Skeletal System — Revision Notes

⚡ 30-Second Revision

Total Bones (Adult): — 206
Axial Skeleton: — 80 bones (Skull 29, Vertebral Column 26, Rib Cage 25)
Appendicular Skeleton: — 126 bones (Pectoral Girdle 4, Upper Limbs 60, Pelvic Girdle 2, Lower Limbs 60)
Bone Cells: — Osteoblasts (build), Osteoclasts (resorb), Osteocytes (maintain)
Joint Types: — Fibrous (immovable, e.g., sutures), Cartilaginous (slightly movable, e.g., intervertebral discs), Synovial (freely movable, e.g., knee, shoulder)
Ligaments: — Bone to Bone
Tendons: — Muscle to Bone
Functions: — Support, Protection, Movement, Mineral Storage ($Ca^{2+}, PO_4^{3-}$), Hematopoiesis (Red Bone Marrow)

2-Minute Revision

The skeletal system, our body's internal framework, consists of 206 bones in adults, along with cartilage, ligaments, and tendons. It's divided into the axial skeleton (80 bones: skull, vertebral column, rib cage) for protection and central support, and the appendicular skeleton (126 bones: limbs and girdles) for movement.

Bones are dynamic, living tissues, serving functions like support, protection of organs, aiding movement, storing minerals (calcium, phosphorus), and producing blood cells in red bone marrow. Bone remodeling involves osteoblasts (bone-forming) and osteoclasts (bone-resorbing).

Joints, where bones meet, are classified by mobility: fibrous (immovable, like skull sutures), cartilaginous (slightly movable, like intervertebral discs), and synovial (freely movable, like the knee or shoulder).

Synovial joints are crucial for extensive movement due to their fluid-filled cavity. Ligaments connect bone to bone, stabilizing joints, while tendons connect muscle to bone, facilitating muscle-driven movement.

Common disorders include osteoporosis (brittle bones) and various forms of arthritis (joint inflammation).

5-Minute Revision

The skeletal system is the robust internal framework of the human body, comprising 206 bones in adults, along with associated cartilage, ligaments, and tendons. It's functionally and structurally divided into two main parts: the axial skeleton and the appendicular skeleton.

The axial skeleton, with its 80 bones (skull, vertebral column, sternum, and ribs), forms the central axis, providing crucial protection for vital organs like the brain, spinal cord, heart, and lungs, while also offering core support.

The appendicular skeleton, consisting of 126 bones (pectoral and pelvic girdles, and the bones of the upper and lower limbs), is primarily responsible for locomotion and manipulation, enabling a wide range of movements.

Bones themselves are living, dynamic tissues with multiple vital functions. They provide the fundamental support for the body's structure and protection for delicate internal organs. By acting as levers, bones facilitate movement when muscles contract and pull on them via tendons.

Furthermore, bones are critical for mineral storage, particularly calcium and phosphorus, which are essential for nerve function, muscle contraction, and blood clotting. Inside certain bones, red bone marrow is the site of hematopoiesis, the production of all blood cells, while yellow bone marrow stores fat.

Bone tissue is constantly undergoing remodeling, a balance between bone formation by osteoblasts and bone resorption by osteoclasts.

Joints, or articulations, are where bones meet, and their classification is based on their degree of mobility. Fibrous joints (synarthroses), like the sutures of the skull, are immovable, offering maximum stability.

Cartilaginous joints (amphiarthroses), such as the intervertebral discs and the pubic symphysis, allow slight movement. Synovial joints (diarthroses) are the most common and freely movable, characterized by a joint cavity filled with synovial fluid, articular cartilage, and a fibrous capsule.

Examples include hinge joints (elbow, knee), pivot joints (atlanto-axial), ball-and-socket joints (shoulder, hip), condyloid joints (wrist), saddle joints (thumb), and gliding joints (intercarpals). Ligaments are tough fibrous bands connecting bone to bone, providing joint stability, while tendons connect muscle to bone, transmitting contractile forces for movement.

Common disorders include osteoporosis (reduced bone density), arthritis (joint inflammation), and gout (uric acid crystal deposition in joints).

Prelims Revision Notes

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Skeletal System Overview: — Total 206 bones in adults. Divided into Axial (80 bones) and Appendicular (126 bones).
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Axial Skeleton (80 bones):

* Skull (29 bones): Cranial (8) - Frontal, Parietal (2), Temporal (2), Occipital, Sphenoid, Ethmoid. Facial (14) - Nasal (2), Maxilla (2), Zygomatic (2), Lacrimal (2), Palatine (2), Inferior Nasal Conchae (2), Vomer, Mandible.

Ear Ossicles (6) - Malleus (2), Incus (2), Stapes (2). Hyoid (1). * Vertebral Column (26 bones): Cervical (7), Thoracic (12), Lumbar (5), Sacrum (1 fused from 5), Coccyx (1 fused from 4). Total 33 vertebrae in child, 26 in adult.

* Rib Cage (25 bones): Ribs (12 pairs = 24) - True (1-7), False (8-10), Floating (11-12). Sternum (1).

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Appendicular Skeleton (126 bones):

* Pectoral Girdle (4 bones): Clavicle (2), Scapula (2). * Upper Limbs (60 bones): Humerus (2), Radius (2), Ulna (2), Carpals (16, 8 per wrist), Metacarpals (10, 5 per hand), Phalanges (28, 14 per hand). * Pelvic Girdle (2 bones): Coxal bones (2, each fused from Ilium, Ischium, Pubis). * Lower Limbs (60 bones): Femur (2), Patella (2), Tibia (2), Fibula (2), Tarsals (14, 7 per ankle), Metatarsals (10, 5 per foot), Phalanges (28, 14 per foot).

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Bone Tissue Types:

* Compact (Cortical) Bone: Dense, outer layer, strength. Structural unit: Osteon (Haversian system). * Spongy (Cancellous) Bone: Lighter, inner, trabeculae, contains red bone marrow.

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Bone Cells:

* Osteoblasts: Bone-forming cells (synthesize matrix). * Osteoclasts: Bone-resorbing cells (break down matrix). * Osteocytes: Mature bone cells (maintain matrix).

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Functions of Skeletal System: — Support, Protection, Movement, Mineral Storage (Ca, P), Hematopoiesis (red marrow), Fat Storage (yellow marrow).
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Joints (Articulations):

* Fibrous (Synarthroses): Immovable. Ex: Sutures (skull), Gomphoses (teeth in sockets), Syndesmoses (tibia-fibula). * Cartilaginous (Amphiarthroses): Slightly movable. Ex: Pubic symphysis, Intervertebral discs.

* Synovial (Diarthroses): Freely movable. Characterized by synovial fluid, articular cartilage, joint capsule. Subtypes: * Ball and Socket: Shoulder, Hip (multiaxial). * Hinge: Elbow, Knee, Interphalangeal (uniaxial, flexion/extension).

* Pivot: Atlanto-axial, Radioulnar (uniaxial, rotation). * Condyloid (Ellipsoidal): Wrist, Metacarpophalangeal (biaxial). * Saddle: Carpometacarpal of thumb (biaxial, more freedom). * Gliding (Plane): Intercarpal, Intertarsal (sliding).

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Ligaments: — Connect bone to bone (stabilize joints).
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Tendons: — Connect muscle to bone (facilitate movement).
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Disorders: — Osteoporosis (low bone density), Arthritis (joint inflammation - Osteoarthritis, Rheumatoid Arthritis), Gout (uric acid crystals), Rickets/Osteomalacia (Vitamin D deficiency).

Vyyuha Quick Recall

To remember the functions of the skeletal system, think of 'SPAMM HFS':

Support
Protection
Aid in Movement
Mineral Storage
Hematopoiesis
Fat Storage