# Locomotion and Movement (NEET Biology Class 11): Sliding Filament Theory, 206 Bones & Joints Source: https://tracknation.in/story/locomotion-and-movement-neet-biology-class-11-sliding-filament-theory-206-bones-joints (TrackNation — free UPSC notes with PYQs) > NEET Class 11 Locomotion and Movement, mined from NCERT: the three types of cellular movement and the three muscle types in comparison tables, skeletal muscle structure (fascia, sarcolemma, bands, Z/M lines, sarcomere, H zone), the contractile proteins (actin, tropomyosin, troponin, meromyosin), the sliding filament theory step by step, a red-vs-white fibre table, the complete 206-bone skeletal system with every count (skull, vertebrae, ribs, carpals, tarsals) in tables, the pectoral and pelvic ## Types of Movement & the Three Muscle Types 📌 **NEET priority: Important.** *1–2 questions a year, but they are pure exact-recall — the **206-bone breakdown**, the **sliding filament theory** and the **five synovial joint types with their examples**. All three are tabulated below.* **All locomotions are movements, but all movements are not locomotions.** Animals move in search of food, shelter, mate, breeding grounds, favourable climate, or to escape predators. ### Three types of movement in human cells | **Type** | **How it works** | **Where it occurs** | | Amoeboid | Pseudopodia formed by streaming of protoplasm; cytoskeletal MICROFILAMENTS involved | Macrophages and leucocytes | | Ciliary | Coordinated beating of cilia lining internal tubular organs | Trachea (removes dust/foreign particles); passage of ova through the female reproductive tract | | Muscular | Contractile property of muscle | Limbs, jaws, tongue — needs coordinated muscular, skeletal and neural activity | ### The three muscle types | **Muscle** | **Appearance** | **Control** | **Location / role** | | Skeletal | STRIATED (striped) | VOLUNTARY | Attached to skeletal elements; locomotion and body posture | | Visceral (smooth) | NON-striated, smooth | INVOLUNTARY | Inner walls of hollow visceral organs — moves food through the digestive tract and gametes through the genital tract | | Cardiac | STRIATED | INVOLUNTARY | Heart only; cells assemble in a BRANCHING pattern | Muscle is a specialised tissue of **mesodermal origin**, contributing **40–50% of an adult human's body weight**. Its properties are **excitability, contractility, extensibility and elasticity**. ## Skeletal Muscle Structure & the Sliding Filament Theory ### Skeletal muscle structure — from muscle to sarcomere - Muscle → **fascicles (muscle bundles)** held by connective tissue called **fascia** → **muscle fibres**. Each fibre is lined by the **sarcolemma** enclosing **sarcoplasm**, and is a **syncytium** (many nuclei). The **sarcoplasmic reticulum** is the **store house of calcium ions**. - The sarcoplasm holds parallel **myofibrils** with alternating dark and light bands from two proteins: light **I-band (Isotropic) contains ACTIN**; dark **A-band (Anisotropic) contains MYOSIN**. Actin is **thin**, myosin **thick**. - The **Z line** bisects each I-band and anchors the thin filaments. The **M line** holds thick filaments in the middle of the A-band. The portion between two successive **Z lines is the SARCOMERE** — the functional unit of contraction. The central part of the thick filament NOT overlapped by thin filaments is the **H zone**. ### The contractile proteins - **Actin (thin) filament:** two **F (filamentous) actins** helically wound; each F-actin is a polymer of **G (globular) actin** monomers. Two **tropomyosin** filaments run alongside, and **troponin** sits at regular intervals on the tropomyosin. **At rest, a troponin subunit MASKS the myosin-binding sites on actin.** - **Myosin (thick) filament:** a polymer of **meromyosin** monomers. Each has a globular head + short arm — the **heavy meromyosin (HMM)**, which projects out as the **cross arm** — and a tail, the **light meromyosin (LMM)**. **The globular head is an active ATPase** with binding sites for both ATP and actin. ### Sliding filament theory — the mechanism, step by step - Contraction occurs by **thin filaments SLIDING over thick filaments**. A signal from the CNS travels via a **motor neuron**; a motor neuron plus the fibres it serves is a **motor unit**. The junction with the sarcolemma is the **neuromuscular junction (motor-end plate)**. - The signal releases the neurotransmitter **acetylcholine** → action potential in the sarcolemma → **release of Ca²⁺** into the sarcoplasm → Ca²⁺ binds troponin → **unmasks the active sites** on actin → using energy from **ATP hydrolysis**, the myosin head binds actin forming a **cross bridge** → pulls the actin towards the centre of the A-band → **Z lines pulled inwards → sarcomere shortens**. - **Crucially: during contraction the I-bands SHORTEN, but the A-bands RETAIN their length.** Myosin then releases ADP and Pi, a new ATP binds, and the cross bridge **breaks**. Relaxation occurs when Ca²⁺ is **pumped back into the sarcoplasmic cisternae**, re-masking the actin sites. - Repeated activation causes **lactic acid accumulation from anaerobic glycogen breakdown → fatigue**. ### Red vs white muscle fibres | **Feature** | **Red fibres** | **White fibres** | | Myoglobin | HIGH — gives the reddish colour | Very LOW — appear pale/whitish | | Mitochondria | Plenty | Few | | Sarcoplasmic reticulum | Less | High amount | | Energy source | AEROBIC — uses stored oxygen for ATP | ANAEROBIC process | ## The Skeletal System — 206 Bones ### The skeletal system — 206 bones - **Bone** has a very hard matrix due to **calcium salts**; **cartilage** has a slightly pliable matrix due to **chondroitin salts**. Both are specialised connective tissues. | **Division** | **Total bones** | **Breakdown** | | AXIAL skeleton | 80 | Skull (22: 8 cranial + 14 facial), vertebral column (26), sternum (1), ribs (24 = 12 pairs). Plus the hyoid and 3 ear ossicles per middle ear | | APPENDICULAR skeleton | 126 | Limbs (30 bones EACH limb) plus the pectoral and pelvic girdles | | TOTAL | 206 | Axial 80 + Appendicular 126 | ### Skull, vertebral column and rib cage — the exact numbers - **Skull = 22 bones**: **8 cranial** (forming the protective cranium) + **14 facial**. A single U-shaped **hyoid** bone sits at the base of the buccal cavity. Each middle ear has **3 ear ossicles — malleus, incus, stapes**. The skull articulates with the vertebral column via **two occipital condyles — a DICONDYLIC skull**. - **Vertebral column = 26 vertebrae**: **cervical 7, thoracic 12, lumbar 5, sacral 1 (fused), coccygeal 1 (fused)**. The first vertebra is the **atlas**, articulating with the occipital condyles. **Cervical vertebrae are 7 in almost ALL mammals**, humans included. - **12 pairs of ribs**, each **bicephalic** (two articulation surfaces dorsally). **Pairs 1–7 = TRUE ribs** (joined to sternum via hyaline cartilage). **Pairs 8, 9, 10 = vertebrochondral (FALSE) ribs** — they join the 7th rib, not the sternum directly. **Pairs 11–12 = FLOATING ribs** — not connected ventrally at all. ### Limb and girdle bones - **Fore limb:** humerus, radius and ulna, **carpals (8)**, **metacarpals (5)**, **phalanges (14)**. **Hind limb:** **femur — the longest bone**, tibia and fibula, **tarsals (7)**, **metatarsals (5)**, **phalanges (14)**. The **patella** (knee cap) covers the knee ventrally. - **Pectoral girdle** (each half) = **clavicle** (collar bone, long slender with two curvatures) + **scapula** (large triangular flat bone between the 2nd and 7th ribs, with a ridge ending in the **acromion**; below it the **glenoid cavity** receives the humerus head to form the shoulder joint). - **Pelvic girdle** = two **coxal bones**, each formed by fusion of **ilium, ischium and pubis**. Their fusion point forms the **acetabulum**, where the thigh bone articulates. The two halves meet ventrally at the **pubic symphysis**. ## Joints, Disorders & Why This Matters for NEET ### Joints — three structural types | **Joint type** | **Movement allowed** | **Example** | | Fibrous | NONE | Sutures between flat skull bones forming the cranium | | Cartilaginous | LIMITED | Between adjacent vertebrae in the vertebral column | | Synovial | CONSIDERABLE — has a fluid-filled synovial cavity | See the five sub-types below | ### The five synovial joints — memorise the example with the name | **Synovial joint** | **Where it occurs** | | Ball and socket | Between humerus and pectoral girdle (shoulder) | | Hinge | Knee joint | | Pivot | Between atlas and axis | | Gliding | Between the carpals | | Saddle | Between carpal and metacarpal of the THUMB | ### Disorders of the muscular and skeletal system | **Disorder** | **What it is** | **Cause** | | Myasthenia gravis | Fatigue, weakening and paralysis of skeletal muscle | AUTOIMMUNE disorder affecting the neuromuscular junction | | Muscular dystrophy | Progressive degeneration of skeletal muscle | Mostly GENETIC | | Tetany | Rapid spasms (wild contractions) in muscle | LOW Ca²⁺ in body fluid | | Arthritis | Inflammation of joints | — | | Osteoporosis | Decreased bone mass, increased fracture risk | Age-related; DECREASED ESTROGEN is a common cause | | Gout | Inflammation of joints | Accumulation of URIC ACID crystals | ### Why this matters for NEET - High-value one-liners: **206 bones** = **80 axial + 126 appendicular**; skull **22** (8 cranial + 14 facial); vertebrae **26** (7-12-5-1-1); carpals **8**, tarsals **7**; **femur is the longest bone**; during contraction **I-band shortens, A-band does not**; sarcomere = between two **Z lines**. - **Trap:** the **A-band retains its length** during contraction — only the I-band and H zone shrink. Students who assume "everything shortens" lose this mark. Also, **tetany is from LOW calcium** while **gout is from uric acid** — both cause joint/muscle problems but for entirely different reasons. And **cardiac muscle is striated but INVOLUNTARY** — striation does not imply voluntary control. ## Test Yourself: MCQs, PYQs & Active Recall Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one. ## Practice Questions **Q1.** Amoeboid movement in human macrophages and leucocytes involves: - (a) Pseudopodia formed by protoplasmic streaming and microfilaments - (b) Flagella - (c) Cilia - (d) Muscle contraction **Answer: (a)** — Amoeboid movement is effected by pseudopodia formed by streaming of protoplasm, with cytoskeletal microfilaments involved. **Q2.** Which muscle type is striated but involuntary? - (a) Cardiac - (b) Skeletal - (c) All skeletal muscles - (d) Visceral (smooth) **Answer: (a)** — Cardiac muscle is striated in appearance but involuntary — the nervous system does not directly control it. **Q3.** Muscle contributes approximately what percentage of an adult human's body weight? - (a) 40-50 per cent - (b) 80 per cent - (c) 10-20 per cent - (d) 60-70 per cent **Answer: (a)** — About 40-50 per cent of the body weight of a human adult is contributed by muscles. **Q4.** The store house of calcium ions in a muscle fibre is the: - (a) Sarcoplasmic reticulum - (b) Sarcolemma - (c) Sarcoplasm - (d) Z line **Answer: (a)** — The sarcoplasmic reticulum, the endoplasmic reticulum of muscle fibres, stores calcium ions. **Q5.** The dark A-band of a myofibril contains: - (a) Actin only - (b) Myosin - (c) Troponin only - (d) Tropomyosin only **Answer: (b)** — The dark A or Anisotropic band contains myosin (thick filaments); the light I band contains actin. **Q6.** The functional unit of muscle contraction, lying between two successive Z lines, is the: - (a) Fascicle - (b) M line - (c) H zone - (d) Sarcomere **Answer: (d)** — The portion of a myofibril between two successive Z lines is the sarcomere, the functional unit of contraction. **Q7.** In the resting state, the myosin-binding sites on actin are masked by a subunit of: - (a) Troponin - (b) Meromyosin - (c) Myoglobin - (d) Tropomyosin **Answer: (a)** — A subunit of troponin, distributed at regular intervals on tropomyosin, masks the active binding sites in the resting state. **Q8.** The globular head of meromyosin functions as an active: - (a) Protease - (b) ATPase enzyme - (c) Dehydrogenase - (d) Lipase **Answer: (b)** — The globular head is an active ATPase enzyme with binding sites for ATP and active sites for actin. **Q9.** During muscle contraction: - (a) The A band shortens while the I band retains its length - (b) The I band shortens while the A band retains its length - (c) Both A and I bands shorten - (d) Neither band changes **Answer: (b)** — During contraction the I bands get reduced while the A bands retain their length — the sliding filament theory. **Q10.** The neurotransmitter released at the neuromuscular junction is: - (a) Adrenaline - (b) Serotonin - (c) Dopamine - (d) Acetylcholine **Answer: (d)** — A neural signal at the motor-end plate releases acetylcholine, generating an action potential in the sarcolemma. **Q11.** Red muscle fibres differ from white fibres in having: - (a) Less myoglobin and fewer mitochondria - (b) High myoglobin, plenty of mitochondria, aerobic metabolism - (c) No myoglobin at all - (d) More sarcoplasmic reticulum **Answer: (b)** — Red fibres have high myoglobin and plenty of mitochondria, relying on aerobic metabolism; white fibres are the opposite. **Q12.** The total number of bones in the adult human skeletal system is: - (a) 256 - (b) 186 - (c) 226 - (d) 206 **Answer: (d)** — The human skeletal system is made up of 206 bones — 80 axial and 126 appendicular. **Q13.** The human skull is composed of how many bones? - (a) 22 - (b) 8 - (c) 26 - (d) 14 **Answer: (a)** — The skull has 22 bones — 8 cranial bones forming the cranium and 14 facial bones. **Q14.** The human vertebral column consists of how many vertebrae? - (a) 33 - (b) 24 - (c) 31 - (d) 26 **Answer: (d)** — There are 26 serially arranged vertebrae: cervical 7, thoracic 12, lumbar 5, sacral 1 (fused) and coccygeal 1 (fused). **Q15.** The 8th, 9th and 10th pairs of ribs are called: - (a) True ribs - (b) Floating ribs - (c) Vertebrochondral or false ribs - (d) Bicephalic ribs **Answer: (c)** — These pairs join the 7th rib via hyaline cartilage rather than the sternum directly, so they are false or vertebrochondral ribs. **Q16.** Each coxal bone of the pelvic girdle is formed by the fusion of: - (a) Femur, tibia and fibula - (b) Ilium, ischium and pubis - (c) Clavicle and scapula - (d) Sacrum and coccyx **Answer: (b)** — Each coxal bone is formed by the fusion of ilium, ischium and pubis; the fusion point forms the acetabulum. **Q17.** A pivot joint is found between: - (a) Humerus and pectoral girdle - (b) The carpals - (c) Carpal and metacarpal of the thumb - (d) Atlas and axis **Answer: (d)** — The pivot joint occurs between the atlas and axis vertebrae; ball and socket is shoulder, gliding is between carpals. **Q18.** Tetany, characterised by rapid muscle spasms, is caused by: - (a) Uric acid crystals - (b) An autoimmune reaction - (c) A genetic mutation - (d) Low Ca2+ in body fluid **Answer: (d)** — Tetany results from low calcium levels in body fluid; gout is caused by uric acid crystal accumulation. ### NEET Previous Year Questions (PYQs) *Real NEET previous-year questions on this chapter, with explanations in our own words.* **Q19.** Myasthenia gravis is best described as: (NEET PYQ) - (a) Loss of bone mass - (b) A genetic degeneration of muscle - (c) An autoimmune disorder affecting the neuromuscular junction - (d) Inflammation of joints **Answer: (c)** — Myasthenia gravis is an autoimmune disorder affecting the neuromuscular junction, causing fatigue and paralysis of skeletal muscle. **Q20.** Gout is caused by the accumulation of: (NEET PYQ) - (a) Lactic acid - (b) Cholesterol - (c) Calcium crystals - (d) Uric acid crystals **Answer: (d)** — Gout is inflammation of joints due to accumulation of uric acid crystals. **Q21.** The number of carpal and tarsal bones respectively is: (NEET PYQ) - (a) 8 and 7 - (b) 5 and 5 - (c) 7 and 8 - (d) 14 and 14 **Answer: (a)** — There are 8 carpals (wrist) and 7 tarsals (ankle). **Q22.** Osteoporosis is commonly associated with decreased levels of: (NEET PYQ) - (a) Estrogen - (b) Thyroxine - (c) Testosterone - (d) Insulin **Answer: (a)** — Osteoporosis is an age-related disorder of decreased bone mass; decreased estrogen levels are a common cause. **Q23.** The number of cervical vertebrae in almost all mammals, including humans, is: (NEET PYQ) - (a) 26 - (b) 7 - (c) 12 - (d) 5 **Answer: (b)** — Cervical vertebrae number seven in almost all mammals including human beings. **Q24.** Each F-actin of a thin filament is a polymer of: (NEET PYQ) - (a) Tropomyosin - (b) Monomeric G (globular) actins - (c) Troponin - (d) Meromyosin **Answer: (b)** — Each F (filamentous) actin is a polymer of monomeric G (globular) actins; two F-actins wind helically. **Q25.** The saddle joint in the human body is found between: (NEET PYQ) - (a) Carpal and metacarpal of the thumb - (b) The carpals - (c) Humerus and pectoral girdle - (d) Atlas and axis **Answer: (a)** — The saddle joint occurs between the carpal and metacarpal of the thumb. **Q26.** Muscle fatigue after repeated activation is due to accumulation of: (NEET PYQ) - (a) Lactic acid from anaerobic glycogen breakdown - (b) Calcium ions - (c) Myoglobin - (d) Uric acid **Answer: (a)** — Repeated activation leads to accumulation of lactic acid due to anaerobic breakdown of glycogen, causing fatigue. ### Active Recall Prompt Write everything you can recall about **Locomotion and Movement**, naming each part first: the **three types of movement** in human cells; the **three muscle types** (appearance, control, location); **skeletal muscle structure** (fascia, sarcolemma, sarcoplasmic reticulum, the bands, Z line, M line, sarcomere, H zone); the **contractile proteins** (actin, tropomyosin, troponin, meromyosin, HMM/LMM); the **sliding filament theory** step by step; **red vs white fibres**; the **skeletal system** (206 bones, axial vs appendicular, and every count — skull, vertebrae, ribs, carpals, tarsals); the **girdles**; the **joint types** with examples; and the **disorders**. Begin each fact with its topic and end it with a full stop.