Locomotion and Movement

2.1 Core concepts

• Movement is one of the significant features of living beings; streaming of protoplasm in unicellular organisms like Amoeba is its simplest form, while voluntary movements that change place or location are called locomotion — walking, running, climbing, flying and swimming (NCERT §17, p. 217).
• Locomotory structures need not be different from those affecting other movements: Paramoecium cilia move food through the cytopharynx and also drive locomotion; Hydra uses tentacles for both prey capture and locomotion; hence all locomotions are movements but all movements are not locomotions (NCERT §17, p. 217).
• Cells of the human body show three types of movement: amoeboid (macrophages, leucocytes — via pseudopodia driven by protoplasmic streaming and microfilaments of the cytoskeleton), ciliary (in ciliated epithelium of internal tubular organs — clears dust in trachea, moves ova in the female reproductive tract), and muscular (limbs, jaws, tongue — requires coordinated action of muscular, skeletal and neural systems) (NCERT §17.1, p. 217–218).
• Muscle is a specialised tissue of mesodermal origin contributing 40–50% of adult body weight and shows the properties excitability, contractility, extensibility and elasticity; on location, muscles are classified as Skeletal, Visceral and Cardiac (NCERT §17.2, p. 218).
• Skeletal muscles are striated and voluntary (locomotion, posture). Visceral / smooth muscles are non-striated and involuntary (alimentary canal, reproductive tract — move food and gametes). Cardiac muscles are striated, branched and involuntary (NCERT §17.2, p. 218–219).
• A skeletal muscle is built of fascicles (muscle bundles) wrapped by a collagenous fascia; each fascicle contains many muscle fibres, each fibre lined by sarcolemma enclosing sarcoplasm, which is a syncytium (many nuclei); the sarcoplasmic reticulum stores Ca++ (NCERT §17.2, p. 219, Fig. 17.1).
• Each muscle fibre contains parallel myofibrils showing alternate dark (A, anisotropic — contains myosin) and light (I, isotropic — contains actin) bands; the Z line bisects the I band and anchors thin filaments; the M line holds thick filaments at the centre of the A band; the unit between two successive Z lines is the sarcomere, the functional unit of contraction; the central part of thick filaments not overlapped by thin filaments is the H zone (NCERT §17.2, p. 219–220, Fig. 17.2).
• Thin filament = two helically wound F-actins (each F-actin is a polymer of G-actin monomers) + two tropomyosin strands + troponin at regular intervals; in the resting state a subunit of troponin masks the myosin-binding sites on actin (NCERT §17.2.1, p. 221, Fig. 17.3a).
• Thick filament = polymerised myosin made of monomers called meromyosins, each with a globular head + short arm (heavy meromyosin, HMM — the "cross arm") and a tail (light meromyosin, LMM); the head bears an ATPase with binding sites for ATP and active sites for actin (NCERT §17.2.1, p. 221, Fig. 17.3b).
• Sliding-filament theory: contraction = thin filaments slide over thick filaments. A signal from the CNS travels along a motor neuron to the neuromuscular junction / motor-end plate; acetylcholine is released, an action potential spreads along the sarcolemma, Ca++ is released from the sarcoplasmic reticulum, Ca++ binds troponin, unmasks the active sites on actin; the myosin head (using ATP hydrolysis) binds actin, forms a cross-bridge, pulls the thin filaments toward the centre of the A band — the I band shortens, the A band length is unchanged, the Z lines move inward and the sarcomere shortens; a new ATP breaks the cross-bridge and the cycle repeats until Ca++ is pumped back to the sarcoplasmic cisternae (NCERT §17.2.2, p. 222–223, Fig. 17.4, 17.5).
• A motor unit = one motor neuron + all the muscle fibres it innervates. Repeated stimulation causes fatigue from accumulation of lactic acid due to anaerobic breakdown of glycogen (NCERT §17.2.2, p. 222–223).
• Red fibres = high myoglobin, plenty of mitochondria, aerobic; White fibres = low myoglobin, few mitochondria, abundant sarcoplasmic reticulum, anaerobic (NCERT §17.2.2, p. 223).
• The human skeleton has 206 bones — 80 axial + 126 appendicular. The axial skeleton comprises skull, vertebral column, sternum and ribs. The skull has 22 bones (8 cranial + 14 facial), plus a U-shaped hyoid; each middle ear has three ear ossicles — Malleus, Incus, Stapes. The skull articulates with the vertebral column via two occipital condyles (dicondylic skull) (NCERT §17.3, p. 224).
• Vertebral column = 26 vertebrae arranged as cervical (7), thoracic (12), lumbar (5), sacral (1 fused) and coccygeal (1 fused); the atlas (1st vertebra) articulates with the occipital condyles; the column protects the spinal cord, supports the head and gives attachment for ribs and back muscles (NCERT §17.3, p. 225, Fig. 17.7).
• Sternum is a flat bone on the ventral midline of the thorax. 12 pairs of ribs, each bicephalic (two articulation surfaces dorsally) — pairs 1–7 are true ribs (joined directly to sternum by hyaline cartilage), pairs 8–10 are vertebrochondral / false ribs (join the 7th rib via cartilage), and pairs 11–12 are floating ribs (not connected ventrally); together with thoracic vertebrae they form the rib cage (NCERT §17.3, p. 225, Fig. 17.8).
• Appendicular skeleton = limb bones + girdles, with each limb of 30 bones. Forelimb: humerus, radius, ulna, 8 carpals, 5 metacarpals, 14 phalanges. Hindlimb: femur (longest bone), tibia, fibula, 7 tarsals, 5 metatarsals, 14 phalanges; patella is the kneecap. Pectoral girdle halves = clavicle + scapula (glenoid cavity of scapula receives humerus to form the shoulder joint; acromion of the scapular spine articulates with the clavicle). Pelvic girdle = two coxal bones, each formed by fusion of ilium, ischium and pubis, meeting ventrally at the pubic symphysis (fibrous cartilage); the acetabulum receives the head of the femur (NCERT §17.3, p. 226, Fig. 17.9, 17.10).
• Joints are points of contact between bones, or between bones and cartilages, and act as the fulcrum for muscle-generated force. Three structural classes: Fibrous (no movement — sutures of cranium), Cartilaginous (limited movement — between adjacent vertebrae), and Synovial (fluid-filled cavity, considerable movement — ball-and-socket between humerus and pectoral girdle, hinge at knee, pivot between atlas and axis, gliding between carpals, saddle between carpal and metacarpal of thumb) (NCERT §17.4, p. 226–227).
• Disorders: Myasthenia gravis (autoimmune, attacks neuromuscular junction → fatigue, weakness, paralysis of skeletal muscle); Muscular dystrophy (progressive degeneration of skeletal muscle, mostly genetic); Tetany (rapid spasms / wild contractions due to low Ca++ in body fluid); Arthritis (inflammation of joints); Osteoporosis (age-related, decreased bone mass and higher fracture risk, often from low estrogen); Gout (joint inflammation from uric-acid crystal accumulation) (NCERT §17.5, p. 227).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 17.1, p. 219 — Cross-section of a muscle showing fascicle, muscle fibre, sarcolemma and blood capillary.
• Figure 17.2, p. 220 — Anatomy of a muscle fibre and a single sarcomere; label Z line, A band, I band, H zone.
• Figure 17.3, p. 221 — (a) Thin filament with F-actin, tropomyosin and troponin; (b) myosin monomer (meromyosin) with head, cross arm, actin-binding sites and ATP-binding sites.
• Figure 17.4, p. 222 — Cross-bridge cycle: ATP-loaded myosin → cross-bridge formation → sliding/rotation (release of P + ADP) → ATP rebinding and cross-bridge breaking.
• Figure 17.5, p. 223 — Sliding-filament view of two sarcomeres in relaxed, contracting and maximally contracted states; track I band and H zone shrinking while A band stays constant.
• Figure 17.6, p. 224 — Human skull labels: frontal, parietal, temporal, occipital, sphenoid, ethmoid, lacrimal, nasal, zygomatic, maxilla, mandible, hyoid, occipital condyle.
• Figure 17.7, p. 225 — Vertebral column (right lateral view) with cervical, thoracic and lumbar regions, intervertebral disc, sacrum and coccyx.
• Figure 17.8, p. 225 — Rib cage with true (1–7), false (8–10) and floating (11–12) ribs.
• Figure 17.9, p. 226 — Right pectoral girdle and upper arm: clavicle, scapula, humerus, radius, ulna, carpals, metacarpals, phalanges.
• Figure 17.10, p. 226 — Right pelvic girdle and lower limb: ilium, ischium, pubis, coxal bone, femur, patella, tibia, fibula, tarsals, metatarsals, phalanges.

2.4 Common confusions / NTA trap points

• During contraction the I band shortens and the H zone shrinks, but the A band length is unchanged — NTA loves to flip this and put "A band shortens" as a distractor (p. 222–223).
• It is troponin (not tropomyosin) whose subunit masks the active sites; tropomyosin is the long strand running along F-actin. Ca++ binds troponin, not actin directly (p. 221–222).
• The skull has 22 bones total (8 cranial + 14 facial) — the hyoid and the three ear ossicles per ear are additional and are not counted within the 22 (p. 224).
• The vertebral column has 26 units (7+12+5+1+1), not 33, because the sacrum and coccyx are each counted as one fused bone (p. 225).
• The pivot joint is between atlas and axis; the ball-and-socket joint at the shoulder is between humerus and pectoral girdle (glenoid cavity), while the hip ball-and-socket is between femur and acetabulum — students mix these up (p. 226–227).
• Tetany (low Ca++ spasms) ≠ tetanus (the bacterial disease) — NCERT only names tetany; do not confuse them (p. 227).
• Osteoporosis is linked to decreased estrogen, not decreased calcium intake per se, in NCERT's wording (p. 227).
• Sliding filament theory — actin slides over myosin; the filaments themselves do not shorten or change length, only their overlap increases (p. 222).
• Cervical vertebrae count — All mammals (including the giraffe) have 7 cervical vertebrae; NTA sometimes asks this comparative trap.
• Myasthenia gravis is autoimmune — the neuromuscular junction is attacked; muscle itself is not the primary defect (p. 227).

2.5 Quick comparison table — locomotion & movement at a glance