Comprehensive Study Guide to the Human Skeletal and Articular Systems

Divisions of the Skeletal System and the Vertebral Column

• The Adult Human Skeleton: Consists of $206$ named bones. Most of these are paired (one on the right and one on the left sides).
• Infant vs. Adult Bone Count: Infants and children have more than $206$ bones because several bones fuse later in life. Primary examples of fusion include the hip bones and segments of the vertebral column (sacrum and coccyx).
• Principal Divisions:
  • Axial Skeleton: Bones located around the longitudinal axis (imaginary vertical line through the center of gravity from head to between the feet). Includes the skull bones, auditory ossicles (ear bones), hyoid bone, ribs, sternum (breastbone), and the vertebral column.
  • Appendicular Skeleton: Consists of the bones of the upper and lower limbs (appendages or extremities), alongside the girdles (pectoral and pelvic) that connect these limbs to the axial skeleton.
• The Vertebral Column (Spine/Backbone): Makes up approximately two-fifths ($40\%$) of total body height.
  • Length: In an average adult male, it measures approximately $71\,cm$; in an average adult female, it measures approximately $61\,cm$.
  • Composition: Formed by a series of bones called vertebrae ($\text{plural: vertebrae; singular: vertebra}$). It consists of bone and connective tissue, while the spinal cord it protects consists of nervous and connective tissues.
  • Functions: Acts as a strong, flexible rod capable of forward, backward, sideways, and rotational movement. It encloses and protects the spinal cord, supports the head, and serves as an attachment point for ribs, the pelvic girdle, and muscles of the back and upper limbs.
• Vertebrae Distribution (Adult vs. Development):
  • Early Development: Total of $33$ vertebrae.
  • Adult: Typically $26$ vertebrae due to fusion.
  • Segments:
    1. Cervical: $7$ vertebrae in the neck region.
    2. Thoracic: $12$ vertebrae posterior to the thoracic cavity.
    3. Lumbar: $5$ vertebrae supporting the lower back.
    4. Sacrum: $1$ bone, formed by the fusion of $5$ sacral vertebrae.
    5. Coccyx: $1$ bone, usually formed by the fusion of $4$ coccygeal vertebrae.
  • Mobility: Cervical, thoracic, and lumbar vertebrae are movable; the sacrum and coccyx are immovable.

Curvatures of the Vertebral Column

• Normal Curves: When viewed from the side (sagittal plane), the vertebral column shows four bends that increase strength, maintain balance, absorb shock, and protect against fractures.
  • Cervical and Lumbar Curves: Convex (bulging out), termed Lordosis.
  • Thoracic and Sacral Curves: Concave (cupping in), termed Kyphosis.
• Developmental Stages of Curves:
  • Primary Curves: Thoracic and sacral curves. These retain the original anteriorly concave curvature of the fetus.
  • Secondary Curves: Cervical and lumbar curves (anteriorly convex).
    • Cervical Curve: Develops around $3$ months after birth when the infant begins to hold its head erect.
    • Lumbar Curve: Develops when the child begins to sit, stand, and walk.
  • Maturity: All curves are fully developed by age $10$. Secondary curves may be lost in old age.
• Abnormal Curves:
  • Scoliosis: The most common abnormality; a lateral bending usually in the thoracic region. Causes include congenitally malformed vertebrae, chronic sciatica, muscle paralysis, poor posture, or unequal leg length. Severe cases ($> 70$ degrees) impair breathing and heart efficiency. Treatments include braces, physical therapy, chiropractic care, or surgery (fusion/metal rods).
  • Kyphosis: Exaggerated thoracic curve ("hunchback"). Caused by tuberculosis (partial collapse of vertebral bodies), intervertebral disc degeneration in the elderly, rickets, poor posture, or advanced osteoporosis in females.
  • Lordosis: Exaggerated lumbar curve ("hollow back"). Results from increased abdominal weight (pregnancy, extreme obesity), poor posture, rickets, osteoporosis, or tuberculosis.

Anatomy and Regional Characteristics of Vertebrae

• Typical Vertebra Composition:
  • Vertebral Body: Thick, disc-shaped anterior portion; weight-bearing. Surfaces are rough for intervertebral disc attachment. Contains nutrient foramina for blood vessels.
  • Vertebral Arch: Formed by two short processes called pedicles uniting with flat laminae. Together with the body, it surrounds the vertebral foramen (containing the spinal cord, adipose tissue, areolar tissue, and blood vessels).
  • Vertebral Canal: Formed by the collective vertebral foramina.
  • Intervertebral Foramen: Opening formed between stacked vertebral notches of adjacent pedicles; permits passage of a single spinal nerve.
  • Processes ($7$ total):
    • Transverse Processes ($2$): Extend laterally at the junction of lamina and pedicle.
    • Spinous Process ($1$): Projects posteriorly from the junction of laminae.
    • Articular Processes ($4$): Two superior and two inferior. Surfaces are called facets, covered in hyaline cartilage, forming intervertebral joints.
• Cervical Vertebrae ($C1-C7$): Smallest bodies (excluding coccyx) but largest vertebral arches/foramina to accommodate the cervical enlargement of the spinal cord.
  • Unique Feature: Three foramina (one vertebral, two transverse). Transverse foramina house the vertebral artery, vein, and nerve fibers.
  • Bifid spines: Spinous processes of $C2$ through $C6$ often branch into two tips.
  • Atlas ($C1$): Ring of bone, lacks a body and spinous process. Superior articular facets articulate with occipital condyles (atlanto-occipital joints) to signal "yes."
  • Axis ($C2$): Features a peglike process called the dens (odontoid process) which acts as a pivot for the atlas. Forms the atlanto-axial joint to signal "no." Trauma to the dens into the medulla oblongata is a common death cause in whiplash ("colpo di frusta") injuries.
  • Vertebra Prominens ($C7$): Features a large, non-bifid spinous process palpable at the base of the neck.
• Thoracic Vertebrae ($T1-T12$): Larger and stronger than cervical. Distinguished by costal facets (articular surfaces for ribs).
  • T1-T10: Long, laterally flattened spinous processes directed inferiorly.
  • T11-T12: Shorter, broader, more posteriorly directed spines.
  • Articulations (Vertebrocostal joints): Bodies feature facets or demifacets (where head of rib meets two adjacent bodies). Transverse processes (except $T11, T12$) have facets that articulate with rib tubercles.
• Lumbar Vertebrae ($L1-L5$): Largest/strongest. Tasked with supporting increasing body weight. Features short, thick projections and quadrilateral-shaped spinous processes for back muscle attachment. $L5$ has massive cone-shaped transverse processes for iliolumbar ligament attachment.
• Sacrum ($S1-S5$): Triangular bone. Fusion starts between $16-18$ and finishes by $30$.
  • Features: Sacral promontory (pelvic measurement landmark), sacral ala (fused transverse processes of $S1$), sacral hiatus (inferior entrance to vertebral canal), and auricular surfaces for the sacroiliac joint.
• Coccyx ($Co1-Co4$): Small triangular bone. Fuses between $20-30$ years of age. In females, it points inferiorly (childbirth); in males, it points anteriorly.

Thoracic Cage and Upper Limb Skeletal Anatomy

• Thoracic Cage Components: Sternum, ribs ($12$ pairs), costal cartilages, and thoracic vertebrae.
• Sternum (Breastbone): Approx. $15\,cm$ long. Parts: Manubrium (superior), Body (middle), Xiphoid Process (inferior, hyaline cartilage until age $40$).
  • Landmarks: Suprasternal (jugular) notch, sternal angle (divides mediastinum into superior and inferior at $T4$ level), clavicular notches.
  • CPR Danger: Incorrect hand position can fracture the xiphoid process into internal organs.
• Ribs:
  • True Ribs ($1-7$): Direct anterior attachment to sternum via costal cartilage (sternocostal joints).
  • False Ribs ($8-12$): Indirect or no attachment. Ribs $8-10$ are vertebrochondral. Ribs $11-12$ are floating ribs.
  • Anatomy: Head (with facets), neck, tubercle (articulates with transverse process), body (shaft), costal angle, and costal groove (protects intercostal vessels/nerve).
• Pectoral (Shoulder) Girdles: Clavicle and scapula.
  • Clavicle (Collarbone): S-shaped; medial half convex anteriorly, lateral half concave. Sternal end (rounded) and acromial end (flat). Anchors muscles: sternocleidomastoid, pectoralis major, trapezius, and deltoid.
  • Scapula (Shoulder Blade): Between ribs $2$ and $7$. Features the spine, acromion, glenoid cavity (ball-and-socket joint with humerus), and coracoid process.
• Humerus (Arm Bone): Proximally articulates with scapula; distally with ulna and radius.
  • Features: Anatomical neck, surgical neck (common fracture site), greater/lesser tubercles, deltoid tuberosity, radial groove, capitulum (lateral knob), trochlea (medial spool), and epicondyles. The ulnar nerve is palpable posterior to the medial epicondyle (the "funnybone").

Arthrology: Classification and Structure of Joints

• Definition: A joint (articulation/arthrosis) is a contact point between two bones, bone and cartilage, or bone and teeth.
• Structural Classification:
  • Fibrous: No synovial cavity; bones held by dense irregular connective tissue (collagen).
    • Sutures: Only in skull (gear, flat, or squamous types).
    • Syndesmosis: Linked by a ligament/interosseous membrane (e.g., radius to ulna).
    • Gomphosis: Tooth in bony socket joined by periodontal ligament.
  • Cartilaginous: No synovial cavity; bones held by cartilage.
    • Synchondrosis: Joined by hyaline cartilage (e.g., epiphyseal plate, rib $1$ to manubrium).
    • Symphysis: Ends covered in hyaline, but connected by fibrocartilage disc (e.g., pubic symphysis, intervertebral discs).
  • Synovial: Cavity present; bones united by articular capsule and ligaments.
• Functional Classification (Degree of Movement):
  1. Synarthrosis: Immovable.
  2. Amphiarthrosis: Slightly movable.
  3. Diarthrosis: Freely movable (all synovial joints).
• Synovial Joint Anatomy:
  • Articular Capsule: Outer fibrous membrane and inner synovial membrane (secretes synovial fluid containing hyaluronic acid and interstitial fluid).
  • Articular Cartilage: Hyaline cartilage reduces friction and absorbs shock.
  • Accessory Structures: Ligaments (bone-to-bone), tendons (muscle-to-bone), and articular discs/menisci (fibrocartilage pads for stability/shock absorption).
  • Bursae: Fluid-filled sacs that reduce friction. Inflammation is termed Bursitis.

Types of Synovial Joints and Pathologies

• Joint Categories by Movement:
  1. Plane Joint: Back-and-forth/side-to-side (e.g., intercarpal). Baxial.
  2. Hinge Joint (Ginglymus): Flexion/extension (e.g., elbow, ankle). Uniaxial.
  3. Pivot Joint (Trochoid): Rotation around longitudinal axis (e.g., atlanto-axial). Uniaxial.
  4. Condyloid Joint (Ellipsoidal): Flexion/extension and abduction/adduction (e.g., radiocarpal). Biaxial.
  5. Saddle Joint (Sellar): Same as condyloid (e.g., carpometacarpal of thumb). Biaxial.
  6. Ball-and-Socket (Spheroid): Flexion/extension, abduction/adduction, rotation (e.g., shoulder, hip). Triaxial.
• Range of Motion (ROM) Factors: Shape of bones, ligament strength/tension, muscle tension (e.g., hamstrings restricting hip flexion), soft part contact, hormones, and disuse.
• Arthritis Conditions:
  • Osteoarthritis (OA): Degenerative "wear-and-tear" disease; loss of articular cartilage and formation of bone spurs. Affects large, weight-bearing joints (knees, hips) first.
  • Rheumatoid Arthritis (RA): Autoimmune disorder; immune system attacks synovial membrane, forming pannus (granulation tissue) that erodes cartilage. Affects small joints bilaterally and can cause joint fusion (ossification).
  • Arthroplasty: Surgical replacement of damaged joints with artificial components (typically metal, ceramic, or plastic).

Muscle Tissue Components

• Connective Tissue Layers:
  1. Hypodermis (Subcutaneous layer): Areolar and adipose tissue; separates muscle from skin, provides pathways for nerves and vessels, and stores triglycerides.
  2. Fascia: Dense irregular connective tissue; surrounds muscle groups and carries nerves/vessels.
  3. Specific Muscle Layers: Epimysium (outer layer), perimysium (surrounds fascicles), and endomysium (surrounds individual fibers).
• Microscopic Anatomy: Includes the sarcolemma (plasma membrane), sarcoplasm (cytoplasm), myofibrils, and the Sarcomere (contractile unit marked by Z lines, A bands, I bands, and H bands).