BP104T Unit 2 – Tissue Organization & Integumentary System
Unit 2 details the structural architecture of the human skeleton (206 bones in adult axial and appendicular divisions), bone histology and remodeling, structural and functional classification of joints, sarcomere ultrastructure, excitation-contraction coupling at the neuromuscular junction, the sliding filament theory, and rigor mortis.
📋 Prescribed Syllabus Topics
📖 Detailed Lecture Notes & Solved Examples
11. Skeletal System Architecture, Bone Histology & Remodeling
Dedicated Webpage➔The adult human skeleton comprises 206 bones divided into the Axial Skeleton (80 bones: skull, vertebral column, ribs, sternum) and the Appendicular Skeleton (126 bones: pectoral girdle, upper limbs, pelvic girdle, lower limbs). Bone is a specialized vascular mineralized connective tissue. Compact (cortical) bone is organized into microscopic structural cylinders called Osteons (Haversian Systems) containing central Haversian canals, concentric lamellae, lacunae housing mature Osteocytes, and radiating canaliculi. Bone tissue is dynamically maintained by four cell types: Osteoprogenitor cells (stem cells), Osteoblasts (bone-forming cells synthesizing osteoid matrix), Osteocytes (matrix-maintaining mechanosensory cells), and Osteoclasts (multinucleated bone-resorbing macrophages secreting HCl and cathepsin K).Adult body me 206 haddiyan hoti hain (80 Axial skull/ribs me aur 126 Appendicular haath/pair me). Haddi ke andar Haversian System (Osteon) hota hai. Osteoblasts nayi haddi banate hain, aur Osteoclasts purani haddi ko gholkar calcium khoon me bhejte hain. PTH hormone calcium badhata hai aur Calcitonin haddi me calcium jama karta hai.
▶Hormonal Regulation of Calcium Homeostasis & Bone Remodeling
Dynamic interplay between Parathyroid Hormone (PTH) and Calcitonin.
- •Haversian Canal: Central longitudinal vascular channel containing nerve and blood vessels; connected horizontally by Volkmann's canals.
- •Ossification Types: Intramembranous (cranial flat bones form directly within mesenchymal membranes) and Endochondral (long bones form from hyaline cartilage templates).
- •Bone Remodeling Cycle: Activation $\rightarrow$ Resorption (by osteoclasts) $\rightarrow$ Reversal $\rightarrow$ Formation (by osteoblasts) $\rightarrow$ Mineralization.
22. Classification of Articulations (Joints)
Dedicated Webpage➔An articulation (joint) is a junction between two or more bones or cartilage. Joints are classified structurally based on binding material and presence of a joint cavity, and functionally based on degree of movement allowed.Joints 3 tarah ke hote hain: (1) Fibrous (bina cavity wale, jaise skull ke achal sutures), (2) Cartilaginous (thoda hilne wale, jaise reede ki haddi ke intervertebral discs), aur (3) Synovial (poori tarah hilne wale freely movable joints jaise knee aur shoulder jinme friction kam karne ke liye Synovial Fluid bhara hota hai).
▶Structural & Functional Joint Classification Scheme
The three anatomical classes of joints.
- •Synovial Fluid: Viscous, egg-white consistency fluid rich in Hyaluronic Acid and Lubricin that reduces friction and provides chondrocyte nutrition.
- •Articular Cartilage: Avascular hyaline cartilage covering bone ends (damaged in Osteoarthritis).
33. Sarcomere Ultrastructure, Neuromuscular Junction & Sliding Filament Theory
Dedicated Webpage➔Skeletal muscle fibers are multinucleated cylindrical cells packed with myofibrils. The Sarcomere is the functional contractile unit of a muscle fiber, bounded by two adjacent Z-discs. It contains Thin Filaments (composed of Actin, Tropomyosin, and the Troponin complex: TnT, TnI, TnC) and Thick Filaments (composed of hundreds of Myosin motor molecules with flexible heads containing ATP-binding and actin-binding sites). The Sliding Filament Theory (Huxley, 1954) states that during muscle contraction, thin actin filaments slide past thick myosin filaments toward the central M-line, narrowing the I-band and H-zone while the A-band length remains constant.Muscle ki sabse chhoti contractile unit Sarcomere hai. Nerve signal aate hi Acetylcholine (ACh) release hota hai jisse Sarcoplasmic Reticulum se Calcium (Ca2+) nikalta hai. Calcium Troponin-C se judta hai aur Actin-Myosin ke beech cross-bridge banta hai jisse sliding hoti hai aur muscle sikudti hai. Death ke baad jab ATP khatam ho jata hai, toh cross-bridge alag nahi ho pata jisse shareer akad jata hai (Rigor Mortis).
▶Excitation-Contraction Coupling & Cross-Bridge Cycle Sequence
From somatic nerve impulse to force generation.
- •A-band: Length of thick myosin filaments (remains constant during contraction).
- •I-band: Region containing only thin actin filaments (shortens during contraction).
- •H-zone: Central region containing only thick filaments (narrows/disappears during contraction).
- •Rigor Mortis: After death, cellular ATP synthesis ceases -> $Ca^{2+}$ leaks from SR, binding TnC and forming actin-myosin cross-bridges. Because ATP is required for myosin head detachment, muscles remain locked in a rigid state of rigor mortis for 24-48 hours until autolytic lysosomal enzymes degrade the myofilaments.
- In 10-mark questions on Muscle Contraction, draw the Sarcomere (Z-disc, A-band, I-band, H-zone, M-line) before and after contraction, and list the 4 steps of the cross-bridge cycle.
- Explain the exact biochemical mechanism of Rigor Mortis.