Types of Movement
Movement vs Locomotion: All locomotions are movements, but all movements are not locomotions.
Types of Cellular Movement
- Amoeboid Movement: Exhibited by specialized cells like macrophages and leucocytes in blood. Effected by pseudopodia and involves cytoskeletal elements like microfilaments
- Ciliary Movement: Occurs in internal tubular organs lined by ciliated epithelium. Aids in dust removal in trachea and passage of ova in female reproductive tract
- Muscular Movement: Essential for movement of limbs, jaws, tongue, and locomotion
Muscle Structure and Types
Characteristics and Classification
Muscles are of mesodermal origin and contribute to 40-50% of adult human body weight.
Key Properties: Excitability, contractility, extensibility, and elasticity.
| Feature | Skeletal Muscle | Visceral (Smooth) Muscle | Cardiac Muscle |
|---|---|---|---|
| Location | Associated with skeletal components | Inner walls of hollow visceral organs (alimentary canal, reproductive tract) | Heart |
| Appearance | Striated (striped) | Non-striated, smooth | Striated, branching pattern |
| Control | Voluntary NEET 2011 | Involuntary NEET 2011 | Involuntary |
| Function | Locomotory actions, body postures | Transportation of food and gametes | Pumping of heart |
Microanatomy of Skeletal Muscle
- Skeletal muscle comprises muscle bundles (fascicles) held together by collagenous connective tissue layer called fascia NEET 2023
- Muscle fibres: Lined by sarcolemma enclosing sarcoplasm. They are syncitium (multinucleated)
- Sarcoplasmic Reticulum: Storehouse of calcium ions (Ca⁺⁺) NEET 2023
- Myofibrils (Myofilaments): Contain alternate dark and light bands due to two proteins: Actin and Myosin NEET 2023
Band Structure of Myofibrils
- I-band (Isotropic): Light band, contains Actin (thin filaments)
- A-band (Anisotropic): Dark band, contains Myosin (thick filaments)
- Z-line: Elastic fibre in centre of each I-band that firmly attaches to actin
- M-line: Thin fibrous membrane holding thick filaments together in middle of A-band
- Sarcomere: Portion between two successive Z-lines; the functional unit of contraction 1998, 2001, NEET 2023
- H-zone: Central gap of thick (myosin) filament in A-band not overlapped by thin (actin) filaments NEET 2013
Contractile Proteins
Actin (Thin Filament)
- Made of two helically wound 'F' (filamentous) actins, which are polymers of monomeric 'G' (Globular) actins
- Tropomyosin: Two filaments run close to 'F' actins throughout its length
- Troponin: Complex protein distributed at regular intervals. Crucial function: In resting state, a subunit masks the active binding sites for myosin on actin filaments NEET 2013
Myosin (Thick Filament)
- A polymerised protein made of monomeric proteins called Meromyosins
- Consists of two parts: Heavy Meromyosin (HMM) - globular head + short arm, and Light Meromyosin (LMM) - tail
- The HMM component projects outward to form the cross arm
- High-yield: The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin 2006
Mechanism of Muscle Contraction
Sliding Filament Theory
Contraction occurs by the sliding of thin (actin) filaments over thick (myosin) filaments. Filaments slide past each other; they do not shorten themselves 2015
Steps of Contraction
- Initiation: CNS sends signal via motor neuron. Junction between motor neuron and sarcolemma is the neuromuscular junction (motor-end plate) 2014, 2024 Re
- Action Potential: Release of neurotransmitter (Acetylcholine) generates action potential in sarcolemma 2024 Re
- Calcium Release: Causes release of Ca⁺⁺ from sarcoplasmic reticulum into sarcoplasm
- Unmasking: Ca⁺⁺ binds to subunit of troponin on actin, removing masking of active sites for myosin NEET 2013, 2024 Re
- Cross Bridge Formation: Utilizing energy from ATP hydrolysis, myosin head binds to exposed active sites on actin 2024 Re
- Sliding: Attached actin is pulled towards centre of A-band, pulling Z-lines inward and shortening the sarcomere (contraction)
- Breaking: A new ATP binds, breaking the cross-bridge
Changes During Sarcomere Shortening
2001, 2021, 2022 Re
- I-bands: Reduce in length
- H-zone: Disappears/decreases
- Z-lines: Come closer together
- A-bands: Retain their length (Does not change)
Muscle Fatigue & Fibres
Repeated activation causes anaerobic breakdown of glycogen, leading to lactic acid accumulation and fatigue.
Tetanus: Lack of relaxation between successive stimuli in sustained contraction NEET 2016
| Feature | Red Muscle Fibres | White Muscle Fibres |
|---|---|---|
| Myoglobin Content | High (reddish appearance) | Low (pale or whitish) |
| Mitochondria | High (aerobic muscles) | Few |
| Sarcoplasmic Reticulum | Low | High |
| Energy Dependence | Aerobic processes | Anaerobic processes |
Skeletal System
Consists of 206 bones and a few cartilages. Divided into Axial and Appendicular skeletons.
Axial Skeleton (80 bones)
Skull (22 bones)
- Cranial (8) + Facial (14)
- Hyoid: Single U-shaped bone at base of buccal cavity
- Ear Ossicles: Malleus, Incus, Stapes (3 per ear)
- Dicondylic Skull: Articulates with vertebral column via two occipital condyles
Vertebral Column (26 units)
Formula: Cervical (7), Thoracic (12), Lumbar (5), Sacral (1-fused), Coccygeal (1-fused)
Crucial detail: Number of cervical vertebrae is seven in almost all mammals 1993, 2007
- First vertebra is the Atlas, articulating with occipital condyles
Sternum & Ribs
- Sternum: Flat bone on ventral midline of thorax 2021
- Ribs (12 pairs): Bicephalic (two dorsal articulation surfaces)
- True Ribs (1st-7th pairs): Attached dorsally to vertebrae and ventrally to sternum via hyaline cartilage 2017
- Vertebrochondral / False Ribs (8th, 9th, 10th pairs): Do not articulate directly with sternum; join 7th rib via hyaline cartilage NEET 2019, NEET 2020
- Floating Ribs (11th and 12th pairs): Not connected ventrally 2010, NEET 2019
Appendicular Skeleton (126 bones)
Limbs (30 bones each)
- Forelimb: Humerus, Radius, Ulna, Carpals (8 wrist bones), Metacarpals (5 palm bones), Phalanges (14 digits) NEET 2023
- Hindlimb: Femur (thigh bone; longest), Tibia, Fibula, Tarsals (7 ankle bones), Metatarsals (5), Phalanges (14 digits) NEET 2023
- Patella: Cup-shaped bone covering knee ventrally
Pectoral Girdle (Clavicle + Scapula)
- Scapula: Large triangular flat bone in dorsal thorax (between 2nd and 7th ribs) 2021
- Acromion: Flat expanded process of scapula spine. Clavicle articulates with it NEET 2020
- Glenoid Cavity: Depression below acromion; articulates with head of humerus to form shoulder joint 2015, NEET 2020, 2024 Re
Pelvic Girdle
- Consists of two coxal bones. Each coxal bone forms by fusion of three bones: Ilium, Ischium, and Pubis 2009
- Acetabulum: Cavity at fusion point where femur articulates
- Pubic symphysis contains fibrous cartilage
Joints
| Joint Type | Characteristics | Examples / High-Yield Matches |
|---|---|---|
| Fibrous Joints | Do not allow any movement 1994, 2015, 2023 | Flat skull bones fusing to form cranium (sutures) 2021, 2023 |
| Cartilaginous Joints | Limited movement; joined by cartilages 2015, 2023, 2024 | Joint between adjacent vertebrae in vertebral column 2021, 2023, 2024 |
| Synovial Joints Fluid-filled synovial cavity; considerable movement |
Ball and Socket: Extensive rotation 2023, 2024 Re | Between Humerus and Pectoral girdle |
| Hinge Joint: Uni-directional movement 2009, 2024 Re | Knee joint, Elbow joint | |
| Pivot Joint: Rotational movement 1999, NEET 2017, 2024 Re | Between Atlas and Axis | |
| Gliding Joint: Sliding movement 2014, 2024 Re | Between the Carpals | |
| Saddle Joint: Greater freedom of movement Odisha 2019, 2023 | Between Carpal and Metacarpal of thumb |
Disorders of Muscular and Skeletal System
- Myasthenia Gravis: Autoimmune disorder affecting neuromuscular junction. Leads to fatigue, weakening, and paralysis of skeletal muscle NEET 2016, 2021, 2024
- Muscular Dystrophy: Progressive degeneration of skeletal muscle, mostly due to genetic disorder NEET 2019, NEET 2020
- Tetany: Rapid spasms (wild contractions) in muscle due to low Ca⁺⁺ in body fluid NEET 2020
- Arthritis: Inflammation of joints
- Osteoporosis: Age-related disorder characterized by decreased bone mass and increased chances of fractures. Crucial detail: Decreased levels of estrogen is a common cause 2010, NEET 2013, NEET 2020
- Gout: Inflammation of joints due to accumulation of uric acid crystals NEET 2013, NEET 2020, 2022 Re
Note: Systemic Lupus Erythematosus (SLE) and Rheumatoid arthritis are mentioned in PYQs as Autoimmune disorders alongside Myasthenia gravis 2024