Skeletal System Notes

Exoskeletons:
- Encases the whole organism.
- Common in arthropods.
- Protects soft tissues effectively.
- Must be shed and regrown for growth.
Endoskeletons:
- Internal skeletons.
- Found in vertebrates, including humans.
- Does not protect soft tissues as well as exoskeletons.
- Better at accommodating the growth of larger organisms.

Consists of:
- Skull
- Vertebral column
- Rib cage
- Hyoid bone (for swallowing)
Provides basic central framework for the body.

Compact Bone:
- Dense and strong.
- Provides characteristic strength.
Spongy (Cancellous) Bone:
- Lattice structure visible under microscopy.
- Composed of bony spicules known as trabeculae.
- Cavities filled with bone marrow (red or yellow).
  - Red marrow: Hematopoietic stem cells (generates blood cells).
  - Yellow marrow: Primarily fat, relatively inactive.
Long Bones:
- Found in the appendicular skeleton.
- Cylindrical shafts called diaphyses.
- Ends swell to form metaphyses.
- Terminate in epiphyses.
- Outermost part is compact bone, internal core is spongy bone.
- Diaphysis and metaphysis contain bone marrow.
- Epiphyses use spongy cores for force dispersion at joints.
- Epiphyseal growth plate:
  - Cartilaginous structure at the internal edge of the epiphysis.
  - Site of longitudinal growth.
  - Filled with mitotic cells during puberty.
  - Closes at adulthood, halting vertical growth.
- Periosteum:
  - Fibrous sheath surrounding long bones.
  - Protects bone and serves as muscle attachment site.
  - Contains cells that can differentiate into bone-forming cells.
  - Necessary for bone growth and repair.
- Tendons: attach muscle to bone
- Ligaments: hold bones together at joints

Bone Matrix:
- Strength of compact bone comes from here.
- Organic components: collagen, glycoproteins, peptides.
- Inorganic components: calcium, phosphate, hydroxide ions (form hydroxyapatite crystals).
  - Formula for hydroxyapatite: $Ca<em>{10}(PO</em>4)<em>6(OH)</em>2$
- Minerals stored: sodium, magnesium, potassium.
- Uniform distribution of organic and inorganic materials is essential for strong bones.
Osteons (Haversian systems):
- Structural units of bony matrix.
- Lamellae: Concentric circles of bony matrix surrounding a central channel.
- Haversian canals: Longitudinal channels (axis parallel to bone) containing blood vessels, nerve fibers, and lymph vessels.
- Volkmann's canals: Transverse channels (axis perpendicular to bone) containing blood vessels, nerve fibers, and lymph vessels.
- Lacunae: Small spaces between lamellar rims housing mature bone cells (osteocytes).
- Canaliculi: Tiny channels interconnecting lacunae, allowing nutrient and waste exchange between osteocytes and Haversian/Volkmann's canals.

Osteoblasts: Build bone using calcium and phosphate from the blood.
Osteoclasts: Resorb bone (polynucleated resident macrophages of bone), releasing calcium and phosphate back into the bloodstream.
Occurs in response to stress, remodeling to accommodate repetitive stresses.
Endocrine Hormones affecting bone metabolism:
- Parathyroid hormone (PTH):
  - Released by parathyroid glands in response to low blood calcium.
  - Promotes bone resorption, increasing blood calcium and phosphate.
  - Activates vitamin D.
- Vitamin D:
  - Promotes bone resorption.
  - Encourages the growth of new, stronger bone, overcompensating for initial resorption.
- Calcitonin:
  - Released by parafollicular cells of the thyroid in response to high blood calcium.
  - Promotes bone formation, lowering blood calcium levels.

Softer and more flexible than bone.
Chondrin: Firm but elastic matrix secreted by chondrocytes.
Fetal skeletons are mostly cartilage.
Adults have cartilage in flexible or cushioning body parts (external ear, nose, larynx and trachea walls, intervertebral discs, joints).
Avascular (no blood/lymphatic vessels) and not innervated.
Endochondral Ossification: Hardening of cartilage into bone (most long bones).
Intramembranous Ossification: Transformation of embryonic mesenchymal tissue into bone (skull bones).

Made of connective tissue.
Immovable Joints:
- Fused bones forming sutures or fibrous joints.
- Primarily in the head (skull).
Movable Joints:
- Hinge joints (elbow, knee).
- Ball and socket joints (shoulder, hip).
- Permit bone movement relative to each other.
- Strengthened by ligaments (connect bones to one another).
- Synovial Capsule: Encloses joint cavity (articular cavity).
- Synovium: Secretes synovial fluid to lubricate joint movement.
- Articular Cartilage: Coats articular surfaces of bones to restrict impact to lubricated joint cartilage.
- Origin: End of muscle with larger attachment to bone (usually proximal).
- Insertion: End of muscle with smaller attachment to bone (usually distal).

Muscles work in antagonistic pairs (one relaxes while the other contracts).
- Example: biceps brachii and triceps brachii in the arm.
  - Biceps contracts, triceps relaxes: elbow flexes.
  - Triceps contracts, biceps relaxes: elbow extends.
Muscles can also be synergistic (working together to accomplish the same function).
Types of Movement:
- Flexor: Decreases the angle across a joint (e.g., biceps brachii).
- Extensor: Increases or straightens the angle across a joint (e.g., triceps brachii).
- Abductor: Moves a body part away from the midline (e.g., deltoid).
- Adductor: Moves a body part toward the midline (e.g., pectoralis major).
- Medial Rotator: Rotates the axis of the limb toward the midline (e.g., subscapularis).
- Lateral Rotator: Rotates the axis of the limb away from the midline (e.g., infraspinatus).

Organ systems work together to achieve a desired effect.
Musculoskeletal system is responsible for movement, calcium storage, protection of internal organs, and body support.
Muscle tissue moves bones, pumps blood, and is key to respiration, digestion, blood pressure, vascular tone, and reproduction.
It is important to understand how each organ system interacts with the others as it is often tested in the MCAT.