Skeletal System: Structure, Function, and Anatomy Study Notes

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Parts, Regions, and Structures of a Long Bone

Overview of Long Bone Anatomy

Long bones are characterized by a central shaft and two expanded ends, each with distinct structural features and functions. These bones are essential for movement and support in the human body.

Diaphysis: The shaft of a long bone, composed of compact bone surrounding a medullary cavity filled with yellow marrow (in adults).
Epiphyses: The ends of the bone, made of spongy bone covered by compact bone and containing red marrow in children.
Epiphyseal Plate: Growth plate in children, made of cartilage; becomes the epiphyseal line when growth stops.

Membranes:

Periosteum: Outer dense irregular connective tissue with Sharpey's fibers anchoring tendons/ligaments.
Endosteum: Lines internal surfaces and trabeculae of spongy bone.

Types of Cartilage and Composition

Classification and Properties of Cartilage

Cartilage is an avascular connective tissue composed of chondrocytes in lacunae and extracellular matrix with fibers and ground substance. There are three main types:

Hyaline Cartilage: Most abundant; fine collagen; supports and cushions; found in nose, costal cartilage, trachea, articular ends of long bones.
Elastic Cartilage: Contains elastic fibers; flexible; found in ear, epiglottis.
Fibrocartilage: Thick collagen; resists compression; found in intervertebral discs, pubic symphysis, menisci.

Bone Markings

Types and Functions of Bone Markings

Bones display various markings that serve as attachment points, passageways, or depressions for anatomical structures.

Projections: Outward bulges for muscle or ligament attachment (e.g., tubercle, spine, process).
Depressions: Grooves or pits (e.g., fossa).
Openings: Holes for vessels/nerves (e.g., foramen, meatus).

Functions of Osteoblasts, Osteoclasts, and Osteocytes

Roles in Bone Formation and Remodeling

Bone cells have specialized functions in the growth, maintenance, and remodeling of bone tissue.

Osteoblasts: Build bone by secreting collagen and osteoid (unmineralized matrix).
Osteocytes: Mature cells in lacunae maintaining bone matrix; act as stress sensors.
Osteoclasts: Large multinucleate cells that break down bone matrix during remodeling.

Microscopic Anatomy of Bone

Osteon, Lamellae, and Related Structures

Compact bone is organized into structural units called osteons, which facilitate strength and nutrient delivery.

Osteon (Haversian system): Cylinder-shaped structural unit of compact bone.
Lamellae: Concentric rings of matrix around central canal.
Haversian canal: Central canal with blood vessels and nerves.
Volkmann's canals: Perforating canals connecting central canals.
Canaliculi: Tiny canals allowing communication and nutrient transfer between osteocytes.

Axial vs. Appendicular Skeleton

Major Divisions of the Human Skeleton

The skeleton is divided into axial and appendicular components, each with distinct functions and bone groups.

Axial (80 bones): Skull, vertebral column, ribs, sternum. Supports and protects organs.
Appendicular (126 bones): Limbs, shoulder girdle, pelvic girdle. For movement.

Appositional vs. Interstitial Growth

Mechanisms of Bone Growth

Bones grow in both width and length through different processes.

Appositional: Growth in width by new bone added to surface.
Interstitial: Growth in length through cartilage at the epiphyseal plate. Bones appear to grow interstitially only because cartilage at the plate grows and is replaced by bone.

Hormones Regulating Blood Calcium

Endocrine Control of Bone and Calcium Homeostasis

Blood calcium levels are tightly regulated by hormones affecting bone resorption and deposition.

PTH (Parathyroid Hormone): Increases blood Ca2+ by stimulating osteoclasts.
Calcitonin: Decreases blood Ca2+ by stimulating osteoblasts to deposit Ca2+ in bone.

Glenohumeral Joint

Structure and Function of the Shoulder Joint

The glenohumeral joint is a ball-and-socket joint between the humeral head and glenoid cavity of the scapula, allowing a wide range of motion.

Location: Shoulder—between humeral head and glenoid cavity of scapula.
Type: Ball-and-socket; very mobile but least stable due to shallow socket and reliance on ligaments/muscles.

Nonaxial, Uniaxial, Biaxial, Multiaxial Joint Movements

Classification of Synovial Joint Movements

Joints are classified by the number of axes around which movement occurs.

Nonaxial: No plane movement—plane joints (carpals).
Biaxial: Two planes—condylar (wrist), saddle (thumb).
Multiaxial: All planes—ball-and-socket (shoulder, hip).

Angular and Special Movements

Types of Movements at Synovial Joints

Joints allow various angular and special movements essential for daily activities.

Angular: Flexion, extension, hyperextension, abduction, adduction, circumduction.
Special: Rotation, supination/pronation, dorsiflexion/plantar flexion, inversion/eversion, protraction/retraction, elevation/depression, opposition.

Six Synovial Joint Types

Classification and Range of Motion

Synovial joints are classified by shape and movement allowed.

Type	Example
Plane	Intercarpal joints
Hinge	Elbow
Pivot	Radioulnar joint
Condylar	Wrist
Saddle	Thumb (carpometacarpal)
Ball-and-socket	Shoulder, hip

Functional vs. Structural Joint Classifications

Comparison of Joint Types

Joints are classified by their structure and the degree of movement they allow.

Classification	Type	Movement
Structural	Fibrous	No cavity; immovable
Structural	Cartilaginous	No cavity; slightly movable
Structural	Synovial	Cavity + capsule; freely movable
Functional	Synarthroses	Immovable
Functional	Amphiarthroses	Slightly movable
Functional	Diarthroses	Freely movable

Zones of Epiphyseal Plate

Growth Plate Structure and Function

The epiphyseal plate is responsible for longitudinal bone growth during development. It consists of several zones:

Resting (quiescent)
Proliferation (growth)
Hypertrophic
Calcification
Ossification (osteogenic): Each contributes to bone lengthening as cartilage cells divide and ossify.

Functions of the Skeletal System

Major Roles of Bone in the Body

The skeletal system provides support, protection, movement, mineral storage, blood formation, triglyceride storage, and hormone production (osteocalcin).

Support: Framework for the body
Protection: Shields vital organs
Movement: Leverage for muscles
Mineral Storage: Reservoir for calcium and phosphorus
Blood Formation: Hematopoiesis in marrow
Triglyceride Storage: Energy reserve in yellow marrow
Hormone Production: Osteocalcin regulates bone formation and glucose metabolism

Cranial and Facial Bones

Classification and Examples

The skull is composed of cranial and facial bones, each with specific functions and locations.

Cranial (8): Frontal, 2 Parietal, 2 Temporal, Occipital, Sphenoid, Ethmoid.
Facial (14): Mandible, Vomer, 2 Maxillae, 2 Zygomatic, 2 Nasal, 2 Lacrimal, 2 Palatine, 2 Inferior Nasal Conchae.

Sella Turcica

Structure and Function

The sella turcica is a depression in the sphenoid bone housing the pituitary gland.

Differences in Vertebrae

Regional Characteristics of Vertebrae

Vertebrae differ in structure and function depending on their location in the spine.

Cervical: Small body, transverse foramina, bifid spinous process.
Thoracic: Articulate with ribs, long downward spinous process.
Lumbar: Large, thick body, short blunt spinous process.

"Yes" and "No" Head Movements

Atlas and Axis Function

Specific cervical vertebrae allow nodding and rotation of the head.

"Yes": Atlas (C1) + occipital condyles.
"No": Axis (C2) + atlas via dens pivot.

Rib Articulations

Connections of Ribs to Vertebrae and Sternum

Ribs articulate posteriorly with thoracic vertebrae and anteriorly with the sternum via costal cartilage.

Bones of Arm, Forearm, Thigh, Leg (Medial/Lateral)

Major Limb Bones and Their Locations

Arm: Humerus
Forearm: Radius (lateral), Ulna (medial)
Thigh: Femur
Leg: Tibia (medial), Fibula (lateral)