Locomotion and Movement
How the human body produces movement and locomotion through the coordinated action of muscles, the 206-bone skeleton and joints, centred on the sliding-filament mechanism of muscle contraction.
This is high-frequency Prelims General Science: UPSC sets direct factual MCQs on bone counts, muscle classification, the sliding-filament sequence, ear ossicles, joint types and skeletal-muscular disorders. For Mains it sits in GS-III (science and technology / health literacy) and supports decoding news on osteoporosis, arthritis, gout and muscular dystrophy. Strong static recall here is a quick, reliable scoring zone.
Understand the chapter
Movement vs Locomotion; Three Types of Cellular Movement
Movement is a defining feature of life, but only movements that change an organism's place or location qualify as locomotion - hence the rule 'all locomotions are movements, but all movements are not locomotions.' The same structure may serve both (Paramoecium cilia move food and the organism; Hydra tentacles both capture prey and aid locomotion). Locomotion is driven by needs - food, shelter, mate, breeding grounds, favourable climate, or escape from predators. Human cells show three movement types: amoeboid, ciliary and muscular.
- Amoeboid: macrophages and leucocytes move via pseudopodia formed by protoplasmic streaming, using cytoskeletal microfilaments.
- Ciliary: ciliated epithelium of internal tubes - clears dust in the trachea and moves ova in the female tract.
- Muscular: contraction moves limbs, jaws, tongue; locomotion needs coordinated muscular + skeletal + neural systems.
Muscle: Properties and Three Types by Location
Muscle is a contractile tissue of mesodermal origin forming 40-50% of adult body weight, with four properties: excitability, contractility, extensibility and elasticity. By location it is skeletal, visceral or cardiac, which also differ in striation and nervous control. The decisive exam point is that striation does NOT equal voluntary control.
- Skeletal: striated + voluntary; attached to the skeleton; drives locomotion and posture.
- Visceral (smooth): non-striated + involuntary; in alimentary canal and reproductive tract; moves food and gametes.
- Cardiac: striated BUT involuntary; branched cells forming the heart muscle.
Ultrastructure of Skeletal Muscle: The Sarcomere
A skeletal muscle is bundles (fascicles) of muscle fibres wrapped by collagenous fascia; each fibre is a syncytium (multinucleate) bounded by the sarcolemma, with sarcoplasmic reticulum storing calcium. Myofibrils show alternating dark A-bands (thick myosin) and light I-bands (thin actin). The sarcomere - the segment between two successive Z-lines - is the functional unit of contraction.
- I-band (Isotropic, light) = actin/thin, bisected by the Z-line; A-band (Anisotropic, dark) = myosin/thick, held by the M-line.
- H-zone = central thick-filament region not overlapped by thin filaments.
- Actin: two F-actins (polymers of G-actin) + tropomyosin + troponin, which masks myosin-binding sites at rest.
- Myosin: meromyosins with a globular head (HMM - ATPase + actin-binding) forming the cross-arm, and a tail (LMM).
Sliding Filament Theory and Muscle Fibre Types
Contraction occurs when thin filaments slide over thick filaments - the filaments themselves do not shorten. A CNS signal travels down a motor neuron to the neuromuscular junction, releasing acetylcholine; the resulting action potential triggers Ca++ release, which binds troponin and unmasks actin's binding sites. Myosin heads use ATP to form cross-bridges, pull actin toward the A-band centre and shorten the sarcomere, then detach when fresh ATP binds; pumping Ca++ back drives relaxation.
- During contraction the I-band and H-zone shorten while the A-band length stays constant.
- Fatigue = lactic acid build-up from anaerobic breakdown of glycogen.
- Red fibres: high myoglobin + many mitochondria = aerobic; White fibres: low myoglobin, more sarcoplasmic reticulum = anaerobic.
Skeletal System: Axial Division
The human skeleton has 206 bones split into axial (80) and appendicular (126); bone has a hard calcium-salt matrix while cartilage has a pliable chondroitin-salt matrix. The axial skeleton - skull, vertebral column, sternum and ribs - runs along the body's main axis. The skull (22 bones) is dicondylic and the vertebral column has 26 vertebrae.
- Skull = 8 cranial + 14 facial = 22; plus a U-shaped hyoid and 3 ear ossicles per ear (malleus, incus, stapes).
- Vertebrae 26: cervical 7, thoracic 12, lumbar 5, sacral 1 (fused), coccygeal 1 (fused); atlas is the first vertebra.
- 12 pairs of ribs, each bicephalic: 7 true, 3 vertebrochondral (false, 8th-10th), 2 floating (11th-12th).
- Rib cage = thoracic vertebrae + ribs + sternum.
Skeletal System: Appendicular Division and Girdles
The appendicular skeleton is the limbs plus their girdles, with 30 bones per limb. The pectoral girdle (clavicle + scapula) links the arm to the axis at the glenoid cavity, while the pelvic girdle (two coxal bones) receives the femur at the acetabulum. The femur is the longest bone of the body.
- Forelimb: humerus, radius, ulna, carpals (8), metacarpals (5), phalanges (14).
- Hindlimb: femur, tibia, fibula, tarsals (7), metatarsals (5), phalanges (14); patella = knee cap.
- Coxal bone = ilium + ischium + pubis fused; the two halves meet at the pubic symphysis.
- Clavicle = collar bone; glenoid cavity (scapula) + humerus head = shoulder joint.
Joints and Common Disorders
Joints are points of contact between bones (or bone and cartilage) and act as the fulcrum for muscle-generated movement. They are classified as fibrous (immovable, e.g., skull sutures), cartilaginous (slightly movable, e.g., between adjacent vertebrae) and synovial (freely movable, with a fluid-filled synovial cavity). The chapter also lists muscular and skeletal disorders that are frequently asked in Prelims.
- Myasthenia gravis: autoimmune block at the neuromuscular junction causing fatigue and paralysis.
- Tetany: rapid muscle spasms from low Ca++; Muscular dystrophy: genetic progressive muscle degeneration.
- Arthritis: joint inflammation; Gout: inflammation from uric-acid crystals; Osteoporosis: age/estrogen-linked bone loss.
Key terms
- Locomotion
- Voluntary movement that causes a change of place or location (walking, swimming, flying).
- Sarcomere
- Segment between two successive Z-lines; the functional contractile unit of a myofibril.
- Sarcolemma
- Plasma membrane that encloses the sarcoplasm of a muscle fibre.
- Sarcoplasmic reticulum
- The muscle fibre's endoplasmic reticulum; store-house that releases calcium ions for contraction.
- Sliding filament theory
- Contraction model in which thin actin filaments slide over thick myosin filaments.
- Motor unit
- A motor neuron together with all the muscle fibres it innervates.
- Neuromuscular junction
- Motor-end plate where a motor neuron meets the sarcolemma; signals via acetylcholine.
- Myoglobin
- Red oxygen-storing pigment; abundant in aerobic red muscle fibres.
- Acetabulum
- Cavity of the coxal bone where the femur (thigh bone) articulates.
- Glenoid cavity
- Scapular depression that receives the head of the humerus to form the shoulder joint.
Must-know facts exam-ready
- Human skeleton = 206 bones: 80 axial + 126 appendicular.
- Muscle is of mesodermal origin and forms about 40-50% of adult body weight.
- Four muscle properties: excitability, contractility, extensibility, elasticity.
- Cardiac muscle is the only striated-yet-involuntary muscle.
- Sarcomere (Z-line to Z-line) is the functional unit of contraction.
- I-band = thin actin (light), A-band = thick myosin (dark); only the I-band and H-zone shorten during contraction.
- Acetylcholine is the neurotransmitter released at the neuromuscular junction.
- The sarcoplasmic reticulum is the store-house of Ca++ ions.
- Skull = 22 bones (8 cranial + 14 facial); ear ossicles = malleus, incus, stapes.
- Vertebral column = 26 vertebrae (7 cervical, 12 thoracic, 5 lumbar, 1 sacral, 1 coccygeal); atlas is the first vertebra.
- 12 pairs of ribs: 7 true, 3 false (vertebrochondral), 2 floating; each rib is bicephalic.
- Femur is the longest bone; each limb has 30 bones; the coxal bone = ilium + ischium + pubis.
Memory tricks remember it for good
Traps to avoid
- Cardiac muscle is striated but involuntary - striation does NOT imply voluntary control.
- During contraction the A-band length is unchanged; only the I-band and H-zone shorten (filaments slide, they do not contract).
- Do not swap proteins: actin = thin filament = light I-band; myosin = thick filament = dark A-band.
- Troponin (not tropomyosin) masks the myosin-binding sites on actin at rest.
- False ribs (vertebrochondral, 8th-10th) are different from floating ribs (11th-12th).
- The sarcoplasmic reticulum - not mitochondria - is the calcium store of the muscle fibre.
Exam focus
🧠 Prelims angles
- Match muscle type with striation and control: skeletal (striated/voluntary), visceral (smooth/involuntary), cardiac (striated/involuntary).
- Numerical recall: 206 bones, 80 axial, 22 skull, 26 vertebrae, 12 rib pairs, 30 bones per limb.
- Sequence the sliding-filament steps and the roles of acetylcholine, Ca++, troponin and ATP.
- Identify named bones: femur (longest), clavicle (collar bone), patella (knee cap), ear ossicles.
- Match joints: sutures (fibrous), intervertebral (cartilaginous), shoulder/hip (synovial).
- Red vs white fibres (myoglobin, mitochondria, aerobic/anaerobic) and disorder-matching (myasthenia gravis, gout, osteoporosis, tetany).
✍️ Mains angles GS-III
- Explain the sliding-filament mechanism of skeletal-muscle contraction.Trace motor signal -> ACh at NMJ -> Ca++ release -> troponin unmasking -> cross-bridge cycling -> sarcomere shortening -> relaxation.
- Bone and muscle disorders as emerging public-health concerns in an ageing India.Link osteoporosis (estrogen/age), arthritis and gout to demography, diet and the NCD burden; suggest screening and nutrition policy (GS-III/GS-II).
- How does skeletal-muscle fibre type (red vs white) shape athletic performance?Contrast aerobic, myoglobin-rich red fibres with anaerobic white fibres in endurance vs sprint events.
Last-minute revision tick as you recall
- All locomotion is movement; not all movement is locomotion.
- 3 movements: amoeboid (WBCs/macrophages), ciliary (trachea/oviduct), muscular.
- Muscle: mesoderm, 40-50% body weight; ECEE properties.
- Cardiac = striated + involuntary.
- Sarcomere = Z to Z; in contraction I-band and H-zone shrink, A-band constant.
- ACh at NMJ -> Ca++ -> troponin unmasks actin -> ATP cross-bridge -> sliding.
- 206 bones = 80 axial + 126 appendicular.
- Vertebrae 26: 7-12-5-1-1; ribs 12 pairs = 7 true + 3 false + 2 floating.
- Joints: fibrous (none), cartilaginous (limited), synovial (free).
Distilled from NCERT Class 11 · Biology (Class 11) for UPSC. Always cross-check facts with the original NCERT.