Lecture Notes on Articulations in Human Anatomy and Physiology I

HUMAN ANATOMY AND PHYSIOLOGY I

LECTURE 18: ARTICULATIONS

  • Instructor: Dr. Shaun D. Cain
  • Course: Biological Sciences 231, Fall 2025
  • Institution: Eastern Oregon University

OVERVIEW OF JOINTS

  • Functions of Joints:
    • Enable Movement:
    • Joints serve as a link between bones, allowing movement when muscles and tendons exert force across the joint.
    • Provide Stability:
    • Some joints allow limited or no movement, providing critical stability for certain structures (e.g., skull) and protecting underlying structures.
    • Allow Long Bones to Lengthen:
    • The epiphyseal plate is where long bones grow in length during skeletal development; this plate acts as a temporary joint.

CLASSIFICATION OF JOINTS

BY MOTION

  • Synarthrosis:
    • No movement between articulating bones.
  • Amphiarthrosis:
    • Small amount of movement between articulating bones.
  • Diarthrosis:
    • Freely movable joints, allowing a wide variety of specific movements.

BY STRUCTURAL FEATURES

  • Fibrous Joints:
    • Connected by dense regular collagenous connective tissue with no joint space between articulating bones. They can be classified as:
    • Synarthroses
    • Amphiarthroses
  • Cartilaginous Joints:
    • Connected by cartilage with no joint space. They can be classified as:
    • Synarthroses
    • Amphiarthroses
  • Synovial Joints:
    • Classified as diarthrosis, these joints possess a layer of hyaline cartilage on the articulating surface of each bone; they feature a fluid-filled joint space between articulating bones.

FIBROUS AND CARTILAGINOUS JOINTS

FIBROUS JOINTS

  • Composed of dense regular collagenous connective tissue; features include:
    • Lends stability but allows little to no motion.
    • Types of Fibrous Joints:
    • Sutures:
      • Immovable joints found between the edges of bones, mainly in the cranium. Fully fused sutures are stable and protective of the brain.
    • Gomphoses:
      • Immovable joints (synarthroses) between each tooth and the bony socket of the jaw, linked by the periodontal ligament, a strong fibrous membrane that firmly anchors the tooth to the jawbone.
    • Syndesmoses:
      • Joints between tibia and fibula, and ulna and radius, where bones are connected by an interosseous membrane or ligament made of dense regular collagenous connective tissue, allowing for a small amount of movement (classified as amphiarthroses).

CARTILAGINOUS JOINTS

  • The articulating bones in cartilaginous joints are held together by cartilage; features include:
    • Lack of joint cavity and minimal motion.
    • Types of Cartilaginous Joints:
    • Synchondroses:
      • Bones linked together by hyaline cartilage; classified as synarthroses. Notable examples include:
      • Epiphyseal plates connecting the diaphysis and epiphysis of developing long bones, replaced by bone during maturation. If disrupted, could impair function and development.
      • First sternocostal and costochondral joints, which stabilize the rib cage and exist into adulthood.
    • Symphyses:
      • Bones united by a fibrocartilaginous pad or plug, functionally classified as amphiarthroses. Best suited for regions that resist compression and tension while allowing slight motion. Examples include:
      • Intervertebral joints between adjacent vertebral bodies of the spinal column.
      • Pubic symphysis between the pubic bones of the pelvic girdle.

STRUCTURE OF SYNOVIAL JOINTS

STRUCTURAL ELEMENTS

  • Joint Cavity (Synovial Cavity):
    • The space between articulating bones.
  • Articular Capsule:
    • A double-layered structure consisting of:
    • Outer Fibrous Layer:
      • Prevents the articulating bones from being pulled apart and isolates the joint from the rest of the body.
    • Inner Synovial Membrane:
      • Lines the inner surface of the capsule, except where hyaline cartilage is present, and secretes synovial fluid.
  • Synovial Fluid:
    • A viscous liquid with three main functions:
    • Lubrication:
      • Reduces friction between articulating surfaces.
    • Transportation:
      • Moves nutrients and waste products in areas lacking blood vessels within the joint.
    • Shock Absorption:
      • Distributes stress and force evenly across articular surfaces during movement.
  • Articular Cartilage:
    • Thin layer of hyaline cartilage covering all exposed articulating bones, providing a smooth surface for interaction and reducing wear from friction. Notable characteristics include:
    • Avascularity, relying on synovial fluid for oxygen, nutrients, and waste removal.
  • Other Components of Synovial Joint:
    • Include adipose tissue, nerves, and blood vessels.

STABILIZING AND SUPPORTING FACTORS

  • Synovial joints offer greater mobility but less stability than other types of joints. The following elements enhance stability:
    • Ligaments:
    • Dense, regular collagenous connective tissue strands linking one bone to another, adding strength and reinforcement to the joint.
    • Tendons:
    • Structural components of skeletal muscles made of dense regular collagenous connective tissue; connect muscles to bones. Tendons that cross joints contribute to stabilization during muscle contraction.
    • Muscle Tone:
    • Continual contraction of muscles serving to stabilize joints.
  • Bursae and Tendon Sheaths:
    • Additional stabilizing elements in high-stress regions.
    • Bursa:
    • Synovial fluid-filled fibrous structures minimizing friction between moving joints.
    • Tendon Sheath:
    • An elongated bursa that surrounds tendons, protecting them as they slide across joints.

FUNCTION OF SYNOVIAL JOINTS

FUNCTIONAL CLASSES OF SYNOVIAL JOINTS

  • Bones in a synovial joint can move in various planes around different axes. The configurations include:
    • Nonaxial Joints:
    • Allow movements in one or more planes without rotating around an axis.
    • Uniaxial Joints:
    • Allow motion around only one axis.
    • Biaxial Joints:
    • Allow motion around two axes.
    • Multiaxial (Triaxial) Joints:
    • Allow motion around three axes.

CONCEPT BOOST: UNDERSTANDING AXES OF MOTION

  • Elbow Joint:
    • A uniaxial joint functioning as a hinge, permitting motion in one plane perpendicular to its single axis.
    • Allows the forearm and hand to move upward toward the shoulder or away from it.
  • Metacarpophalangeal Joints:
    • Biaxial joints situated between proximal phalanges and metacarpals.
    • Axis 1:
    • Allows proximal phalanges to move towards and away from the palm (similar movement to the elbow).
    • Axis 2:
    • Facilitates finger actions such as squeezing together or fanning out.
  • Multiaxial Joints:
    • Allow additional movements not achievable by uniaxial or biaxial joints; the shoulder acts as an example:
    • Humerus can move back and forth around Axis 1 (humeral swing while walking).
    • Humerus can move away from and toward the body around Axis 2 (jumping jacks).
    • Humerus can rotate around Axis 3 (e.g., throwing a frisbee).

MOVEMENTS AT SYNOVIAL JOINTS

  • Four general types of movement can occur at synovial joints:
    • Gliding Movements:
    • Sliding motions occurring between articulating surfaces; classified as nonaxial.
    • Angular Movements:
    • Insert or restrict the angle between articulating bones.
    • Rotation:
    • A nonangular movement where one bone rotates along an imaginary line running down its longitudinal axis.
    • Special Movements:
    • Refer to various other types of motions distinct from the aforementioned categories.