Articulations - Human Anatomy and Physiology I

HUMAN ANATOMY AND PHYSIOLOGY I

Synarthrosis:
- No movement occurs between the articulating bones.
Amphiarthrosis:
- Small amount of movement allowed between articulating bones.
Diarthrosis:
- Freely movable joints that allow a wide variety of specific movements.

Fibrous Joints:
- Fastened together by dense regular collagenous connective tissue without joint space; can be synarthroses or amphiarthroses.
Cartilaginous Joints:
- Joined by cartilage without a joint space; can also be synarthroses or amphiarthroses.
Synovial Joints:
- Classified as diarthrosis; have a layer of hyaline cartilage on the articulating surfaces of each bone, featuring a fluid-filled joint space between the bones.

Contain dense regular collagenous connective tissue, offering stability mainly, permitting little to no motion.
Types of Fibrous Joints:
1. Sutures:
- Immovable joints between the edges of bones, primarily found in the cranium; when fully fused, they provide stability and protection to the brain.
1. Gomphoses:
- Immovable joint (synarthrosis) between each tooth and its bony socket in the jaw; secured by the periodontal ligament, a robust fibrous membrane that links the tooth to the jaw bone.
1. Syndesmoses:
- A joint between the tibia and fibula or ulna and radius; bones united by an interosseous membrane or ligament made of dense regular collagenous connective tissue, permitting small movements (amphiarthroses).

In cartilaginous joints, articulating bones are held together by cartilage, lacking a joint cavity and generally allowing limited movement.
Types of Cartilaginous Joints:
1. Synchondroses:
  - Bones linked by hyaline cartilage; classified as synarthroses. Examples include:
  - Epiphyseal Plates: Hyaline cartilage connects the diaphyses and epiphyses of developing long bones, being replaced by bone during maturation. Movement occurring at the epiphyseal plate could disrupt normal structure and function, potentially affecting development.
  - First Sternocostal and Costochondral Joints: Stabilize the rib cage and persist into adulthood as synchondroses.
2. Symphyses:
  - Bones united by a fibrocartilaginous pad or plug, functionally categorized as amphiarthroses. Examples include:
  - Intervertebral Joints: Situated between adjacent vertebral bodies of the spinal column, allowing for slight motion while resisting compression.
  - Pubic Symphysis: Located between the pubic bones of the pelvic girdle, similarly allowing slight motion and providing stability.

Joint Cavity (Synovial Cavity):
- The space between articulating bones in synovial joints.
Articular Capsule:
- A double-layered structure:
  - Outer Fibrous Layer: Keeps articulating bones from being pulled apart and isolates the joint from the body's rest.
  - Inner Layer (Synovial Membrane): Lines the inner surface except where hyaline cartilage is present; secretes synovial fluid.

A thick liquid with three primary functions:
- Lubrication: Reduces friction between the articulating surfaces, allowing for smoother movement.
- Transportation: Moves nutrients and waste products where blood vessels are absent within the joint.
- Shock Absorption: Distributes stress and force evenly on articular surfaces during movement.
Articular Cartilage:
- A thin layer of hyaline cartilage that covers all exposed articulating bones, providing a smooth surface for interaction; being avascular, it relies on synovial fluid for oxygen, nutrients, and waste removal.
Other Components: Include adipose tissue, nerves, and blood vessels for additional support.

Synovial joints permit more mobility but have less stability than other joint types; the following structures provide extra stabilization:
- Ligaments:
- Dense, regular collagenous connective tissue that links one bone to another, enhancing joint strength and reinforcement.
- Tendons:
- Structural components of skeletal muscle that connect muscle to bone; tendons crossing associated joints provide stabilization when the associated muscles contract.
- Muscle Tone:
- The continuous level of muscle contraction that provides a stabilizing force underlying joint function.
Bursae and Tendon Sheaths:
- Bursa: A synovial fluid-filled fibrous structure that minimizes friction between moving parts of joints.
- Tendon Sheath: A long bursa which surrounds tendons, offering protection as they slide across joints, especially in high-stress regions.

Elbow Joint:
- Represents a uniaxial joint (Axis 1 in figure), akin to a hinge; movement occurs in one plane perpendicular to the axis. This allows the forearm and hand to move either upward toward the shoulder or downward away from the shoulder.
Metacarpophalangeal Joints:
- Classified as biaxial joints (between proximal phalanges and metacarpals):
  - Axis 1: Allows proximal phalanges to move toward and away from the palm of the hand.
  - Axis 2: Allows fingers to squeeze together or fan out.
Multiaxial Joint Example (Shoulder):
- More complex than uniaxial and biaxial joints, allowing movement in three distinct axes:
  - Axis 1: Forward and backward movement (humerus swinging back and forth).
  - Axis 2: Movement away from and towards the body (like in jumping jacks).
  - Axis 3: Rotational movement (as in throwing a Frisbee).