Summary Notes: Musculoskeletal, Circulatory, Respiratory, and Digestive Systems

Musculoskeletal System

• Components: bones, skeletal muscles, tendons, joints, ligaments, cartilage, and other connective tissues.

• Functions of bones:

  • Framework and support: Provides the structural framework for the body and supports soft tissues.

  • Protection of organs: Encloses and protects vital organs (e.g., skull protects brain, rib cage protects heart and lungs).

  • Mineral storage: Serves as a reservoir for essential minerals, primarily calcium (Ca) and phosphate, which can be released into the bloodstream to maintain homeostasis.

  • Triglyceride storage: Yellow marrow, primarily found in the medullary cavities of long bones, stores triglycerides (fats) as a chemical energy reserve.

  • Hematopoiesis: Red marrow, found in spongy bone, is the site of blood cell formation, producing red blood cells, white blood cells, and platelets.

  • Levers for movement: Bones act as rigid levers, and with muscle attachments, they facilitate body movement across joints.

• Bone tissue composition:

  • Osseous tissue: Consists of a unique extracellular matrix and specialized cells.

  • Matrix: Composed of organic components (collagen fibers giving flexibility and tensile strength, ground substance) and inorganic components (calcium phosphate crystals, primarily hydroxyapatite, providing hardness and compressional strength).

  • Specialised cells:

    • Osteoprogenitor cells: Undifferentiated stem cells that can develop into osteoblasts.

    • Osteoblasts: Bone-forming cells that synthesize and secrete collagen fibers and other organic components of bone matrix (osteoid), initiating calcification.

    • Osteocytes: Mature bone cells, derived from osteoblasts, that maintain the bone matrix and sense mechanical stress.

    • Osteoclasts: Large, multinucleated cells responsible for bone resorption (breakdown of bone matrix), releasing minerals into the blood.

• Bone structure:

  • Outer compact bone: Dense, solid bone tissue forming the outer layer, providing strength and protection. Organized into osteons (Haversian systems).

  • Inner spongy bone (cancellous bone): Lighter, porous bone tissue found in the interior of bones. Consists of trabeculae (thin columns of bone) that provide strength with minimal weight and house red bone marrow.

  • Periosteum: A tough, fibrous membrane covering the outer surface of bone (except at articular surfaces). Contains osteoprogenitor cells, osteoblasts, blood vessels, and nerves. Involved in bone growth, repair, and nutrition.

  • Endosteum: A thinner membrane lining the medullary cavity and trabeculae of spongy bone. Contains osteoprogenitor cells and osteoclasts.

  • Compact bone organization: Osteons consist of concentric lamellae (rings of matrix) around a central Haversian canal containing blood vessels and nerves.

  • Spongy bone organization: Trabeculae are irregularly arranged lamellae and osteocytes, lacking Haversian systems.

• Bone growth and remodelling:

  • Interstitial growth: Lengthening of long bones at the epiphyseal plates (growth plates) through cartilage cell division and subsequent ossification.

  • Appositional growth: Increase in bone diameter (thickness) through the addition of new bone matrix by osteoblasts on the bone surface beneath the periosteum.

  • Bone remodelling: Continuous process throughout life involving the coordinated action of osteoclasts (resorption) and osteoblasts (formation) to repair microfractures, adapt to mechanical stress, and maintain mineral homeostasis.

• Growth regulation:

  • Growth hormone (GH): Produced by the pituitary gland, stimulates liver to produce insulin-like growth factors (IGFs), which promote bone growth (especially epiphyseal plate activity).

  • Thyroid hormone (TH): Modulates the activity of GH, ensuring proper proportions during skeletal development.

  • Sex hormones (estrogen, testosterone): Promote significant growth spurts during puberty and eventually cause the ossification and closure of the epiphyseal plates, halting longitudinal bone growth.

• Bone shapes: Classified based on their general morphology.

  • Long bones: Longer than they are wide; primarily weight-bearing (e.g., femur, humerus).

  • Short bones: Roughly cube-shaped; provide stability and some movement (e.g., carpals, tarsals). The patella is a sesamoid bone, a type of short bone forming within tendons.

  • Flat bones: Thin, flattened, and often curved; provide protection and broad surfaces for muscle attachment (e.g., skull bones, sternum, scapulae).

  • Irregular bones: Complex shapes that don't fit other categories (e.g., vertebrae, hip bones).

• Fractures and repair (brief):

  • Types: Categorized by the nature of the break and skin integrity.

    • Closed (simple): Bone breaks, but the skin remains intact.

    • Open (compound): Bone breaks and pierces through the skin, increasing infection risk.

    • Comminuted: Bone fragments into three or more pieces, common in older individuals.

    • Compression: Bone is crushed, common in porous bones (osteoporosis) subjected to extreme trauma (e.g., in vertebrae).

    • Greenstick: Incomplete break, like a green twig. Common in children whose bones are more flexible.

    • Spiral: Ragged break occurs when excessive twisting forces are applied to a bone, common in sports injuries.

    • Epiphyseal: Fracture along the epiphyseal plate, potentially affecting bone growth in children.

  • Repair stages:

    1. Haematoma formation: Blood vessels rupture, forming a large blood clot (haematoma) at the fracture site within hours. Bone cells deprived of nutrition die.

    2. Fibrocartilaginous callus formation: Within days, phagocytes clean up debris. Fibroblasts and osteoprogenitor cells invade the fracture site, forming a soft callus of collagen fibers and cartilage that bridges the broken ends.

    3. Bony callus formation: Osteoblasts begin to form spongy bone within the fibrocartilaginous callus, converting it into a hard, bony callus over several weeks.

    4. Bone remodelling: Over months to years, the bony callus is remodelled by osteoclasts and osteoblasts. Excess material is removed, and compact bone is laid down to reconstruct the shaft walls, restoring the bone's original structure and strength.

• Axial vs appendicular skeleton:

  • Axial skeleton: Forms the central axis of the body, including the skull (cranium and facial bones), vertebral column (spine), and rib cage (sternum and ribs). Provides protection for vital organs and sites for muscle attachment.

  • Appendicular skeleton: Consists of the limbs (upper and lower) and their associated girdles (pectoral/shoulder girdle and pelvic girdle) that attach the limbs to the axial skeleton. Primarily involved in locomotion and manipulation of the environment.

Joints

• Functional classification by movement:

  • Synarthrosis: Immovable joints, providing strong protection (e.g., sutures between skull bones, gomphosis of teeth in sockets).

  • Amphiarthrosis: Slightly movable joints, offering both stability and limited flexibility (e.g., intervertebral discs, pubic symphysis).

  • Diarthrosis: Freely movable joints, characteristic of all synovial joints, allowing a wide range of motion.

• Structural classification by connective tissue and joint cavity:

  • Fibrous joints: Bones joined by dense fibrous connective tissue; no joint cavity. Mostly immovable (e.g., sutures, syndesmoses between tibia and fibula) or slightly movable.

  • Cartilaginous joints: Bones united by cartilage (hyaline or fibrocartilage); no joint cavity. Immovable (e.g., epiphyseal plates, synchondroses) or slightly movable (e.g., pubic symphysis, intervertebral discs, symphyses).

  • Synovial joints: Bones separated by a fluid-filled joint cavity; freely movable. The most common joint type in the appendicular skeleton.

• Synovial joint features and function:

  • Articular capsule: Encloses the joint cavity. Composed of an outer fibrous layer (dense irregular connective tissue for strength) and an inner synovial membrane (areolar connective tissue that produces synovial fluid).

  • Joint cavity: A space filled with synovial fluid, reducing friction.

  • Synovial fluid: A viscous, egg-white-like fluid that lubricates the articular cartilages, nourishes chondrocytes, and absorbs shock.

  • Articular cartilage: Smooth layer of hyaline cartilage covering the ends of bones within the joint, minimizing friction and absorbing compression.

  • Reinforcing ligaments: Bands of fibrous connective tissue that strengthen the joint capsule and prevent excessive movements.

  • Sensory nerve supply and blood vessels: Nerves detect pain and joint position; blood vessels supply nutrients to surrounding tissues.

  • Additional structures:

    • Menisci (articular discs): Pads of fibrocartilage that improve the fit between bone ends, stabilize the joint, and reduce wear (e.g., in the knee).

    • Bursae: Flattened fibrous sacs lined with synovial membrane and containing synovial fluid, reducing friction where ligaments, muscles, skin, bones, or tendons rub together.

    • Tendon sheaths: Elongated bursae that wrap completely around a tendon subjected to friction (e.g., in the wrist and ankle).

    • Fat pads: Cushions of adipose tissue found in some joints (e.g., hip, knee), providing additional cushioning.

• Six types of synovial joints and movements:

  • Plane joints: Allow only gliding (translation) movements between flat articular surfaces (e.g., intercarpal, intertarsal joints).

  • Pivot joints: Allow rotation around a single longitudinal axis (e.g., proximal radioulnar joint for pronation/supination; atlantoaxial joint between atlas and axis for head rotation).

  • Condylar (ellipsoidal) joints: Oval articular surface of one bone fits into an oval depression of another, allowing flexion/extension, adduction/abduction, and circumduction (e.g., metacarpophalangeal joints/knuckles, radiocarpal/wrist joint).

  • Saddle joints: Each articular surface has both concave and convex areas, resembling a saddle; allows flexion/extension, adduction/abduction, and circumduction (e.g., carpometacarpal joint of the thumb, giving the thumb its unique opposition capability).

  • Hinge joints: Cylindrical projection of one bone fits into a trough-shaped surface on another, allowing angular movement in one plane (flexion/extension) (e.g., elbow, knee, interphalangeal joints).

  • Ball-and-socket joints: Spherical head of one bone articulates with a cup-like socket of another, allowing greatest range of motion: flexion/extension, abduction/adduction, circumduction, and rotation (e.g., shoulder, hip joints).

• Movements and special terms:

  • Flexion: Decreasing the angle of a joint.

  • Extension: Increasing the angle of a joint.

  • Abduction: Moving a limb away from the midline of the body.

  • Adduction: Moving a limb toward the midline of the body.

  • Circumduction: Multiplanar movement: flexion, abduction, extension, and adduction sequentially to create a cone-shaped path.

  • Rotational: Turning a bone around its own long axis (e.g., medial/lateral rotation of the humerus).

  • Pronation/Supination: Rotational movements of the forearm. Pronation: palm faces posteriorly. Supination: palm faces anteriorly (anatomical position).

  • Protraction/Retraction: Non-angular movements in the anterior/posterior direction (e.g., jaw).

  • Elevation/Depression: Lifting/lowering a body part superiorly/inferiorly (e.g., shrugging/dropping shoulders).

  • Inversion/Eversion: Movements of the sole of the foot medial/lateral.

  • Opposition: Movement of the thumb to touch the tips of other fingers.

Skeletal Muscles

• Structure:

  • Skeletal muscles are organs composed of muscle fibres (individual muscle cells), connective tissue sheaths, blood vessels, and nerves.

  • Connective tissue sheaths organize and protect the muscle:

    • Endomysium: Delicate connective tissue surrounding each individual muscle fibre.

    • Perimysium: Fibrous connective tissue enclosing bundles of muscle fibres called fascicles.

    • Epimysium: Dense irregular connective tissue surrounding the entire skeletal muscle.

  • Myofibrils: Rod-like cytoplasmic organelles densely packed within each muscle fibre. They are the contractile elements.

  • Sarcomeres: The fundamental contractile units of a myofibril, extending from one Z disc to the next.

  • Thick filaments: Composed primarily of myosin protein, possessing myosin heads that form cross-bridges with actin.

  • Thin filaments: Composed primarily of actin protein, along with regulatory proteins troponin and tropomyosin.

  • Troponin-tropomyosin complex: In a relaxed muscle, tropomyosin blocks the myosin-binding sites on actin; troponin binds to calcium ions, moving tropomyosin away from these sites during contraction.

• Neuromuscular apparatus:

  • Somatic nervous system: Contains motor neurons that exclusively control skeletal muscle contraction.

  • Lower motor neurons: Their axons extend from the spinal cord to muscle fibres, forming specialized synapses called neuromuscular junctions (NMJs).

  • Acetylcholine (ACh) release: At the NMJ, the motor neuron releases the neurotransmitter acetylcholine into the synaptic cleft.

  • Depolarisation: ACh binds to receptors on the muscle fibre's sarcolemma (cell membrane), causing an influx of sodium ions ($Na^+$) and depolarizing the sarcolemma (end-plate potential), which triggers an action potential and muscle contraction.

• Contraction mechanism (Sliding Filament Model):

  • Calcium ($Ca^{2+}$) release: The muscle action potential propagates along the sarcolemma and into T-tubules, triggering the release of $Ca^{2+}$ from the sarcoplasmic reticulum (SR) into the sarcoplasm.

  • Cross-bridge cycling: $Ca^{2+}$ binds to troponin, causing a conformational change that moves tropomyosin away from the myosin-binding sites on actin.

    1. Myosin heads bind actin: Energized myosin heads (carrying ADP and $P_i$) attach to the exposed binding sites on actin, forming a cross-bridge.

    2. Power stroke: As ADP and $P_i$ are released, the myosin head pivots and pulls the thin filament toward the M line (center of the sarcomere).

    3. ATP binding and detachment: A new ATP molecule binds to the myosin head, causing it to detach from actin.

    4. ATP hydrolysis and re-cocking: ATP is hydrolyzed by myosin ATPase into ADP and $P_i$, re-energizing (

Respiratory System

• Overall function: Facilitates gas exchange between the body and the external environment, primarily bringing in oxygen ($O<em>2$) and expelling carbon dioxide ($CO</em>2$).

• Components: Divided into upper and lower respiratory tracts.

  • Upper respiratory tract: Nose, pharynx, and associated structures.

  • Lower respiratory tract: Larynx, trachea, bronchi, and lungs.

• Key functions:

  • Gas exchange: In the lungs, $O<em>2$ diffuses from the alveoli into the blood, and $CO</em>2$ diffuses from the blood into the alveoli to be exhaled.

  • Regulation of blood pH: By altering $CO<em>2$ exhalation, the respiratory system can quickly adjust blood $ext{pH}$. Increased $CO</em>2$ leads to lower $ext{pH}$ (more acidic); decreased $CO_2$ leads to higher $ext{pH}$ (more alkaline).

  • Voice production: Larynx contains vocal cords that produce sounds as air passes over them.

  • Olfaction: Receptors for the sense of smell located in the nasal cavity.

  • Filtering, warming, and moistening air: Nasal passages, cilia, and mucus humidify and clean incoming air.

• Respiratory pathway:

  1. Nose/Nasal cavity: Air enters, is warmed, filtered, and moistened.

  2. Pharynx (throat): Passageway for air and food.

  3. Larynx (voice box): Contains vocal cords; epiglottis prevents food from entering the trachea.

  4. Trachea (windpipe): Extends from the larynx, branches into bronchi. C-shaped cartilage rings prevent collapse.

  5. Bronchi: Primary bronchi branch into secondary and tertiary bronchi, leading to smaller bronchioles.

  6. Lungs: Contain alveoli for gas exchange.

  7. Alveoli: Tiny air sacs where gas exchange occurs across the alveolar-capillary membrane. Surrounded by capillaries.

• Mechanics of breathing (Ventilation):

  • Inspiration (inhalation): Diaphragm contracts and flattens, external intercostal muscles contract, expanding the thoracic cavity. This decreases intrapulmonary pressure, drawing air into the lungs.

  • Expiration (exhalation): Diaphragm and external intercostal muscles relax, decreasing thoracic volume and increasing intrapulmonary pressure, expelling air from the lungs. Forced exhalation involves internal intercostals and abdominal muscles.

Digestive System

• Overall function: Ingests food, breaks it down into absorbable nutrients, absorbs these nutrients into the blood, and eliminates indigestible waste.

• Two main groups of organs:

  • Gastrointestinal (GI) tract (alimentary canal): Continuous tube extending from the mouth to the anus. Includes mouth, pharynx, esophagus, stomach, small intestine, and large intestine.

  • Accessory digestive organs: Teeth, tongue, salivary glands, liver, gallbladder, and pancreas. These organs contribute to breakdown and absorption but are not part of the GI tract itself.

• Digestive processes:

  1. Ingestion: Taking food into the mouth.

  2. Propulsion: Movement of food through the GI tract (e.g., swallowing, peristalsis).

  3. Mechanical digestion: Physical breakdown of food (e.g., chewing/mastication, churning in stomach, segmentation in small intestine).

  4. Chemical digestion: Enzymatic breakdown of complex food molecules into their chemical building blocks.

  5. Absorption: Passage of digested nutrients from the GI tract into the blood or lymph.

  6. Defecation: Elimination of indigestible substances from the body via the anus as feces.

• Key organs and their roles:

  • Mouth:

    • Ingestion, mechanical digestion (mastication), chemical digestion (salivary amylase for carbohydrates).

    • Saliva contains water, electrolytes, mucus, enzymes (amylase, lipase), and antibacterial compounds.

  • Pharynx & Esophagus:

    • Propulsion (swallowing/deglutition and peristalsis).

    • Esophagus is a muscular tube connecting pharynx to stomach.

  • Stomach:

    • Mechanical digestion (churning), chemical digestion (pepsin for proteins, gastric lipase for fats).

    • Produces gastric juice containing hydrochloric acid (HCl), pepsinogen (converts to pepsin), and intrinsic factor (for $B_{12}$ absorption).

    • Converts food into chyme.

  • Small Intestine:

    • Major site of chemical digestion and absorption.

    • Divided into duodenum, jejunum, and ileum.

    • Receives bile from the liver/gallbladder (for fat emulsification) and digestive enzymes from the pancreas (amylase, proteases, lipases, nucleases).

    • Villi and microvilli increase surface area for absorption.

  • Large Intestine:

    • Absorbs water and electrolytes, compacts indigestible material into feces.

    • Houses beneficial bacteria (gut microbiome).

    • Includes cecum, appendix, colon (ascending, transverse, descending, sigmoid), rectum, and anal canal.

  • Liver:

    • Produces bile (important for fat digestion).

    • Metabolically diverse functions: nutrient processing, detoxification, protein synthesis.

  • Gallbladder:

    • Stores and concentrates bile produced by the liver.

  • Pancreas:

    • Exocrine function: Produces a wide range of digestive enzymes (amylases, lipases, proteases, nucleases) and bicarbonate to neutralize acidic chyme.

    • Endocrine function: Produces hormones like insulin and glucagon.