Core Definitions
Movement — Any change in position of body or its parts. It does NOT necessarily involve change of location.
Locomotion — Movement that causes a change in location/place. All locomotion is movement, but all movement is NOT locomotion.
3 Types of Movements in Humans
- Amoeboid Movement — Occurs by pseudopodia (cytoplasmic extensions). Seen in WBCs (leucocytes, macrophages) and Amoeba. Also plays role in phagocytosis.
- Ciliary Movement — Caused by cilia or flagella. Seen in movement of ovum in fallopian tube, removal of dust particles in trachea by cilia, movement of sperm by flagella.
- Muscular Movement — Caused by muscles. Responsible for locomotion of limbs, movement of tongue, eyelids, jaws, and all voluntary body movements.
Q. Which type of movement is shown by WBCs in human body?
✅ Answer: Amoeboid movement
Explanation
WBCs (White Blood Cells), particularly macrophages and neutrophils, move using pseudopodia — temporary cytoplasmic projections. This type of movement is called amoeboid movement because it resembles the locomotion of Amoeba. This helps them travel to infection sites in the body.
Q. Movement of ovum in fallopian tube is caused by?
✅ Answer: Ciliary movement
Explanation
The inner lining of the fallopian tube (oviduct) is lined with ciliated epithelium. The beating of these cilia creates a current that helps move the ovum from ovary towards the uterus. This is a classic example of ciliary movement in the human body.
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Muscle — Overview & Properties
Key Facts About Muscles
- Muscle is a specialized contractile tissue of mesodermal origin
- Adult human body has 639 muscles — 317 paired + 5 unpaired
- Muscles constitute 40–50% of body weight
- Each skeletal muscle fibre is formed by fusion of 100 or more myoblast cells → becomes Syncytium (multinucleated)
- Muscle fibres are non-dividing (cannot undergo mitosis) — only satellite cells can divide for repair
4 Special Properties of Muscle
- Contractility — Ability to shorten/contract when stimulated
- Extensibility — Ability to be stretched beyond normal resting length
- Elasticity — Ability to return to original length after stretch or contraction
- Excitability — Ability to respond to a stimulus (chemical or electrical)
Comparison of 3 Muscle Types
| Feature | Skeletal (Striated) | Smooth (Non-striated) | Cardiac |
|---|
| Shape | Cylindrical, unbranched | Spindle-shaped | Cylindrical, branched |
| Nuclei | Multinucleated (peripheral) | Uninucleated (central) | Uninucleated (central) |
| Striation | Present (light & dark bands) | Absent | Present + Intercalated discs |
| Control | Voluntary (CNS) | Involuntary (ANS) | Involuntary (CNS + ANS) |
| Fatigue | Fatigues fast | Does not fatigue easily | Never fatigues (rhythmic) |
| Location | Limbs, body wall | Gut, uterus, blood vessels | Heart only |
| Example | Biceps, quadriceps | Intestinal wall, iris | Myocardium |
Red Fibre vs White Fibre (Skeletal Muscle)
| Property | Red Fibres (Slow / Type I) | White Fibres (Fast / Type II) |
|---|
| Diameter | Small | Large |
| Myoglobin | More → Red colour | Less |
| Mitochondria | More | Less |
| Sarcoplasmic Reticulum | Less | More (faster Ca²⁺ release) |
| Respiration type | Aerobic | Anaerobic (glycolytic) |
| Lactic acid | No deposition | Lactic acid deposited → fatigue |
| Contraction speed | Slow, sustained (long period) | Fast, short burst only |
| Example | Back extensor muscles, chicken leg | Eyeball muscles, chicken breast, sparrow |
Q. Which muscle fibres show aerobic oxidation without deposition of lactic acid?
✅ Answer: Red muscle fibres (Slow fibres / Type I)
Explanation
Red muscle fibres are rich in myoglobin (oxygen-storing protein) and contain many mitochondria. They rely on aerobic (oxidative) metabolism for energy production. Since they use oxygen efficiently, lactic acid is NOT produced, and they can sustain contractions for a long time without fatigue. In contrast, white fibres use anaerobic glycolysis and accumulate lactic acid, leading to quick fatigue.
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Muscle Fibre Structure & Sarcomere
Coverings of Muscle (Outside to Inside)
- Fascia — Outermost fibrous covering of entire muscle
- Epimysium — Covers the whole muscle belly
- Perimysium — Covers a bundle of muscle fibres (fascicle)
- Endomysium — Covers each individual muscle fibre
- Sarcolemma — Plasma membrane of muscle fibre
- Sarcoplasm — Cytoplasm of muscle fibre (contains glycogen, myoglobin)
- T-Tubules — Transverse tubules; extensions of sarcolemma deep into fibre; carry action potential to interior
- Sarcoplasmic Reticulum (S.R.) — Smooth ER of muscle; stores Ca²⁺ ions
Sarcomere — The Functional Unit of Muscle
Sarcomere = the region between two consecutive Z-lines. It is the basic structural and functional contractile unit of a myofibril.
- I-Band (Isotropic) → Light band → contains only Actin (thin filaments); bisected by Z-line
- A-Band (Anisotropic) → Dark band → contains both Actin + Myosin (overlapping); middle portion is H-zone
- H-Zone → Central region of A-band → contains only Myosin (no actin)
- M-Line → Middle of H-zone → where myosin filaments are anchored
- Z-Line → Boundary of sarcomere → Actin filaments are attached here
⚡ Changes During Muscle Contraction — MOST IMPORTANT
- I-Band → Decreases (actin slides in, so light band shortens)
- H-Zone → Decreases / Disappears (actin covers myosin-only region)
- A-Band → UNCHANGED ✔ (myosin filament length doesn't change)
- Z-lines → Move closer together
- Sarcomere length → Decreases
Key Rule: Only A-band remains constant. I-band and H-zone both decrease.
Q. Which band remains unchanged during muscle contraction?
✅ Answer: A-Band (Dark / Anisotropic band)
Explanation
During contraction, actin filaments slide over myosin (Sliding Filament Theory by Huxley). The thick myosin filaments do NOT change their length — they stay the same. Since A-band represents the length of myosin filaments, its width remains constant. Only I-band (actin only region) and H-zone (myosin only region) decrease in width as actin moves inward and covers H-zone.
Q. What happens to H-zone during contraction?
✅ Answer: H-zone decreases and may completely disappear
Explanation
H-zone is the central portion of A-band that contains only myosin (no actin overlap) in a relaxed muscle. When muscle contracts, actin filaments slide inward and start overlapping the myosin-only region (H-zone). As contraction increases, actin covers the entire H-zone — so H-zone first decreases and in full contraction it may completely disappear. This is a classic exam question.
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Contractile & Regulatory Proteins
Thin Filament — Actin (6 molecules surround each myosin)
- Made of 2 F-actin (Filamentous actin) strands twisted helically
- Each F-actin is a polymer of G-actin (Globular actin) monomers
- G-actin monomers have active sites for myosin binding
- Regulatory proteins sit on actin: Tropomyosin + Troponin complex
Thick Filament — Myosin (1 in centre, 6 actin surround it)
- Each myosin molecule = 6 polypeptide chains (2 heavy + 4 light)
- Structurally divided into: Heavy Meromyosin (HMM) + Light Meromyosin (LMM)
- HMM = Head (2 globular heads) + Short arm → forms the Cross bridge
- LMM = Tail (rod-like portion)
- Myosin head contains ATPase enzyme — hydrolyzes ATP to generate force
- Myosin head has actin-binding site
Regulatory Proteins on Thin Filament
- Tropomyosin — Lies along the length of actin, physically masks the active sites of G-actin in resting state → prevents myosin binding
- Troponin-I (TnI) — Inhibitory subunit; inhibits actin-myosin interaction by helping tropomyosin stay in blocking position
- Troponin-T (TnT) — Tropomyosin-binding subunit; binds to Tropomyosin, anchors troponin complex on the thin filament
- Troponin-C (TnC) — Calcium-binding subunit; when Ca²⁺ binds here, causes conformational change → tropomyosin moves away → active sites on actin exposed
Q. Which protein has ATPase activity and is responsible for power stroke during contraction?
✅ Answer: Myosin head (Heavy Meromyosin / HMM)
Explanation
The globular head of myosin (part of Heavy Meromyosin) contains the enzyme ATPase. When a cross bridge is formed between myosin head and actin, ATP binds to the myosin head and is hydrolyzed into ADP + Pi. This energy release causes the myosin head to bend (power stroke), pulling the actin filament toward the M-line. After the power stroke, a new ATP molecule must bind to release the myosin head from actin.
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Mechanism of Muscle Contraction — Sliding Filament Theory
Step-by-Step Mechanism
- Neural signal from CNS travels through motor neuron to Neuromuscular Junction (NMJ) / Motor End Plate
- Motor neuron releases neurotransmitter Acetylcholine (ACh) into synaptic cleft
- ACh binds to receptors on Sarcolemma → opens Na⁺ channels (Ligand-gated) → action potential generated
- Action potential spreads along sarcolemma and into T-Tubules → reaches Sarcoplasmic Reticulum (S.R.)
- Ca²⁺ released from S.R. into sarcoplasm (via Voltage-gated Ca²⁺ channels)
- Ca²⁺ binds to Troponin-C → changes shape of Troponin-Tropomyosin complex → Tropomyosin shifts away → active sites on actin exposed
- Myosin head (energized by ATP) binds to actin → Cross bridge formed
- ATP hydrolyzed by myosin ATPase → ADP + Pi released → myosin head bends toward M-line → Power stroke → actin slides toward centre
- New ATP binds to myosin head → cross bridge detaches from actin
- ATP hydrolyzed again → myosin re-cocks to original position → ready for next cycle
- Cycle repeats → many sarcomeres shorten simultaneously → whole muscle contracts
Relaxation — How Muscle Relaxes
- ACh broken down by enzyme Acetylcholinesterase → Choline + Acetate → action potential stops
- Ca²⁺ actively pumped back into Sarcoplasmic Reticulum (requires ATP)
- Without Ca²⁺, Troponin-C releases Ca²⁺ → Tropomyosin moves back over active sites → Active sites masked again
- Myosin can no longer bind actin → cross bridges cannot form → Muscle relaxes
- Actin filaments slide back → I-band and H-zone return to original size
Energy Source for Contraction
- Direct energy: ATP (hydrolyzed by myosin ATPase)
- Immediate backup: Creatine Phosphate (CP) → donates phosphate to ADP → ATP (very fast, limited store)
- Sustained energy: Anaerobic glycolysis during intense activity → produces ATP + Lactic acid → lactic acid causes muscle soreness/fatigue
- ATP is needed for: (1) cross bridge formation, (2) power stroke, (3) cross bridge detachment, (4) Ca²⁺ re-uptake into S.R.
Q. What is the role of Ca²⁺ in muscle contraction?
✅ Answer: Ca²⁺ binds to Troponin-C → shifts Tropomyosin → exposes active sites on Actin → allows cross bridge formation
Explanation
In a resting muscle, Tropomyosin physically blocks the myosin-binding sites (active sites) on actin. When a nerve impulse arrives, Ca²⁺ is released from the Sarcoplasmic Reticulum. Ca²⁺ binds to Troponin-C (the calcium-sensitive component of the Troponin complex). This binding causes a conformational change that moves Tropomyosin away from the active sites on actin. Now, myosin heads can bind to actin, forming cross bridges, and contraction begins.
Q. What happens if ATP is completely exhausted in a muscle?
✅ Answer: Rigor Mortis occurs — muscle stays in locked contracted state
Explanation
ATP is needed not just for the power stroke but also for detaching the myosin head from actin. When ATP is completely absent (as in death), the myosin heads bind to actin but CANNOT detach. All cross bridges remain permanently locked. This causes the characteristic stiffening of muscles after death, known as Rigor Mortis. It begins a few hours after death and lasts until decomposition of proteins begins.
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Skeletal System — 206 Bones
Division of Human Skeleton
- Axial Skeleton = 80 bones — forms the central axis (skull + vertebral column + ribs + sternum)
- Appendicular Skeleton = 126 bones — limbs and girdles
- Total = 206 bones in adult human body
Axial Skeleton — 80 Bones (Detailed)
| Part | Count | Important Details |
|---|
| Cranial bones | 8 | Frontal(1), Parietal(2), Temporal(2), Occipital(1), Sphenoid(1), Ethmoid(1) |
| Facial bones | 14 | Mandible(1), Maxilla(2), Nasal(2), Lacrimal(2), Zygomatic(2), Vomer(1), Palatine(2), Inferior Nasal Conchae(2) |
| Ear ossicles | 6 (3×2) | Malleus, Incus, Stapes in each ear. Stapes = smallest bone |
| Hyoid | 1 | U-shaped; only bone NOT articulated with any other bone; supports tongue |
| Vertebral column | 26 | Cervical(7) + Thoracic(12) + Lumbar(5) + Sacrum(1) + Coccyx(1) |
| Ribs | 24 | 12 pairs — True(1–7), False(8–10), Floating(11–12) |
| Sternum | 1 | Manubrium + Body + Xiphoid process; Manubrium articulates with clavicle & 1st rib |
Vertebral Column — All Important Facts
- Total = 26 bones (33 in infant — Sacral & Coccygeal fuse later)
- Cervical = 7 — Present in ALL mammals (even giraffe & whale have 7)
- 1st cervical = Atlas (supports skull); 2nd = Axis (has Odontoid peg/process)
- Atlas-Axis joint = Pivot joint (rotation of head)
- Thoracic = 12 — each has extra articular facet for rib attachment
- Lumbar = 5 — largest and thickest; responsible for lumbar curve and bearing body weight
- Sacrum = 1 (formed by fusion of 5 sacral vertebrae)
- Coccyx = 1 (formed by fusion of 4 coccygeal vertebrae) — vestigial tail
- Human skull is Dicondylic — 2 occipital condyles at base of skull
Ribs — Classification (12 Pairs = 24 Ribs)
- True Ribs (Pairs 1–7) = Vertebrosternal ribs — attached directly to sternum via costal cartilage (hyaline)
- False Ribs (Pairs 8–10) = Vertebrochondral ribs — NOT attached directly; share costal cartilage of 7th rib
- Floating Ribs (Pairs 11–12) = Vertebral ribs — attached to vertebrae only; FREE anterior end, NOT attached to sternum at all
- Each rib has 2 articular surfaces — Capitulum (head) + Tuberculum — both on dorsal side
- Sternum = 3 parts: Manubrium (top) + Body + Xiphoid process (bottom)
Q. How many bones are present in the human skull (cranium + facial)?
✅ Answer: 29 bones (8 cranial + 14 facial + 6 ear ossicles + 1 hyoid)
Explanation
The skull is made of 29 bones total: 8 cranial bones that form the braincase (protect the brain), 14 facial bones that form the facial structure, 6 ear ossicles (3 in each ear — malleus, incus, stapes) for sound conduction, and 1 hyoid bone that supports the tongue. Among these, Stapes is the smallest bone in the body, and Mandible (lower jaw) is the only movable bone in the skull.
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Appendicular Skeleton — Girdles & Limbs (126 Bones)
Pectoral Girdle (Shoulder Girdle) — 4 Bones
- Clavicle (Collar bone) × 2 — S-shaped; connects sternum to scapula
- Scapula (Shoulder blade) × 2 — triangular flat bone; has Acromion process and Glenoid cavity
- Glenoid cavity (on scapula) + Head of Humerus = Ball and Socket joint (shoulder)
- Scapula is attached to ribs 2–7 on dorsal side
Forelimb Bones — 30 per side = 60 total
- Humerus (1) — upper arm bone
- Radius (1) + Ulna (1) — forearm bones; Radius-Ulna joint = Pivot joint (pronation/supination)
- Carpals (8) — wrist bones arranged in 2 rows
- Metacarpals (5) — palm bones (one per finger)
- Phalanges (14) — 2 in thumb (pollex), 3 in each other finger
Pelvic Girdle — 2 Coxal Bones
- Each Coxal (Hip) bone = fusion of 3 bones: Ilium + Ischium + Pubis
- Acetabulum = deep socket formed at junction of ilium, ischium & pubis → head of Femur fits here → Ball and Socket joint (hip)
- Obturator foramen = oval opening formed by Ischium + Pubis
- Pubic Symphysis = cartilaginous joint between left and right pubis bones (slightly movable)
- Two coxal bones + Sacrum + Coccyx = Pelvis
Hindlimb Bones — 30 per side = 60 total
- Femur (1) — thigh bone; longest and strongest bone in human body
- Patella (1) — kneecap; a sesamoid bone (formed by ossification of tendon); protects knee joint
- Tibia (1) + Fibula (1) — leg bones; Tibia is medial and weight-bearing
- Tarsals (7) — ankle bones; largest = Calcaneus (heel bone)
- Metatarsals (5) — foot bones
- Phalanges (14) — 2 in big toe, 3 in each other toe
Q. Patella is an example of which type of bone? How is it formed?
✅ Answer: Sesamoid bone; formed by ossification (hardening) of a tendon
Explanation
Patella (kneecap) is a sesamoid bone — a bone that develops within a tendon where the tendon wraps around a joint. In this case, it develops within the quadriceps tendon. Sesamoid bones protect tendons from wear and stress, and they improve the mechanical advantage of muscles. Patella is also the only bone in the body located within a tendon, and it protects the knee joint from direct impact.
3 Main Categories of Joints
- Fibrous Joints (Synarthroses) — Immovable; bones connected by dense fibrous connective tissue; e.g., Skull sutures
- Cartilaginous Joints (Amphiarthroses) — Slightly movable (semimovable); bones connected by cartilage; e.g., Pubic symphysis, Intervertebral discs, Rib-sternum joint (Hyaline cartilage)
- Synovial Joints (Diarthroses) — Freely movable; characterized by fluid-filled synovial cavity between articulating surfaces
Synovial Joint — Key Features
- Articular cartilage (Hyaline) — covers bone ends, reduces friction
- Synovial cavity — space between articulating bones, filled with synovial fluid
- Synovial membrane — lines the cavity; secretes synovial fluid (lubricant)
- Ligaments — connect bone to bone, provide stability
- Tendons — connect muscle to bone (NOT part of joint itself)
Types of Synovial Joints — Complete
| Joint Type | Movement | Example |
|---|
| Ball & Socket | Movement in all planes (most mobile) | Shoulder (Scapula + Humerus), Hip (Acetabulum + Femur) |
| Hinge | Movement in one plane only (flexion/extension) | Knee, Elbow, Ankle, Interphalangeal joints |
| Pivot | Rotation only around a central axis | Atlas-Axis (rotation of head), Radius-Ulna (pronation/supination) |
| Gliding | Gliding movement in multiple planes | Between Carpals, between Tarsals, between pre- and post-zygapophyses |
| Saddle | Movement in two planes (biaxial) | Between Thumb metacarpal and Trapezium (carpometacarpal of thumb) |
| Condyloid / Ellipsoid | Oval-shaped movement (biaxial, no rotation) | Metacarpal + Phalanges, Metatarsal + Phalanges |
Q. Which type of joint allows movement in all directions and is the most freely movable?
✅ Answer: Ball and Socket joint
Explanation
In a ball and socket joint, the rounded head (ball) of one bone fits into a cup-like depression (socket) of another. This design allows movement in all planes — flexion, extension, abduction, adduction, rotation, and circumduction. The shoulder joint (Humerus into Glenoid cavity of Scapula) and hip joint (Femur into Acetabulum of Pelvis) are the two ball and socket joints in the human body. The hip joint is more stable but less mobile than the shoulder joint.
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Disorders of Muscular & Skeletal System
| Disorder | Cause | Main Features |
|---|
| Myasthenia Gravis | Autoimmune — body makes antibodies against Acetylcholine receptors at NMJ | Progressive fatigue, weakness, paralysis of skeletal muscles |
| Muscular Dystrophy | Genetic disorder (e.g., mutation in dystrophin gene) | Progressive degeneration and weakening of skeletal muscles |
| Tetany | Low blood Ca²⁺ (Hypocalcemia); Ca²⁺ channels remain open | Rapid involuntary spasms (wild contractions) throughout body muscles |
| Arthritis | Inflammation of joints (many causes — auto-immune, wear & tear, infection) | Pain, swelling, stiffness, reduced mobility of joints |
| Osteoporosis | Age-related; low estrogen in post-menopausal women; low Ca²⁺ absorption | Decreased bone density → bones become brittle → increased fracture risk |
| Gout | Excess uric acid in blood (hyperuricemia) → deposits as crystals in joints | Severe inflammation, pain, swelling in joints (especially big toe) |
Myasthenia Gravis — Detailed (Frequently Asked)
- Type: Autoimmune disease
- Body produces antibodies against Nicotinic Acetylcholine receptors (nAChR) at the neuromuscular junction
- Result: ACh released by motor neuron cannot bind effectively → no action potential in muscle → muscle cannot contract properly
- Symptoms: Easy fatigue, drooping eyelids (ptosis), difficulty swallowing, progressive weakness — worsens with activity, improves with rest
- The NMJ is affected, NOT the muscle or motor neuron directly
Q. Tetany is caused due to deficiency of which ion?
✅ Answer: Calcium ions (Ca²⁺) — condition called Hypocalcemia
Explanation
Normally, low extracellular Ca²⁺ causes voltage-gated Na⁺ channels to become more excitable (threshold lowers). When blood Ca²⁺ falls below normal (Hypocalcemia), nerve and muscle membranes become hyperexcitable — they fire spontaneously without any stimulus. This causes rapid, involuntary, repetitive muscle contractions (spasms) throughout the body — this condition is called Tetany. It is different from "tetanus" (caused by a bacterium). Tetany is corrected by administering calcium.
Q. What is Gout and which substance accumulates in joints?
✅ Answer: Gout is inflammation of joints caused by deposition of Uric Acid crystals
Explanation
Uric acid is the end product of purine metabolism in humans. Normally, uric acid is dissolved in blood and excreted by kidneys. In Gout, either too much uric acid is produced or kidneys cannot excrete it fast enough → blood uric acid rises (hyperuricemia). Uric acid crystallizes as monosodium urate crystals and gets deposited in joints, especially the metatarsophalangeal joint of the big toe. These crystals cause intense inflammatory response — severe pain, redness, and swelling.
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Quick Revision — Must-Know Points
Longest bone → Femur
Smallest bone → Stapes (ear ossicle)
Sesamoid bone → Patella
No articulation → Hyoid bone
A-band → Unchanged during contraction
Mammals always have → 7 cervical vertebrae
Human skull → Dicondylic (2 occipital condyles)
Functional unit of muscle → Sarcomere
Anatomical unit → Muscle Fibre
Total muscles → 639 (317 paired + 5 unpaired)
ATPase activity → Myosin head
Ca²⁺ stored in → Sarcoplasmic Reticulum
Ca²⁺ binds to → Troponin-C
ACh broken by → Acetylcholinesterase
Axial skeleton → 80 bones
Appendicular → 126 bones
Carpals → 8 (wrist)
Tarsals → 7 (ankle)