Locomotion and Movement — NEET notes

Types of Movement — Amoeboid, ciliary, muscular, and why locomotion isn't the same as movement

Movement is a change in body position (can be at cellular level), while locomotion is voluntary movement from one place to another. Three main types: amoeboid (pseudopodia, e.g., macrophages), ciliary (cilia, e.g., tracheal epithelium, Fallopian tube), and muscular (skeletal muscles, the main locomotory system). Not all organisms that show movement show locomotion.

Movement vs Locomotion

Movement is any change in position of body parts or the whole organism. Locomotion specifically means voluntary displacement of the entire body from one place to another. Plants show movement but NOT locomotion.

Movement is any change in position — it can be at the level of cells, organs, or the entire organism. Locomotion is specifically the voluntary movement of the entire body from one place to another. The key distinction: plants show movement (leaves folding in Mimosa, roots growing toward water) but they do NOT show locomotion.

• Movement = change in position of body part or whole body (broader term)
• Locomotion = voluntary movement of entire body from place A to place B (subset of movement)
• All locomotion is movement, but all movement is NOT locomotion
• Plants show movement (nastic, tropic) but NOT locomotion
• Animals show BOTH movement and locomotion (except sponges — adults are fixed/sessile)
• Locomotion requires a well-coordinated musculoskeletal system in higher animals

Amoeboid Movement

Movement by forming temporary cytoplasmic extensions called pseudopodia. Involves streaming of cytoplasm (sol-gel interconversion). Seen in Amoeba, macrophages, WBCs (leucocytes), and cytoskeletal elements like microfilaments.

Amoeboid movement uses pseudopodia (false feet) — temporary extensions of the cell membrane and cytoplasm. The cytoplasm streams in the direction of movement through sol-gel transformation: the inner cytoplasm (endoplasm/sol) flows forward, hits the front, and converts to the outer gel state (ectoplasm). This pushes the cell forward.

• Powered by microfilaments (actin filaments) of the cytoskeleton
• Involves cytoplasmic streaming and sol-gel interconversion of cytoplasm
• Seen in: Amoeba, macrophages, leucocytes (WBCs), and some other cells
• WBCs use amoeboid movement to squeeze through blood vessel walls (diapedesis) to reach infection sites
• Cytoskeletal elements like microfilaments are essential for this movement

Ciliary Movement

Movement by coordinated beating of cilia (short, numerous hair-like projections). Cilia contain microtubules in a 9+2 axonemal arrangement. Seen in tracheal epithelium (mucus clearance), Fallopian tubes (egg transport), and ciliated protozoans like Paramecium.

Ciliary movement uses cilia — short, hair-like projections present in large numbers on the cell surface. They beat in coordinated waves (like a Mexican wave in a stadium) to move substances across cell surfaces. In the human body, ciliary movement is crucial in the respiratory tract and reproductive tract.

• Cilia are powered by microtubules (9+2 axonemal arrangement — 9 peripheral doublets + 2 central singlets)
• Trachea/bronchi: ciliated epithelium sweeps mucus (with trapped dust/microbes) upward toward the throat → keeps lungs clean
• Fallopian tube (oviduct): cilia push the ovum from ovary toward uterus after ovulation
• Ciliated protozoans: Paramecium uses cilia for locomotion
• Motor protein dynein (not myosin!) powers ciliary beating by sliding microtubule doublets

Muscular Movement

The primary mechanism of locomotion in vertebrates. Skeletal muscles attached to bones via tendons contract to produce movement at joints. Also includes movement of internal organs (smooth muscle) and heart pumping (cardiac muscle).

Muscular movement is the primary locomotory mechanism in most animals, especially vertebrates. Skeletal muscles are attached to bones by tendons. When a muscle contracts, it pulls on the tendon → the tendon pulls on the bone → the bone moves at a joint. This is how you walk, run, write, and dance.

• Skeletal muscles (voluntary, striated) → locomotion and body movements
• Smooth muscles (involuntary) → peristalsis in gut, blood vessel constriction, iris adjustment
• Cardiac muscle (involuntary, striated) → heart pumping blood
• Muscles can only pull (contract), never push — that's why they work in antagonistic pairs (e.g., biceps + triceps)
• Muscle-bone coordination requires: muscles + bones + joints + nervous system (all four!)

Quick recall

What are the three types of movement seen in cells/organisms as per NCERT?

Amoeboid (pseudopodia, microfilaments), Ciliary (cilia, microtubules), and Muscular (actin-myosin, most common in vertebrates). Additionally, flagellar movement exists (sperm).

Skeletal System — 206 bones organized into axial and appendicular divisions

The human skeleton has 206 bones in adults divided into axial skeleton (80 bones: skull, vertebral column, sternum, ribs) and appendicular skeleton (126 bones: limbs + girdles). Bones provide framework, protect organs, produce blood cells (haematopoiesis), and store minerals. Cartilage, ligaments, and tendons are accessory structures.

Axial Skeleton

The axial skeleton forms the central vertical axis of the body. It includes the skull (22 bones), hyoid (1), auditory ossicles (6), vertebral column (26), sternum (1), and ribs (24). Total: 80 bones. Primarily protective in function.

The axial skeleton is the central pillar of the body — it runs along the longitudinal axis from skull to tailbone. Its primary role is protection of vital organs (brain, spinal cord, heart, lungs) and providing the central framework to which the appendicular skeleton attaches.

• Skull (22 bones): 8 cranial bones (protect brain) + 14 facial bones (form face)
• Cranial bones: Frontal (1), Parietal (2), Temporal (2), Occipital (1), Sphenoid (1), Ethmoid (1) = 8 total
• Hyoid bone (1): U-shaped, in the neck — the ONLY bone NOT articulating with any other bone (floating bone!)
• Auditory ossicles (6): Malleus, Incus, Stapes — 3 per ear × 2 ears. Stapes = smallest bone in the body
• Vertebral column (26 bones): Cervical (7) + Thoracic (12) + Lumbar (5) + Sacral (1, fused from 5) + Coccygeal (1, fused from 4)
• Sternum (1): breastbone — manubrium + body + xiphoid process
• Ribs (24): 12 pairs — True ribs (1-7), False ribs (8-10), Floating ribs (11-12)

Quick recall

What is the smallest bone in the human body and where is it found?

Stapes (stirrup bone) — one of the three auditory ossicles in the middle ear. It's about 3 mm long and transmits sound vibrations from incus to the oval window of the inner ear.

Appendicular Skeleton

The appendicular skeleton includes the bones of the upper limbs (60), lower limbs (60), pectoral girdle (4), and pelvic girdle (2). Total: 126 bones. Primarily locomotory in function.

The appendicular skeleton is everything that appends (attaches) to the axial skeleton. It includes the limbs and the girdles that connect limbs to the trunk. While the axial skeleton protects, the appendicular skeleton moves. It accounts for 126 of the 206 bones.

• Pectoral girdle (4 bones): 2 clavicles (collarbones) + 2 scapulae (shoulder blades). Connects upper limbs to axial skeleton
• Each upper limb (30 bones): Humerus (1) → Radius + Ulna (2) → Carpals (8) → Metacarpals (5) → Phalanges (14). Total per arm = 30 × 2 = 60
• Pelvic girdle (2 bones): 2 coxal/hip bones (each formed by fusion of ilium + ischium + pubis). They fuse at the acetabulum (hip socket)
• Each lower limb (30 bones): Femur (1) → Patella (1) → Tibia + Fibula (2) → Tarsals (7) → Metatarsals (5) → Phalanges (14). Total per leg = 30 × 2 = 60
• Femur = longest and strongest bone in the body
• Patella (kneecap) = largest sesamoid bone (a bone embedded within a tendon)

Bone and Cartilage

Bones are made of ossein (organic) and hydroxyapatite (inorganic mineral). Cartilage is softer, made of chondrin. Bones provide rigidity; cartilage provides flexibility at joints, nose, ear, trachea. Ligaments connect bone-to-bone; tendons connect muscle-to-bone.

Bone is a hard connective tissue with a matrix of ossein protein (organic, gives flexibility) + calcium hydroxyapatite (inorganic mineral, gives hardness). About 70% mineral and 30% organic. Cartilage is softer with a matrix of chondrin — it's found where flexibility is needed.

• Bone functions: framework, protection, blood cell production (in red bone marrow — haematopoiesis), mineral storage (Ca²⁺ and PO₄³⁻)
• Cartilage locations: tip of nose, external ear (pinna), epiglottis, tracheal rings, intervertebral discs, joint surfaces
• Ligaments: connect bone to bone at joints (strong, slightly elastic connective tissue)
• Tendons: connect muscle to bone (tough, inelastic, made of dense regular connective tissue/collagen)
• At birth: ~300 bones (many cartilaginous). By adulthood: 206 bones (cartilage ossifies/fuses)

Joints — Where bones meet, from immovable sutures to freely movable synovial joints

Joints (articulations) are points where two or more bones meet. Three types based on mobility: fibrous/synarthroses (immovable, e.g., skull sutures), cartilaginous/amphiarthroses (slightly movable, e.g., intervertebral discs, pubic symphysis), and synovial/diarthroses (freely movable, e.g., knee, shoulder, elbow). Synovial joints have 6 subtypes — ball-and-socket, hinge, pivot, gliding, saddle, condyloid.

Fibrous Joints

Fibrous joints (synarthroses) have bones joined by dense fibrous connective tissue with NO joint cavity and NO movement. Examples: sutures of the skull, tooth-in-socket (gomphosis).

In fibrous joints, the bones are held together by dense fibrous connective tissue — they're basically fused. There is no joint cavity and no movement (or negligible movement). These joints exist where movement would be dangerous — like in the skull, where the brain needs rock-solid protection.

• Also called synarthroses (syn = together, arthrosis = joint) or immovable joints
• Sutures of the skull — the wavy interlocking lines between cranial bones. In newborns, gaps called fontanelles allow brain growth; they ossify by age 1-2
• Gomphosis — tooth in its bony socket (peg-and-socket joint). Held by periodontal ligament
• No synovial fluid, no joint cavity — bones are essentially cemented together
• NCERT calls these 'fibrous joints' — the name tells you the joining material (fibrous tissue)

Cartilaginous Joints

Cartilaginous joints (amphiarthroses) have bones joined by cartilage, allowing limited/slight movement. Examples: intervertebral discs (between vertebrae), pubic symphysis, and the joint between the first rib and sternum.

In cartilaginous joints, the bones are joined by cartilage (hyaline or fibrocartilage). They allow limited movement — not free like your shoulder, but not locked like your skull. Think of them as the shock absorbers of the skeleton.

• Also called amphiarthroses (amphi = on both sides) or slightly movable joints
• Intervertebral discs — fibrocartilage pads between adjacent vertebrae. Allow slight bending/twisting. Together, small movements at each disc add up to significant spinal flexibility
• Pubic symphysis — fibrocartilage pad between the two pubic bones of the pelvic girdle. Loosens slightly during pregnancy (hormonal effect of relaxin) to widen the birth canal
• No joint cavity or synovial fluid — cartilage is the cushion
• The joint between adjacent vertebrae is the most commonly asked example

Synovial Joints

Synovial joints (diarthroses) have a joint cavity filled with synovial fluid, lined by a synovial membrane, and allow free movement. They have 6 subtypes based on movement pattern: ball-and-socket, hinge, pivot, gliding, saddle, and condyloid (ellipsoid).

Synovial joints are the most common and most mobile joints in the body. They have a fluid-filled cavity (synovial cavity) between the articulating bones. The cavity is lined by a synovial membrane that secretes synovial fluid — a viscous lubricant that reduces friction and nourishes the articular cartilage.

• Also called diarthroses or freely movable joints
• Key structural features: articular cartilage (covers bone ends), synovial membrane (lines cavity), synovial fluid (lubricant), joint capsule (encloses the joint), ligaments (hold bones together)
• Synovial fluid is thick and egg-white-like (syn = with, ovia = egg — 'with egg')
• These joints allow the widest range of movements: flexion, extension, rotation, abduction, adduction, circumduction

Quick recall

A joint allows movement in all directions including rotation. What type is it?

Ball-and-socket joint — the ball-shaped head of one bone fits into the cup-shaped socket of another. Examples: shoulder (humerus head in glenoid cavity of scapula) and hip (femur head in acetabulum of hip bone).

Muscle Types & Structure — From whole muscle down to the sarcomere

Skeletal muscle has a hierarchical structure: Muscle → Fascicle (bundle of fibers) → Muscle fiber (single cell, multinucleated) → Myofibril → Sarcomere (functional unit of contraction). Each sarcomere has thick filaments (myosin) and thin filaments (actin, with tropomyosin and troponin). Bands visible under microscope: dark A-band (anisotropic, myosin), light I-band (isotropic, actin only), H-zone (myosin only, central), M-line (center), Z-line (sarcomere boundary).

Muscle Organization

Skeletal muscle is organized hierarchically. Each muscle is wrapped in epimysium, contains fascicles wrapped in perimysium, each fascicle has many muscle fibers (cells) wrapped in endomysium, each fiber has many myofibrils, and each myofibril is a chain of sarcomeres.

Skeletal muscle has a beautiful Russian-doll organization — structures within structures, each level smaller than the last. Understanding this hierarchy is essential for NEET because questions can ask about any level.

• Muscle (whole organ) — wrapped by epimysium (dense connective tissue sheath)
• Fascicle (bundle of fibers) — wrapped by perimysium. Each muscle has many fascicles
• Muscle fiber (single cell) — wrapped by endomysium. Each fascicle has many fibers. Each fiber is a single multinucleated cell (10-100 μm diameter, up to 30 cm long!)
• Myofibril — thread-like structure running the length of the fiber. Each fiber contains 1000-2000 myofibrils
• Sarcomere — the repeating unit within a myofibril (Z-line to Z-line). The functional unit of contraction
• Cell membrane of muscle fiber = Sarcolemma. Cytoplasm = Sarcoplasm. ER = Sarcoplasmic reticulum (SR)

Sarcomere Anatomy

The sarcomere (Z-line to Z-line) shows alternating bands under the microscope. A-band (dark, contains myosin ± overlapping actin). I-band (light, actin only). H-zone (lighter stripe within A-band, myosin only). M-line (center of sarcomere). Z-line (boundary of sarcomere).

When you look at skeletal muscle under a microscope, you see alternating dark and light bands — this is what gives it the name 'striated' muscle. These bands arise from the organized arrangement of thick (myosin) and thin (actin) filaments within each sarcomere.

• Sarcomere = Z-line to Z-line (functional unit). Resting length: ~2.0-2.5 μm
• A-band (dark band) = Anisotropic band. Contains the entire length of thick (myosin) filaments + overlapping portions of thin filaments. Length does NOT change during contraction
• I-band (light band) = Isotropic band. Contains only thin (actin) filaments (no myosin overlap). Bisected by Z-line. Shortens during contraction
• H-zone = lighter central region within the A-band. Contains only thick (myosin) filaments (no actin overlap). Shortens during contraction
• M-line = dark line at the very center of the H-zone. Holds myosin filaments in place
• Z-line (Z-disc) = boundary of the sarcomere. Thin filaments are anchored here. 'Z' from German 'Zwischen' (between)

Quick recall

During muscle contraction, which bands/zones change in length?

Decrease: I-band (shrinks), H-zone (shrinks or disappears), sarcomere length (shortens). Stays the same: A-band (constant — myosin doesn't change length). This is the hallmark of the sliding filament model.

Actin and Myosin Filaments

Thin filaments: F-actin (two twisted chains of G-actin monomers) + tropomyosin (covers active sites on actin) + troponin (regulatory protein with 3 subunits: TnC binds Ca²⁺, TnT binds tropomyosin, TnI inhibits actin-myosin interaction). Thick filaments: myosin molecules with tail, head (cross-bridge with ATPase activity), and neck.

The sarcomere's contractile machinery consists of two types of protein filaments. Thin filaments are mainly actin (plus regulatory proteins tropomyosin and troponin). Thick filaments are made of myosin — a motor protein with a head that acts as the engine of contraction.

• Thin filament components: (1) F-actin — two helically wound chains of globular G-actin monomers. Each G-actin has a myosin-binding site (active site)
• (2) Tropomyosin — long rod-shaped protein that lies in the groove of F-actin and covers/masks the active sites on actin at rest (prevents unwanted contraction)
• (3) Troponin — a complex of 3 subunits sitting on tropomyosin at regular intervals: TnC (binds Ca²⁺), TnT (binds tropomyosin), TnI (inhibitory, prevents actin-myosin binding)
• Thick filament: made of ~200 myosin molecules. Each myosin has a tail (forms the filament backbone), a head (globular, projects outward as cross-bridge), and ATPase activity on the head
• Myosin head = cross-bridge — it physically connects thick and thin filaments during contraction

Mechanism of Muscle Contraction — Sliding filament theory and the cross-bridge cycle

Muscle contraction follows the sliding filament theory: actin slides over myosin, sarcomere shortens, but filament lengths stay the same. Steps: nerve impulse → neuromuscular junction → Ca²⁺ release from SR → Ca²⁺ binds troponin C → tropomyosin shifts → active sites exposed → myosin cross-bridge binds actin → power stroke → ATP-driven detachment and re-cocking. When ATP depletes after death, cross-bridges lock → rigor mortis.

Sliding Filament Theory

Proposed by H.E. Huxley and A.F. Huxley (1954). During contraction, thin (actin) filaments slide over thick (myosin) filaments toward the center of the sarcomere. Individual filament lengths DO NOT change — the overlap increases. This shortens the sarcomere, I-band, and H-zone while A-band stays constant.

The sliding filament theory (Huxley & Huxley, 1954) is one of biology's most elegant explanations. During contraction, actin filaments are pulled inward over stationary myosin filaments, toward the M-line. The filaments themselves don't get shorter — the overlap between them increases, which shortens the sarcomere.

• Thin (actin) filaments slide inward toward the center of the sarcomere (M-line)
• Thick (myosin) filaments remain stationary — they don't move or shorten
• Sarcomere shortens (Z-lines move closer together)
• I-band shortens (less actin-only region as actin slides into A-band)
• H-zone shortens or disappears (actin overlaps myosin completely in maximal contraction)
• A-band stays the same (myosin length unchanged) — the #1 NEET fact from this theory
• Energy source: ATP (hydrolyzed by myosin ATPase)

Steps of Muscle Contraction

The complete sequence: (1) Motor neuron fires action potential, (2) Neurotransmitter (ACh) released at neuromuscular junction, (3) Impulse spreads along sarcolemma and into T-tubules, (4) Ca²⁺ released from sarcoplasmic reticulum, (5) Ca²⁺ binds troponin-C, (6) Tropomyosin shifts, exposing active sites, (7) Cross-bridge formation, (8) Power stroke, (9) ATP-driven detachment and resetting.

Muscle contraction is a beautifully choreographed sequence of events. It starts with a nerve impulse and ends with a physical shortening of the muscle. Every step requires precise molecular coordination — a failure at any step means no contraction.

• Step 1: Motor neuron fires an action potential that travels to the nerve ending at the muscle
• Step 2: At the neuromuscular junction (motor end plate), the nerve releases acetylcholine (ACh) into the synaptic cleft
• Step 3: ACh binds receptors on the sarcolemma → generates action potential that spreads along sarcolemma and into T-tubules (transverse tubules — deep invaginations of sarcolemma)
• Step 4: T-tubule signal triggers sarcoplasmic reticulum (SR) to release stored Ca²⁺ ions into the sarcoplasm
• Step 5: Ca²⁺ binds to troponin-C (TnC) on the thin filament
• Step 6: Troponin changes conformation → pulls tropomyosin away from actin's active sites → active sites exposed
• Step 7: Myosin head (already energized by ATP hydrolysis, cocked position) binds to exposed active site → cross-bridge forms
• Step 8: Power stroke — myosin head pivots, pulling actin toward M-line. ADP + Pi released
• Step 9: New ATP binds myosin head → cross-bridge detaches → ATP hydrolyzed → head re-cocks (ready for next cycle)

Quick recall

What neurotransmitter is released at the neuromuscular junction?

Acetylcholine (ACh) — released by the motor neuron. It binds nicotinic receptors on the sarcolemma, depolarizing the muscle fiber. ACh is quickly broken down by acetylcholinesterase to prevent continuous stimulation.

Cross-Bridge Cycling and ATP

The cross-bridge cycle: (1) Energized myosin binds actin (cross-bridge), (2) Power stroke (head pivots, pulls actin, releases ADP+Pi), (3) ATP binds → cross-bridge detaches, (4) ATP hydrolyzed → head re-cocks. Cycle repeats ~5 times/second. ATP is needed for both contraction (power stroke cocking) and relaxation (detachment + Ca²⁺ re-uptake). Rigor mortis occurs when ATP depletes after death.

The cross-bridge cycle is the molecular engine of contraction. Each cycle of attach-pivot-detach-reset pulls actin a tiny bit (~10 nm). With thousands of cross-bridges cycling simultaneously and repeatedly, these tiny movements add up to powerful macroscopic contraction.

• Attachment: Energized (cocked) myosin head binds to actin's active site → cross-bridge forms
• Power stroke: Myosin head pivots from ~90° to ~45° → pulls actin toward M-line. ADP and Pi released from myosin head
• Detachment: Fresh ATP molecule binds myosin head → reduces myosin's affinity for actin → cross-bridge releases
• Re-cocking: Myosin ATPase hydrolyzes ATP → ADP + Pi. Energy stored in myosin head (returns to 90° cocked position). Ready for next cycle
• Each cross-bridge cycle pulls actin ~10 nm and consumes 1 ATP
• Cycle repeats about 5 times per second per cross-bridge during sustained contraction

Quick recall

Why does rigor mortis occur after death?

After death, cellular respiration stops → ATP is depleted. Without ATP, myosin heads cannot detach from actin filaments → cross-bridges remain permanently locked → muscles become stiff and rigid. Rigor mortis eventually resolves when decomposition enzymes break down the muscle proteins.

Disorders of Muscular and Skeletal System — From autoimmune attacks to brittle bones

Key disorders: Myasthenia gravis (autoimmune, ACh receptors destroyed → muscle weakness), Muscular dystrophy (genetic, progressive muscle degeneration due to defective dystrophin), Tetany (low blood Ca²⁺ → sustained muscle spasms), Arthritis (joint inflammation — osteoarthritis from wear, rheumatoid from autoimmunity), Osteoporosis (decreased bone density, fragile bones, common in postmenopausal women), Gout (uric acid crystals in joints → painful inflammation).

Myasthenia Gravis

An autoimmune disorder where antibodies destroy acetylcholine (ACh) receptors at the neuromuscular junction. Fewer receptors → weaker signal → progressive muscle weakness and fatigue, especially in facial and eye muscles. 'Myasthenia' = muscle weakness, 'gravis' = severe.

Myasthenia gravis is a classic autoimmune neuromuscular disease. Your own immune system produces antibodies that attack and destroy nicotinic ACh receptors on the motor end plate (postsynaptic membrane of the neuromuscular junction). With fewer receptors, the muscle receives weaker signals and contracts weakly.

• Autoimmune — antibodies target ACh receptors at the neuromuscular junction
• Fewer functional receptors → decreased response to ACh → weak contractions
• Symptoms: drooping eyelids (ptosis), double vision, difficulty chewing/swallowing, progressive skeletal muscle weakness
• Fatigue worsens with activity and improves with rest (characteristic feature)
• Facial and eye muscles typically affected first
• Treatment includes anticholinesterase drugs (prevent ACh breakdown → more ACh available to compete for remaining receptors)

Muscular Dystrophy

A group of genetic (hereditary) disorders characterized by progressive degeneration and weakening of skeletal muscles. Most common type: Duchenne muscular dystrophy (DMD) — X-linked recessive, caused by defective/absent dystrophin protein. Affects mainly boys.

Muscular dystrophy refers to a group of genetic diseases where muscles progressively degenerate and weaken over time. Unlike myasthenia gravis (which attacks the junction), muscular dystrophy attacks the muscle fiber itself. The most common and severe form is Duchenne Muscular Dystrophy (DMD).

• Genetic (hereditary) — NOT autoimmune. Caused by mutations in genes coding for muscle structural proteins
• Duchenne MD (DMD): most common type. X-linked recessive → primarily affects boys (1 in 3500 male births)
• DMD is caused by absence/defect of dystrophin — a protein that anchors the cytoskeleton to the cell membrane. Without it, muscle fibers tear during contraction
• Progressive weakness: starts in legs and pelvis, eventually affects all voluntary muscles
• Muscle fibers degenerate and are replaced by fat and fibrous tissue
• Currently no cure — management focuses on physiotherapy and delaying progression

Tetany

Tetany is a condition caused by low blood calcium (hypocalcaemia). Since Ca²⁺ is essential for regulating muscle contraction, low Ca²⁺ makes muscles hyperexcitable, leading to sustained involuntary spasms, especially in hands and feet. Often due to hypoparathyroidism.

Tetany occurs when blood calcium levels drop below normal (hypocalcaemia). Since Ca²⁺ ions are critical for muscle contraction (binding troponin-C) AND for nerve impulse transmission, low Ca²⁺ makes both nerves and muscles hyperexcitable. The result: rapid, sustained, involuntary muscle contractions — spasms.

• Caused by low blood Ca²⁺ (hypocalcaemia) — usually below 4.3 mEq/L
• Common cause: hypoparathyroidism (underactive parathyroid glands → insufficient PTH → low Ca²⁺ absorption)
• Symptoms: involuntary sustained spasms, especially in hands and feet (carpopedal spasm), tingling, numbness
• Low Ca²⁺ → increased membrane permeability to Na⁺ → nerves fire spontaneously → muscles contract uncontrollably
• Not the same as tetanus (tetanus is a bacterial disease caused by Clostridium tetani toxin)

Arthritis, Osteoporosis & Gout

Arthritis (joint inflammation — osteoarthritis from wear-and-tear, rheumatoid from autoimmune attack on synovial membrane). Osteoporosis (decreased bone mineral density → fragile, porous bones, common in postmenopausal women due to estrogen drop). Gout (accumulation of uric acid crystals in joints → intense inflammatory pain, especially in big toe).

These three conditions affect the skeletal system and are NCERT-specified. Each has a distinct mechanism: arthritis inflames joints, osteoporosis weakens bones, and gout deposits crystals in joints. Together they account for most musculoskeletal NEET questions.

• Osteoarthritis: wear-and-tear degeneration of articular cartilage at joints. Affects weight-bearing joints (knees, hips). Common in elderly
• Rheumatoid arthritis: autoimmune — immune system attacks the synovial membrane → chronic inflammation, pain, joint deformity. Can affect any age
• Osteoporosis: decreased bone mineral density → bones become porous, brittle, fracture-prone. Common in postmenopausal women (estrogen drop reduces calcium deposition in bones). Also caused by low Ca²⁺ intake, vitamin D deficiency, sedentary lifestyle
• Gout: excess uric acid in blood (hyperuricaemia) → uric acid crystals deposit in joints → intense pain, swelling, redness. Classically affects the big toe (first metatarsophalangeal joint) first
• Gout is associated with purine-rich diet (red meat, organ meat, beer) and kidney dysfunction

Quick recall

A 65-year-old postmenopausal woman fractures her hip from a minor fall. What is the most likely underlying condition?

Osteoporosis — decreased bone mineral density makes bones fragile and fracture-prone. Postmenopausal women are at highest risk due to estrogen decline (estrogen normally promotes bone calcium deposition).