Comprehensive Study Guide: Human Anatomy and Physiology of the Skeletal System

Learning Objectives of the Skeletal System

• Compare and contrast various types of cartilaginous tissue based on their specific locations and functional roles. Examples include tissue found in:
  • Joints
  • Growth plates
  • Menisci
  • Intervertebral discs
• Provide a detailed description of the structure and function for three main joint categories: fibrous, cartilaginous, and synovial joints.
• Identify and describe the different types of synovial joints, providing anatomical examples for each.
• Explain how joint structure determines the specific type of movement and the range of motion allowed.
• Describe various joint movements in relation to anatomical planes and specific joint types, including:
  • Flexion
  • Extension
  • Abduction
  • Adduction
  • Rotation
• Explain the major functions and structural organization of the skeletal system, distinguishing between the axial and appendicular divisions.
• Identify key bones and anatomical features that facilitate support, protection, movement, and muscle attachment.
• Describe the specific roles that joint-related connective tissues play in joint stability and movement, including:
  • Tendons
  • Ligaments
  • Bursae
  • Sesamoid bones
  • Articular cartilage

The Axial Skeleton

• Definition and Purpose: The axial skeleton forms the vertical, central axis of the body. It houses and protects all organs essential to life.
• Muscle Attachment: It serves as the primary attachment point for muscles that move the head, neck, and back, as well as muscles that cross the shoulder and hip joints to move the limbs.
• Bone Count: The adult axial skeleton consists of exactly $80$ bones.
• Structural Components:
  • The Skull
  • The Vertebral Column
  • The Thoracic Cage
• Functional Focus: Understanding the groupings and functions of these bones is prioritized over memorizing every individual bone, though bones listed in red (within study diagrams) must be memorized.

The Skull: Cranial and Facial Bones

• General Structure: The skull is composed of $22$ bones. Of these, $21$ are fused together via immobile joints called sutures. The only movable bone is the mandible.
• Functional Division: The skull is divided into two primary groups: cranial bones and facial bones.

Cranial Bones (The Brain Case)

• Frontal bone ($1$): Forms the forehead and the superior portion of the orbital cavities (eye sockets).
• Parietal bones ($2$): Form the majority of the superior and lateral surfaces of the skull. The left and right parietal bones are joined at the midline by the sagittal suture.
• Temporal bones ($2$): Form the lower lateral sides of the skull. Key features include:
  • External acoustic meatus: The ear canal.
  • Mastoid process: A bony prominence felt just behind the earlobe.
• Occipital bone ($1$): Forms the posterior and the base of the skull. It contains the foramen magnum, the large opening that allows the spinal cord to connect to the brainstem.
• Sphenoid bone ($1$): A butterfly-shaped bone spanning the width of the skull floor. It is critical as it articulates with nearly every other cranial bone.
• Ethmoid bone ($1$): Positioned between the orbits; it forms part of the nasal cavity and includes the superior and middle nasal conchae.

Facial Bones

• Mandible ($1$): The lower jaw bone. It is the only movable skull bone and articulates with the temporal bone at the temporomandibular joint (TMJ).
• Maxillae ($2$): The upper jaw bones. They form the floor of the orbits, the sides of the nasal cavity, and hold the upper teeth.
• Zygomatic bones ($2$): Commonly known as the cheekbones. They form the lateral wall of the orbit and combine with the temporal bone to form the zygomatic arch.
• Nasal bones ($2$): Two small bones that form the bridge of the nose.
• Lacrimal bones ($2$): Tiny bones located in the medial orbit that house the nasolacrimal canal (the tear duct).
• Inferior nasal conchae ($2$): Independent bones projecting into the nasal cavity. Their function is to increase surface area to warm and humidify inhaled air.
• Vomer ($1$): A single bone forming the inferior portion of the nasal septum.

The Vertebral Column

• General Overview: Also known as the spinal column, it comprises $24$ individual vertebrae, the sacrum, and the coccyx, totaling $26$ bones.
• Functions: Supports the head and trunk, protects the spinal cord, and facilitates neck and back movement.
• Regional Classifications (Superior to Inferior):
  • Cervical (C1–C7): Located in the neck.
  • Thoracic (T1–T12): Located in the upper and mid-back; these articulate with the ribs.
  • Lumbar (L1–L5): Located in the lower back.
  • Sacrum (S1–S5): Consists of five fused vertebrae; forms the posterior wall of the pelvis.
  • Coccyx (C1–C3/4): Consists of three to four fused vertebrae, commonly called the tailbone.
• Curvatures:
  • The column features four curvatures to increase strength, flexibility, and shock absorption.
  • Primary Curves: The thoracic and sacrococcygeal curves (retained from fetal development).
  • Secondary Curves: The cervical and lumbar curves (develop after birth as infants learn to support their heads and walk).

Vertebral Structure and Intervertebral Discs

• Typical Vertebra Components:
  • Body: The anterior, weight-bearing portion.
  • Vertebral arch: The posterior portion enclosing the vertebral foramen (the opening for the spinal cord).
  • Processes: Projections for muscle and ligament attachment.
• Intervertebral Discs: Located between vertebrae; made of fibrocartilage to absorb compression and provide flexibility.
  • Annulus fibrosus: The tough outer ring.
  • Nucleus pulposus: The gel-like center.
  • Clinical Note: A herniated disc occurs when the nucleus pulposus pushes through the annulus, compressing a spinal nerve.

Regional Structural Differences

• Cervical Vertebrae: The smallest vertebrae.
  • C1 (Atlas): Lacks a body; supports the skull and enables the "yes" (nodding) motion.
  • C2 (Axis): Features the dens (odontoid process), a bony projection that serves as a pivot point for the atlas, enabling the "no" (shaking) motion.
• Thoracic Vertebrae: Distinguished by facets on the body and transverse processes for articulation with the ribs. These are the most restricted in movement.
• Lumbar Vertebrae: The largest and sturdiest vertebrae; designed to bear the most body weight.

The Thoracic Cage

• Components: Formed by the sternum (anteriorly), $12$ pairs of ribs (laterally), and thoracic vertebrae (posteriorly).
• Function: Protects the heart, lungs, and major blood vessels.
• The Sternum: Divided into the manubrium (superior), the body (central), and the xiphoid process (inferior tip).
• Costal Cartilage: Connects ribs to the sternum, providing the flexibility necessary for the cage to expand and contract during respiration.
• Rib Classification ($12$ pairs):
  • True ribs ($1$-$-$7)**: Attach directly to the sternum via their own individual costal cartilage.\n - **False ribs (8$-$-$10$): Attach to the sternum indirectly by sharing the costal cartilage of rib $7$.
  • Floating ribs ($11$ and $12$): Have no anterior attachment to the sternum; they end in the abdominal musculature.

The Appendicular Skeleton

• General Overview: Includes all bones of the upper and lower limbs and the girdles that anchor them to the axial skeleton.
• Function: Specifically designed for movement. This contrasts with the axial skeleton (the "chassis"), while the appendicular system acts as the "suspension and wheels."
• Girdle Comparison:
  • Pectoral Girdle: Loosely attached to maximize mobility of the upper limbs.
  • Pelvic Girdle: Firmly fused to the sacrum for superior stability and weight-bearing capability.

Regional Bone Distribution (Per Side)

• Pectoral Girdle ($2$ bones): Clavicle and Scapula.
• Upper Limb ($30$ bones): Humerus, Radius, Ulna, Carpals, Metacarpals, Phalanges.
• Pelvic Girdle ($1$ bone): Coxal bone (composed of Ilium, Ischium, and Pubis).
• Lower Limb ($30$ bones): Femur, Patella, Tibia, Fibula, Tarsals, Metatarsals, Phalanges.

The Pectoral Girdle Details

• Clavicle (Collarbone): Acts as a strut, holding the shoulder joint away from the trunk. It forms the sternoclavicular joint (the only bony connection between the upper limb and axial skeleton).
• Scapula (Shoulder Blade): A flat, triangular bone. Key features:
  • Acromion: The bony tip of the shoulder.
  • Coracoid process: An anterior, hook-shaped projection.
  • Glenoid cavity: A shallow socket for the head of the humerus. Its shallowness allows for the greatest range of motion in the body but increases dislocation risk.

Upper Limb Details

• Arm: Humerus (features: head, tubercles, deltoid tuberosity, and epicondyles).
• Forearm:
  • Radius: Lateral bone; the head rotates on the humerus.
  • Ulna: Medial bone; features the olecranon process (the elbow point) and the trochlear notch.
• Hand:
  • $8$ Carpals: Arranged in two rows of four.
  • $5$ Metacarpals: Forming the palm.
  • $14$ Phalanges: $2$ in the thumb (pollex), $3$ in each finger.

The Pelvic Girdle Details

• Coxal Bone: Formed by the fusion of the ilium (superior/waist), ischium (posteroinferior/sitting bone), and pubis (anteromedial).
• Acetabulum: The deep socket where the three pelvic bones meet; it holds the head of the femur. It sacrifices range of motion for the stability needed to bear body weight.
• The Pelvis: The collective structure of the two hip bones, the sacrum, and the coccyx. The hip bones join at the pubic symphysis (anteriorly) and the sacroiliac joints (posteriorly).

Lower Limb Details

• Thigh: Femur (longest and strongest bone). Features the head, greater trochanter (lateral hip), and the neck (common fracture site).
• Knee: Patella (a sesamoid bone embedded in the quadriceps tendon). It protects the joint and improves muscle leverage.
• Leg (Shank):
  • Tibia: The weight-bearing "shinbone."
  • Fibula: Bears almost no weight; used for muscle attachment and forming the lateral ankle.
• Foot:
  • $7$ Tarsals: The calcaneus (heel) and talus (articulates with the tibia) are the largest.
  • $5$ Metatarsals.
  • $14$ Phalanges.

Long Bone Structure and Composition

Anatomical Regions

• Diaphysis: The shaft of the bone; a hollow tube of dense cortical (compact) bone. Provides maximum strength with minimum weight.
  • Medullary cavity: The hollow center containing yellow bone marrow (fat storage) in adults.
• Epiphyses: The expanded ends of the bone, filled with trabecular (spongy/cancellous) bone.
  • Red bone marrow: Located in the spaces of trabecular bone; site of haematopoiesis (blood cell production).
  • Articular cartilage: Hyaline cartilage covering the epiphysis surface to provide a low-friction interface.
• Metaphysis: The transition zone between the diaphysis and epiphysis.

Bone Tissue Composition

Cartilage Types

• General Characteristics: Cartilage is avascular (no blood supply). Nutrients must diffuse through the matrix, resulting in very slow healing times.
• Chondrocytes: The primary cells of cartilage.

Joint Classification

• Articulation: Any place where bones or bone and cartilage meet. Every bone except the hyoid bone articulates with at least one other.
• Classification Systems:

Structural Classification (Based on what connects them)

• Fibrous: Bones joined by fibrous connective tissue; no joint cavity.
  • Examples: Skull sutures, interosseous membrane of the forearm.
• Cartilaginous: Bones joined by hyaline or fibrocartilage; no joint cavity.
  • Examples: Epiphyseal plates, intervertebral discs.
• Synovial: Characterized by a joint cavity containing synovial fluid and an articular capsule.
  • Examples: Shoulder, hip, knee, elbow.

Functional Classification (Based on degree of movement)

• Synarthrosis: Immobile (e.g., sutures).
• Amphiarthrosis: Slightly movable (e.g., pubic symphysis, intervertebral discs).
• Diarthrosis: Freely movable; includes all synovial joints.

Joint (Body) Movements

Movements are described relative to the anatomical position and occur within specific planes.

• Flexion: Decreases the angle between bones (e.g., bending the elbow).
• Extension: Increases the angle between a joint (e.g., straightening the knee).
• Abduction: Moves a limb away from the body's midline (e.g., raising the arm to the side).
• Adduction: Moves a limb toward the body's midline.
• Rotation: Turning a bone around its own long axis (occurs at pivot and ball-and-socket joints).
• Circumduction: A circular movement that sequentially combines flexion, abduction, extension, and adduction (common in the shoulder and hip).