Skeletal System

Bone Histology

The microscopic structure of bone reveals specialized features that support its function in the body. Understanding bone histology is essential for identifying bone types and their roles.

• Osteon (Haversian System): The fundamental functional unit of compact bone, consisting of concentric lamellae arranged around a central canal containing blood vessels and nerves.

• Spongy Bone vs. Compact Bone:

  • Compact bone is dense and forms the outer layer of bones, providing strength for weight-bearing.

  • Spongy bone (cancellous bone) is found at the ends of long bones and inside flat bones, containing trabeculae that support marrow.

• Location in Long Bones: Compact bone forms the diaphysis (shaft), while spongy bone is found in the epiphyses (ends).

• Connective Tissue Coverings:

  • Periosteum: Dense connective tissue covering the external surface of bones; contains osteoblasts and osteoclasts.

  • Endosteum: Thin membrane lining the internal bone surfaces, including the medullary cavity.

Skeletal Gross Anatomy

The human skeleton is divided into axial and appendicular components, each with distinct bones and functions.

• Axial Skeleton: Includes the skull, vertebral column, ribs, and sternum. Provides central support and protects vital organs.

• Appendicular Skeleton: Comprises the limbs and girdles (pectoral and pelvic), facilitating movement and interaction with the environment.

• Bone Identification: Key bones of the skull (e.g., parietal, temporal, frontal, occipital, sphenoid, ethmoid, mandible, maxilla, zygomatic, nasal, lacrimal) and their landmarks (e.g., sutures, foramina, processes) are essential for anatomical orientation.

• Shape and Function: Bones are classified by shape (long, short, flat, irregular, sesamoid) and serve functions such as support, protection, movement, mineral storage, and hematopoiesis.

Major Bones of the Axial Skeleton

• Skull: Parietal, temporal, frontal, occipital, sphenoid, ethmoid, mandible, maxilla, zygomatic, nasal, lacrimal

• Vertebral Column: 7 cervical, 12 thoracic, 5 lumbar vertebrae, sacrum (5 fused), coccyx

• Thoracic Cage: Sternum (manubrium, body, xiphoid process), ribs

• Hyoid bone

Major Bones of the Appendicular Skeleton

• Pectoral Girdle: Clavicle, scapula

• Upper Limb: Humerus, radius, ulna, carpals, metacarpals, phalanges

• Pelvic Girdle: Ilium, ischium, pubis, sacrum, coccyx, pubic symphysis, acetabulum, iliac crest

• Lower Limb: Femur, patella, tibia, fibula, tarsals, metatarsals, phalanges

Distinguishing Male and Female Skeletons

• Pelvic Differences: Female pelvis is wider and shallower for childbirth; male pelvis is narrower and deeper.

• Other Features: Males generally have more prominent bone markings due to larger muscle attachments.

Spinal Curvatures and Disorders

• Normal Curves: Cervical and lumbar (lordotic), thoracic and sacral (kyphotic)

• Abnormal Curvatures:

  • Kyphosis: Excessive posterior curvature, usually in thoracic region

  • Lordosis: Excessive anterior curvature, usually in lumbar region

  • Scoliosis: Lateral curvature of the spine

Articulations and Movements

Joints (articulations) connect bones and allow for movement. They are classified by structure and function.

• Structural Classification:

  • Fibrous Joints: Bones joined by dense connective tissue (e.g., sutures of the skull)

  • Cartilaginous Joints: Bones joined by cartilage (e.g., intervertebral discs)

  • Synovial Joints: Bones separated by a joint cavity filled with synovial fluid; most movable type

• Functional Classification: Synarthrosis (immovable), amphiarthrosis (slightly movable), diarthrosis (freely movable)

• Synovial Joint Structure: Articular cartilage, joint (synovial) cavity, articular capsule (fibrous and synovial layers), synovial fluid, ligaments, nerves, and blood vessels

Types of Synovial Joints and Movements

• Types: Plane, hinge, pivot, condyloid, saddle, ball-and-socket

• Movements: Flexion/extension, abduction/adduction, circumduction, rotation, pronation/supination, dorsiflexion/plantar flexion, elevation/depression, protraction/retraction, opposition/reposition, eversion/inversion

Muscular System

Muscle Histology

Muscle tissue is specialized for contraction and is classified into three types based on structure and function.

• Skeletal Muscle: Striated, voluntary, multinucleated; attached to bones for movement

• Cardiac Muscle: Striated, involuntary, branched, with intercalated discs; found in the heart

• Smooth Muscle: Non-striated, involuntary, spindle-shaped; found in walls of hollow organs

Sarcomere Structure

• Sarcomere: The functional contractile unit of skeletal muscle, defined by Z-disks

• Key Components: Actin (thin) and myosin (thick) myofilaments, A band, I band, H zone, M line, Z disk

Muscle Organization

• Muscle Fiber: Single muscle cell

• Endomysium: Connective tissue surrounding each muscle fiber

• Perimysium: Surrounds bundles of fibers (fascicles)

• Epimysium: Surrounds the entire muscle

• Fascia: Connective tissue that separates muscles from each other

Muscle Physiology

Muscle contraction is influenced by electrical and mechanical factors.

• Effect of Voltage: Increasing stimulus voltage recruits more muscle fibers, increasing tension (up to a maximum).

• Frequency of Stimulus: Higher frequency leads to summation and tetanus (sustained contraction).

• Muscle Length: Optimal length allows maximal cross-bridge formation and tension generation.

Additional info: Refer to PhysioEx simulations for practical demonstrations of these principles.

Gross Muscle Anatomy

Understanding muscle anatomy involves identifying origins, insertions, shapes, and connective tissue structures.

• Origin: The fixed attachment point of a muscle

• Insertion: The movable attachment point

• Muscle Shapes: Circular (orbicularis), pennate (rectus femoris), parallel (sartorius), convergent (pectoralis major), fusiform (biceps brachii)

• Tendon: Dense connective tissue attaching muscle to bone

• Aponeurosis: Broad, flat tendon (e.g., epicranial aponeurosis)

• Muscle Actions: Flexion, extension, abduction, adduction, rotation, etc.

Muscle Labeling and Actions

• Be able to identify major muscles on diagrams and models

• Know the primary action of each muscle (e.g., gracilis adducts the thigh)

Summary Table: Comparison of Bone and Muscle Tissue Types