. Cardiac muscle cells are branched, interconnected by intercalated discs, and contract rhythmically without fatigue.

Striated muscles (skeletal and cardiac) show alternating light (I bands — actin only) and dark (A bands — actin + myosin overlap) transverse bands when viewed under a microscope, giving a striped appearance.

Smooth muscle cells are spindle-shaped (fusiform), uninucleate, and lack the cross-striations seen in skeletal and cardiac muscle. They are found in blood vessel walls, digestive tract, uterus, and other internal organs.

Tendons are dense, regular connective tissue cords made primarily of collagen fibres. They are non-elastic and very strong, efficiently transmitting contractile force from muscle to bone. The Achilles tendon is the strongest tendon.

Muscle cells (myocytes) are specialised contractile cells evolved in multicellular animals. They contain the contractile proteins actin and myosin organised into myofibrils, enabling generation of force for movement.

After a bone fracture, a haematoma (blood clot) forms at the fracture site as the first step of healing. It is gradually replaced by a soft callus (fibrocartilage), then hard callus (woven bone), and finally remodelled to compact bone.

Arthritis (particularly rheumatoid arthritis) is an inflammatory disease causing joint damage through immune-mediated destruction of the synovial membrane, cartilage, and eventually bone. It causes pain, swelling, and loss of function.

Intervertebral discs are fibrocartilaginous cushions between vertebrae that absorb compressive forces during all physical activities — walking, running, jumping, and even sitting. They prevent vertebrae from grinding together.

Sciatica is pain radiating along the path of the sciatic nerve — from the lower back through the buttock and down the leg. It is commonly caused by a herniated disc pressing on the nerve root.

Slipped (herniated) intervertebral disc is treated progressively: first bed rest and anti-inflammatory medication, then physiotherapy and traction (stretching the spine), and finally surgery (discectomy or spinal fusion) for severe cases.

Rickets is a skeletal disease in children caused by vitamin D deficiency (impairing calcium absorption) or dietary calcium deficiency. Bones become soft and weak, leading to bowed legs, enlarged joints, and deformed pelvis.

Arthritis encompasses over 100 conditions affecting joints — including osteoarthritis (cartilage degeneration), rheumatoid arthritis (autoimmune inflammation), gout (uric acid crystals), and septic arthritis (infection).

Cleft palate (failure of palate bones to fuse), microcephaly (abnormally small skull/brain), polydactyly, syndactyly, and achondroplasia (dwarfism) are all genetic skeletal deformities caused by abnormal gene expression during development.

Skeletal deformities can result from genetic causes (cleft palate, achondroplasia), hormonal imbalances (gigantism, dwarfism from GH disorders), and nutritional deficiencies (rickets from vitamin D deficiency, osteoporosis from calcium deficiency).

Ligaments are strong, flexible bands of fibrous connective tissue that hold bones together at joints, providing stability while allowing appropriate movement. Tendons connect muscles to bones (not bones to bones).

Structural classification of joints: fibrous joints (bones united by fibrous tissue — sutures, syndesmoses), cartilaginous joints (united by cartilage — synchondroses, symphyses), and synovial joints (joint cavity with synovial fluid).

Joints are classified by movement allowed: synarthroses (immovable — skull sutures), amphiarthroses (slightly movable — intervertebral discs, pubic symphysis), and diarthroses/synovial joints (freely movable — hip, knee, shoulder).

The knee joint (largest joint in the body) is formed between the femur (distal end), tibia, and patella. It is primarily a hinge joint allowing flexion and extension. The fibula is not part of the knee joint proper.

The human hindlimb (leg) consists of: femur (thigh), patella (kneecap), tibia and fibula (leg), 7 tarsal bones (ankle/foot), 5 metatarsals (sole), and 14 phalanges (toes — 3 each except big toe with 2).

The wrist joint between the radius/ulna and the carpal bones is a condyloid (ellipsoid) joint allowing flexion, extension, abduction, and adduction. The intercarpal joints are gliding/plane joints allowing slight sliding movements.

The human forelimb (arm) consists of: humerus (upper arm), radius and ulna (forearm), 8 carpal bones (wrist), 5 metacarpals (palm), and 14 phalanges (fingers — 3 each except thumb with 2).

Of the 12 pairs of ribs, 2 pairs (11th and 12th) are floating ribs — they do not connect to the sternum at all. Ribs 8-10 connect via shared costal cartilage; ribs 1-7 connect directly to the sternum.

There are 12 thoracic vertebrae (T1-T12) in the chest region. Each articulates with one pair of ribs. Cervical vertebrae: 7, Lumbar: 5, Sacral: 5 (fused), Coccygeal: 4 (fused).

The human vertebral column consists of 33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 4 fused coccygeal. In adults the sacral vertebrae fuse into the sacrum and coccygeal into the coccyx.

The axial skeleton forms the central axis of the body and includes: skull (29 bones), vertebral column (26 bones), rib cage (12 pairs of ribs + sternum). It protects the brain, spinal cord, and thoracic organs.

Chondrocytes are the only cells present in cartilage, occupying small spaces called lacunae. They secrete and maintain the cartilage matrix (collagen + proteoglycans). Osteocytes are bone cells; osteoblasts build bone.

Ecdysis in arthropods is primarily controlled by the hormone ecdysone (moulting hormone) from the prothoracic gland, triggered by the nervous system (brain→PTTH→ecdysone). Juvenile hormone prevents metamorphosis during larval moults.

Spongy (cancellous/trabecular) bone has a honeycomb-like structure with cavities (medullary spaces) filled with red bone marrow, where haematopoiesis (production of RBCs, WBCs, platelets) occurs.

Both bone and cartilage have living cells (osteocytes and chondrocytes respectively) embedded in an extracellular matrix rich in collagen fibres and ground substance. Bone matrix is also mineralised with calcium hydroxyapatite.

Vertebrate endoskeleton is composed of bones (ossified connective tissue) and cartilage (flexible connective tissue). Cartilage forms the skeleton in embryos and in cartilaginous fish, and persists as joint surfaces and ear/nose in adults.

Ecdysis (moulting) is the periodic shedding of the chitinous exoskeleton in arthropods to allow growth. During the brief vulnerable period between shedding the old cuticle and hardening of the new one, the animal is soft and exposed.

Sea anemones have a hydrostatic skeleton — a fluid-filled cavity (coelenteron) under pressure. Muscles acting against this internal fluid pressure allow the anemone to change shape, extend/retract tentacles, and burrow.

Arthropods have the most complex exoskeleton — a segmented, jointed chitinous cuticle with specialised regions (sclerites) separated by flexible membranes, allowing diverse movements while providing protection.

The skeleton is the tough, rigid supporting framework of an animal's body. It protects organs, gives shape, supports weight, provides attachment points for muscles, and acts as a calcium reservoir.

The epicuticle is the thin outermost layer of arthropod exoskeleton. It is composed of lipoproteins and waxes that provide waterproofing, preventing desiccation. Beneath it lies the thicker procuticle (chitin + protein).

The skeleton provides the framework that gives shape to the body, protects vital organs (skull protects brain, rib cage protects heart and lungs), and provides levers for muscles to work against for movement.

Auxin (IAA) is redistributed to the shaded side of a shoot when light comes from one direction. Higher auxin concentration on the shaded side causes faster cell elongation, bending the shoot toward the light source.

Tulip flower opening and closing in response to temperature is thermonasty. Flowers open when temperature rises (warmer day) and close when temperature falls (cooler night) through differential growth of petal surfaces.

Nastic movements are non-directional plant movements where the direction is determined by the plant's anatomy, not by the direction of the stimulus. Examples include photonasty (light-triggered), thermonasty (temperature-triggered), and seismonasty (touch-triggered).

Roots show positive geotropism — they grow in the direction of gravity (downward). Shoots show negative geotropism (grow upward, against gravity). Gravity is sensed by statocytes containing starch-filled statoliths.

Chemotropism is directional growth in response to a chemical stimulus. The pollen tube grows chemotropically toward the ovule guided by chemicals from the style. Root growth toward mineral-rich soil is also chemotropic.

Chemotaxis is movement in response to a chemical gradient. Fern and moss sperm swim toward archegonia guided by chemicals (malic acid, sucrose) secreted by the egg. This ensures successful fertilization.

Hyponasty is upward bending of a plant part caused by more rapid growth on the lower surface than the upper surface. It keeps flower buds closed by curling petals upward and inward.

Phototactic movement of chloroplasts refers to their repositioning in response to light intensity. In strong light they move edge-on (parallel to light) to reduce damage; in weak light they face the light to maximise absorption.

Phototropism is the directional growth of plant organs toward (positive) or away from (negative) light. Stems are positively phototropic; roots are negatively phototropic. Auxin redistribution mediates phototropism.

Epinasty is downward bending of a plant part (e.g., leaf or petal) caused by more rapid growth on the upper surface than the lower surface. It causes leaves to curl downward and is involved in bud opening.

Callus is undifferentiated parenchymatous tissue that forms rapidly at wound sites on stems and roots. It seals the wound and from it differentiated tissues may regenerate. Callus formation is exploited in plant tissue culture.

Turgor movements result from rapid changes in turgor pressure (water content) in specific cells without cell growth. Examples include stomatal opening/closing, Mimosa leaf folding, and the Venus flytrap snap.

Autonomic (spontaneous) movements in plants are those due to internal stimuli, not external. Differential growth movements (like nutation — the rotating/nodding movement of growing tips) are examples of autonomic movements.

Sapwood (alburnum) is the outer, lighter-coloured, living secondary xylem that actively conducts water. Heartwood (duramen) is the inner, darker, dead non-conducting wood impregnated with resins and tannins for extra strength.

The vascular cambium (a lateral meristem) produces secondary xylem (wood) toward the inside and secondary phloem toward the outside by cell division. This increases the girth of woody stems and roots (secondary growth).

Sclereids (stone cells) are short, irregularly shaped sclerenchyma cells with very thick, heavily lignified walls. They form the hard seed coats, nut shells (like in coconuts), gritty texture of pears, and stone of drupes.

Both collenchyma (with unevenly thickened primary cell walls, living at maturity, flexible support) and sclerenchyma (with uniformly thick, lignified secondary walls, dead at maturity, rigid support) are plant support tissues.

Vessels (vessel elements) are dead, hollow xylem cells with perforated end walls that join end-to-end forming long continuous tubes for water conduction. Their lignified walls also provide structural support.

Sclerenchyma fibres are elongated cells with thick, lignified walls that provide mechanical strength to plants. They occur as bundle caps around vascular bundles, in xylem, and as bast fibres in phloem (e.g., jute, flax, hemp).

Wind is the primary mechanical stress on terrestrial plants — it imposes bending, swaying forces that require strong stems. Plants respond to wind by producing more sclerenchyma (strengthening tissue) and thickening stems.

Skeletal muscles are striated voluntary muscles attached to bones by tendons. They work in antagonistic pairs (one contracts while the other relaxes) to move bones at joints, controlling all voluntary movements.

Starfish move using tube feet (podia) — hydraulic suckers operated by the water vascular system. Each tube foot can extend and attach by suction or retract, enabling slow but coordinated movement across surfaces.

Arthropods are defined by their chitinous exoskeleton and jointed appendages (legs, antennae, mouthparts). Arthropoda is the largest animal phylum including insects, arachnids, crustaceans, and myriapods.

Frogs (and all vertebrates) possess an endoskeleton — an internal skeleton of bones and cartilage. Snails and oysters have exoskeletons (shells). Paramecium has a pellicle but no true skeleton.

Sarcolemma is the plasma membrane of a muscle cell (fibre). It conducts the action potential along the muscle fibre and into the T-tubules, triggering calcium release from the sarcoplasmic reticulum and muscle contraction.

Insects have three pairs of legs (6 legs total) — one of the defining characteristics of class Insecta. The legs are attached to the three thoracic segments. Spiders have four pairs (8 legs) and crustaceans have five or more pairs.

Cardiac muscle is involuntary — it contracts automatically without conscious control. The sinoatrial node generates rhythmic electrical impulses that spread through the heart causing coordinated contractions.

Amoeba moves by forming pseudopodia — temporary cytoplasmic extensions. The cytoplasm flows into the pseudopodium (amoeboid movement) driven by cytoskeletal dynamics. This is used for both locomotion and food capture.

Euglena moves using one long flagellum (with a second shorter one) that beats to propel it through water. It shows positive phototaxis, moving toward light for photosynthesis.

Skeletal (striped/striated) muscles are attached to and move the bones of the limbs and body. Heart muscle is cardiac (striated but involuntary). Lungs and intestines contain smooth (unstriated) muscle.

Calcified (hyaline) cartilage is found in the supra-scapula region of the pectoral girdle in frogs and some other vertebrates. Calcification makes cartilage harder and more bone-like.

Vascular tissue (blood) consists of formed elements (corpuscles — RBCs, WBCs, platelets) suspended in a liquid matrix (plasma). This combination allows transport of gases, nutrients, hormones, and waste products.

The A band in striated muscle is crossed by Hensen's disc (H zone/M line) in its centre. The H zone is a lighter central area within the A band where only thick (myosin) filaments are present with no actin overlap.

The entire skeletal muscle is covered by a tough connective tissue sheath called the epimysium. Within the muscle, bundles of fibres (fascicles) are surrounded by perimysium, and individual fibres by endomysium.

Blood and lymph are liquid connective tissues — they have a fluid matrix (plasma/lymph fluid) containing formed elements (cells). Both transport substances throughout the body and contain cells (blood cells/lymphocytes).

Tendons connect muscles to bones, transmitting the contractile force of muscles to move the skeleton. The Achilles tendon (calcaneal tendon) connecting the calf muscles to the heel bone is the largest tendon in the body.

Ligaments are strong bands of fibrous connective tissue that connect bone to bone at joints. They stabilise joints, limit excessive movement, and prevent dislocation. They contain both collagen and elastin fibres.

Lobe-finned fishes (Sarcopterygii, like Tiktaalik) are considered the ancestors of the first tetrapods — early amphibians that colonised land. Their fleshy, lobed fins were precursors to tetrapod limbs.

Planaria moves by crawling — gliding along surfaces using cilia on its ventral surface combined with muscle contractions. The cilia beat in mucus secreted by gland cells to produce smooth gliding movement.

Ligamentum flava (yellow ligaments) in the vertebral column are made almost entirely of elastin fibres, giving them their yellow colour and exceptional elasticity. They connect adjacent vertebral laminae and assist in spine extension.

The evolution of vertebrate endoskeleton is divided into six major periods based on the geological/evolutionary record, from early jawless fish to modern mammals, showing progressive sophistication of skeletal structures.

Animals show eight main types of locomotion: walking, running, jumping, crawling, swimming, flying, climbing, and burrowing. Some animals can use multiple types depending on the situation.

Tendons are tough, fibrous connective tissue cords that attach muscles to bones. Ligaments connect bone to bone at joints. Tendons transmit the force generated by muscle contraction to move the bone.

Starfish (sea stars) move using tube feet — small hydraulic appendages operated by the water vascular system. Tube feet can extend, attach by suction to surfaces, and retract, pulling the animal along.

Different multicellular organisms have evolved different mechanisms for movement depending on their body plan — cilia and flagella in simple organisms, tube feet in echinoderms, fins in fish, limbs in tetrapods.

Myosin is both a structural protein (forming thick filaments in the sarcomere) and an enzyme (its head domains have ATPase activity, hydrolyzing ATP to ADP + Pi to power the power stroke during muscle contraction).

Actin (thin) filaments are anchored at the Z discs (Z lines) at each end of the sarcomere and extend toward the centre. The I band consists only of actin; the A band contains overlapping actin and myosin filaments.

The mammalian endoskeleton is divided into three parts: axial skeleton (skull, vertebral column, ribs, sternum), appendicular skeleton (limbs and girdles), and visceral skeleton (bones in soft organs, e.g., hyoid bone).

Muscle fatigue during intense anaerobic exercise is caused by accumulation of lactic acid (lactate) — produced when pyruvate is reduced to regenerate NAD⁺. The drop in pH inhibits enzyme function and muscle contraction.

Smooth muscles are also called unstriated (no visible cross-striations), involuntary (not under conscious control), or visceral muscles. They are found in the walls of blood vessels, intestines, uterus, and bladder.

Vertebrate muscles are of three types: skeletal (striated, voluntary — attached to bones), smooth (unstriated, involuntary — in visceral organs), and cardiac (striated, involuntary — heart wall only).

A joint (articulation) is the point where two or more bones meet. Joints can be immovable (sutures in skull), slightly movable (between vertebrae), or freely movable/synovial (hip, knee, shoulder).

The cyton (cell body/soma) of a neuron contains all the usual organelles including a large nucleus, mitochondria (for energy), Nissl granules (ribosomes for protein synthesis), and neurofibrillae (cytoskeletal support).

The skeleton is the outer or inner framework of the body that provides structural support, protects vital organs, enables movement (as lever system for muscles), and in vertebrates produces blood cells in bone marrow.

Special sensory receptors include those for all five special senses: vision (eye), hearing and balance (ear), taste (taste buds on tongue), smell (olfactory receptors in nasal cavity), and touch/pressure in skin.

The central nervous system (CNS) consists of the brain and spinal cord. The peripheral nervous system (PNS) includes all nerves outside the CNS — cranial nerves, spinal nerves, ganglia, and sensory receptors.

Nervous tissue is composed of neurons (nerve cells that transmit electrical signals) and neuroglia (supporting/glial cells that insulate, support, and nourish neurons). Together they form the brain, spinal cord, and nerves.

Striated (striped) muscle is called voluntary muscle because it is under conscious control. It is also called skeletal muscle because it attaches to bones. All three names — striated, voluntary, and skeletal — refer to the same tissue.

Muscle tissue is specialised contractile tissue — its cells contain actin and myosin protein filaments that slide past each other to shorten the cell. This unique property allows generation of force for movement.

Animals possess three types of skeleton: hydrostatic skeleton (fluid pressure, e.g., earthworms, jellyfish), exoskeleton (external hard covering, e.g., arthropods, molluscs), and endoskeleton (internal hard framework, e.g., vertebrates).

Animals can have radial symmetry (cnidarians, echinoderms — body parts arranged around a central axis) or bilateral symmetry (most other animals — left and right halves are mirror images). Some are asymmetric (sponges).

Geotropism (gravitropism) is directional growth in response to gravity. Roots show positive geotropism (grow downward) and shoots show negative geotropism (grow upward). This is regulated by auxin redistribution.

Chemotropism is directional growth in response to a chemical gradient. The most classic example is pollen tube growth toward the ovule, guided by chemicals secreted by the style and ovule.

Fish use their fins for locomotion — the caudal (tail) fin provides forward thrust, dorsal and anal fins provide stability, and paired pectoral and pelvic fins provide steering and braking. Body undulation also drives movement.

Locomotion is the movement of an animal's entire body from one place to another. It requires a skeletal system for support and muscles for generating movement. Different animals use different modes of locomotion.

Thermonasty is a nastic movement in response to temperature changes. Tulip flowers open when temperature rises and close when it falls — this is controlled by differential growth of the inner and outer petal surfaces.

Photonasty is a nastic movement (non-directional) in response to light intensity changes. Examples include the opening of flowers in light (morning) and their closing in darkness (evening), such as in dandelions and chicory.

Nastic movements are non-directional responses to external stimuli — the direction of movement is determined by the plant's own structure, not the direction of the stimulus. They include photonasty, thermonasty, and seismonasty.

Phototropism is the directional growth movement of a plant part in response to light. Stems show positive phototropism (grow toward light) because auxin redistributes to the shaded side causing faster growth there.

Thermotaxis is the directed movement of a motile organism in response to a temperature gradient. Thermotropism is the growth response to temperature in non-motile plant parts. Thermonasty is non-directional response to temperature.

Paratonic movements are classified into three types: tactic movements (movement of whole organism in response to stimulus), tropic movements (directional growth toward/away from stimulus), and nastic movements (non-directional response).

Plant movements are generally too slow to be observed directly with the naked eye in real time. However, their effects (e.g., a sunflower tracking the sun across a day, or roots growing downward) are clearly visible over time.

Plant movements are classified into two main types based on mechanism: autonomic movements (due to internal stimuli) and paratonic movements (due to external stimuli). Paratonic movements include tactic, tropic, and nastic movements.

Phototaxis is the movement of a free-moving organism toward (positive phototaxis) or away from (negative phototaxis) a source of light. Euglena shows positive phototaxis, moving toward light for photosynthesis.

Paratonic movements are plant movements induced by external stimuli such as light, gravity, temperature, touch, or chemicals. They include tactic, tropic, and nastic movements — all triggered by environmental factors.