Locomotion and Movement (NEET Biology Class 11): Sliding Filament Theory, 206 Bones & Joints
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
Show answer
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)
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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.