Skeletal System and Joints: Comprehensive Notes

Functions of the Skeletal System

• The skeletal system has several crucial functions:

  • Protection: Encases and shields vital organs. For example, the skull protects the brain, the rib cage protects the heart and lungs, and the vertebral column protects the spinal cord.

  • Mineral Storage and Acid-Base Homeostasis: Acts as a reservoir for essential minerals, primarily calcium (Ca^{2+}) and phosphate (PO_4^{3-}). These minerals are critical for various physiological processes, including nerve function, muscle contraction, and blood clotting. The bones also play a role in maintaining acid-base balance by releasing or absorbing alkaline salts.

  • Blood Cell Formation (Hematopoiesis): Red bone marrow, found within certain bones, is the primary site of blood cell production, including red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).

  • Fat Storage: Yellow bone marrow, located within the medullary cavity of long bones, serves as a storage site for triglycerides, which can be utilized as an energy source.

  • Movement: Provides a framework for muscle attachment. Muscles contract and exert force on bones, producing body movements and enabling locomotion.

  • Support: Provides structural support, maintaining the body's posture and bearing its weight.

Major Bones of the Skeleton and Bone Markings

• The adult human skeleton typically comprises 206 individual bones. These bones are organized into two principal divisions:

  • Axial Skeleton: Forms the central axis of the body, encompassing the following components:

  • Skull: Protects the brain and houses sensory organs.

  • Vertebral Column: Provides support and flexibility to the trunk.

  • Ribs: Form a protective cage around the thoracic organs.

  • Sternum: Connects the ribs in the anterior chest.

  • Appendicular Skeleton: Consists of the bones of the upper and lower limbs, as well as the girdles that attach them to the axial skeleton:

  • Upper Limb Bones: Scapula (shoulder blade), clavicle (collarbone), humerus (upper arm), radius and ulna (forearm), carpals (wrist), metacarpals (hand), and phalanges (fingers).

  • Lower Limb Bones: Femur (thigh), tibia and fibula (lower leg), foot, os coxae (hip bone), hand, patella (kneecap).

Bone Markings

• Bone markings are distinct surface features on bones that serve various functions. These markings include:

  • Depressions:

  • Fossa: A shallow depression in a bone's surface, often serving as an area for muscle attachment or articulation.

  • Sulcus: A narrow groove or furrow that accommodates blood vessels, nerves, or tendons.

  • Projections:

  • Tubercle: A small, rounded projection or protuberance, typically serving as an attachment site for muscles or ligaments.

  • Trochanter: A massive, rough projection found exclusively on the femur, providing attachment points for powerful hip muscles.

  • Line: A low ridge or elongated elevation on a bone's surface, often indicating muscle attachment.

  • Head: A prominent, rounded epiphysis (end of a long bone) that articulates with another bone.

  • Condyle: A large, smooth, rounded oval surface, usually found at the end of a bone, that articulates with another bone to form a joint.

Structure of Bone

• Bone tissue exists in two primary forms:

  • Compact Bone:

  • Forms the external layer of bones, providing strength and rigidity.

  • Characterized by its solid and relatively dense structure, with few visible spaces.

  • Cancellous Bone (Spongy Bone):

  • Located on the internal aspect of bones, particularly at the epiphyses of long bones.

  • Exhibits an open, lattice-like network of narrow plates called trabeculae, which creates a porous structure.

Compact Bone and the Osteon

• Osteon (Haversian System): The fundamental structural unit of compact bone.

  • Cylindrical structures aligned parallel to each other along the long axis of the bone.

  • Concentric Lamellae: Circular layers of bone matrix arranged concentrically around a central canal.

  • Central Canal (Haversian Canal): A longitudinal channel located at the center of each osteon, containing blood vessels, nerves, and lymphatic vessels.

  • Lacunae: Small cavities or spaces between lamellae that house osteocytes (mature bone cells).

Cancellous Bone and Trabeculae

• Trabeculae: Interconnecting rods or plates of bone tissue that form the lattice-like structure of cancellous bone.

  • Composed of irregularly arranged lamellae and osteocytes interconnected by canaliculi.

  • Unlike compact bone, trabeculae do not contain osteons, and nutrients and waste products are exchanged directly between osteocytes and the marrow.

Bone Cell Types

• Bone tissue is populated by four main cell types, each with distinct functions:

  • Osteoblasts: Bone-forming cells responsible for synthesizing and secreting the organic components of bone matrix (osteoid). Osteoblasts also initiate mineralization, the process by which osteoid becomes hardened bone.

  • Osteocytes: Mature bone cells derived from osteoblasts, residing within lacunae. Osteocytes maintain the bone matrix and communicate with osteoblasts to regulate bone remodeling.

  • Osteoclasts: Large, multinucleated cells responsible for bone resorption (breakdown). Osteoclasts secrete enzymes and acids that dissolve the mineral and organic components of bone, releasing calcium (Ca^{++}) and other minerals into the bloodstream.

Bone Extracellular Matrix (ECM)

• The extracellular matrix (ECM) of bone is a complex composite material that gives bone its unique properties.

  • Inorganic Matrix (65%): Primarily composed of calcium and phosphorus in the form of hydroxyapatite crystals, which confer hardness and rigidity to bone.

  • Organic Matrix (35%): Consists mainly of collagen fibers, which provide flexibility and resistance to tension. The organic matrix also contains various proteins and glycosaminoglycans.

Structure of a Long Bone

• Long bones, such as the femur and humerus, have a characteristic structure:

  • Epiphyses: The expanded ends of a long bone that articulate with other bones to form joints. The epiphyses are composed primarily of spongy bone covered by a thin layer of compact bone.

  • Articular (Hyaline) Cartilage: A smooth, hyaline cartilage layer that covers the articular surfaces of the epiphyses, reducing friction and absorbing shock in movable joints.

  • Diaphysis: The long, cylindrical shaft of the bone, composed mainly of compact bone. The diaphysis surrounds the medullary cavity.

  • Medullary Cavity: A hollow space within the diaphysis that contains yellow (fatty) marrow in adults.

  • Periosteum: A tough, fibrous connective tissue membrane that covers the external surface of the diaphysis (except where articular cartilage is present). The periosteum anchors blood vessels and nerves to the bone and contains osteoprogenitor cells and osteoblasts, which are active in bone remodeling, growth, and fracture repair.

Bone Growth and Remodeling

• Ossification or Osteogenesis: The process of bone formation and development, which begins during embryonic development and continues into adulthood.

  • Two General Patterns of Ossification:

  • Intramembranous Ossification: Bone formation directly from mesenchyme (embryonic connective tissue), producing flat bones of the skull and a few other bones.

  • Endochondral Ossification: Bone formation that begins with a hyaline cartilage model, which is gradually replaced by bone tissue. This process forms the majority of bones in the body.

Bone Remodeling

• Bone is a dynamic tissue that undergoes continuous remodeling throughout life, involving the coordinated actions of osteoblasts and osteoclasts.

  • Bone Deposition by Osteoblasts: Osteoblasts synthesize and secrete osteoid, which then mineralizes to become bone.

  • Bone Resorption by Osteoclasts: Osteoclasts break down bone tissue, creating Howship’s lacunae (resorption lacunae).

• Remodeling is influenced by various factors, including exercise, calcium intake, and hormones (e.g., estrogen).

• Bone remodeling is essential for maintaining bone health, repairing microdamage, and adapting to mechanical stress.

Osteoporosis

• A condition characterized by a decrease in bone mineral density, leading to increased bone fragility and a higher risk of fractures.

  • Bone resorption outpaces bone deposition, resulting in a net loss of bone mass.

  • Cancellous bone is particularly susceptible to osteoporosis.

  • Can lead to vertebral compression fractures and hip fractures.

  • Postmenopausal women are at increased risk due to decreased estrogen levels.

  • Other risk factors include inadequate calcium intake, malabsorption of nutrients, and certain medical conditions.

Articulations Defined

• An articulation, or joint, is the point where two or more bones, cartilage, or teeth come together. Joints provide stability and enable movement.

• Articulations vary in structure, stability, and mobility, depending on their specific functions.

Joint Classification

• Joints are classified based on their structure and function:

  • Structural Classification:

  • Fibrous Joints: Bones connected by dense connective tissue.

  • Cartilaginous Joints: Bones connected by cartilage.

  • Synovial Joints: Bones separated by a fluid-filled joint cavity.

  • Functional Classification:

  • Synarthrosis: Immovable or slightly movable joints.

  • Amphiarthrosis: Slightly movable joints.

  • Diarthrosis: Freely movable joints.

Fibrous Joints

• Bones are joined by dense irregular connective tissue, lacking a joint cavity.

• Most fibrous joints are synarthrotic or amphiarthrotic.

• Three specific types:

  • Sutures: Interlocking, irregular edges of skull bones joined by a thin layer of dense fibrous connective tissue, permitting very little movement (synarthrotic).

  • Syndesmoses: Bones connected by ligaments or interosseous membranes, allowing limited movement (amphiarthrotic).

  • Gomphoses: Peg-in-socket joints, such as teeth held in the alveolar sockets of the maxilla and mandible by periodontal ligaments (synarthrotic).

Cartilaginous Joints

• Articulating bones are tightly connected by hyaline cartilage or fibrocartilage, lacking a joint cavity.

• Cartilaginous joints are synarthrotic or amphiarthrotic.

• Two types:

  • Synchondroses: Bones connected by hyaline cartilage, usually temporary and eventually ossify (synarthrotic).

  • Symphyses: Bones connected by a pad of fibrocartilage, providing both strength and flexibility (amphiarthrotic).

Synovial Joints

• Freely movable (diarthrotic) joints characterized by a fluid-filled joint cavity between the articulating bones.

  • Articular Capsule: A double-layered capsule enclosing the joint cavity:

  • Outer Fibrous Layer: Provides strength and support.

  • Inner Synovial Membrane: Lines the joint cavity, secreting synovial fluid.

• Synovial Fluid: A viscous, lubricating fluid that fills the joint cavity, reducing friction, nourishing articular cartilage, and absorbing shock during joint movement.

• Hyaline (Articular) Cartilage: A smooth, hyaline cartilage layer that covers the articulating surfaces of the bones, preventing bone-to-bone contact and reducing friction.

Function of Articular Cartilage

• Articular cartilage plays a critical role in joint function by reducing friction and absorbing shock during movement.

• Absence or damage to articular cartilage, as seen in osteoarthritis, leads to increased friction, pain, and joint degeneration.

Types of Synovial Joints

• Synovial joints are classified based on the shape of the articulating surfaces and the type of movement they allow:

  • Plane Joints: Flat or slightly curved surfaces that allow gliding or sliding movements (e.g., intercarpal joint).

  • Hinge Joints: Convex surface of one bone fitting into the concave surface of another, allowing flexion and extension movements (e.g., elbow joint).

  • Pivot Joints: Rounded end of one bone fitting into a ring formed by another bone, allowing rotational movements (e.g., radioulnar joint).

  • Condylar Joints: Oval-shaped condyle of one bone fitting into the elliptical cavity of another, allowing flexion, extension, abduction, and adduction movements (e.g., radiocarpal joint).

  • Saddle Joints: Both articulating surfaces have a saddle shape, allowing a wide range of movements, including flexion, extension, abduction, adduction, and circumduction (e.g., carpometacarpal joint of the thumb).

  • Ball-and-Socket Joints: Spherical head of one bone fitting into the cup-shaped socket of another, allowing the greatest range of motion, including flexion, extension, abduction, adduction, rotation, and circumduction (e.g., glenohumeral joint).

Movements at Synovial Joints

• The degree and direction of movement at synovial joints are determined by the shape of the articulating bones and the ligaments and tendons crossing the joints.

  • Movements:

  • Flexion: Bending a joint, decreasing the angle between the bones.

  • Extension: Straightening a joint, increasing the angle between the bones.

  • Abduction: Moving a bone away from the midline of the body.

  • Adduction: Moving a bone toward the midline of the body.

  • Rotation: Turning a bone around its longitudinal axis.

  • Gliding: Sliding or gliding movements between flat or slightly curved surfaces.

  • Special Movements: Unique movements that occur at specific joints, such as pronation, supination, inversion, eversion, dorsiflexion, and plantarflexion.

Rotational Movements

• Rotation involves the movement of a bone around its longitudinal axis.

  • In Limbs:

  • Medial Rotation (Internal Rotation): Turning the anterior surface of a limb toward the midline of the body.

  • Lateral Rotation (External Rotation): Turning the anterior surface of a limb away from the midline of the body.