13.6:
Structural Joints: Synovial Joints
Synovial joints are diarthroses or articluations formed between bones that move freely against each other, such as the joints in the limbs.
Articular cartilage, a type of hyaline cartilage, covers the adjoining bones and reduces friction during joint movement.
The articular or joint capsule is a fibrous connective tissue enclosing the joint. Its dense outer tissue attaches to the periosteum of the articulating bones, whereas the loose inner tissue forms the synovial membrane that produces the synovial fluid.
The synovial membrane is supplied by a network of blood vessels. These vessels act as a source of blood plasma which is filtered to form the synovial fluid.
The synovial fluid fills the cavity between the articulating bone surfaces and functions as a lubricant and shock absorber. It also helps to distribute nutrients to the surrounding cells.
Bands of thickened fibrous tissue outside the articulating surfaces form the capsular ligaments that strengthen the synovial joint.
The capsule and ligaments contain sensory nerves that monitor the joint movement and sense pain.
13.6:
Structural Joints: Synovial Joints
Synovial joints are the most common type of joint in the body. A key structural characteristic for a synovial joint is the presence of a joint cavity. This fluid-filled space is where the articulating surfaces of the bones contact each other. Also, unlike fibrous or cartilaginous joints, the articulating bone surfaces at a synovial joint are not directly connected to each other with fibrous connective tissue or cartilage. This gives the bones of a synovial joint the ability to move smoothly against each other, allowing for increased joint mobility.
The walls of the joint cavity are formed by the articular capsule, a fibrous connective tissue structure attached to each bone just outside the area of the bone's articulating surface. Friction between the bones at a synovial joint is prevented by the presence of the articular cartilage that covers the entire articulating surface of each bone. Lining the inner surface of the articular capsule is a thin synovial membrane. The cells of this membrane secrete synovial fluid, a thick, slimy fluid that provides lubrication to further reduce friction between the joint's bones. This fluid also provides nourishment to the articular cartilage, which does not contain blood vessels. The ability of the bones to move smoothly against each other within the joint cavity, and the freedom of joint movement this provides, means that each synovial joint is functionally classified as a diarthrosis.
Outside their articulating surfaces, the bones are connected by ligaments, which are strong bands of fibrous connective tissue. These strengthen and support the joint by anchoring the bones together and preventing their separation. Ligaments allow for normal movements at a joint but limit the range of these motions, thus preventing excessive or abnormal joint movements. At many synovial joints, additional support is provided by the muscles and their tendons that act across the joint. A tendon is a dense connective tissue structure that attaches a muscle to bone. As forces acting on a joint increase, the body will automatically increase the overall strength of contraction of the muscles crossing that joint, thus allowing the muscle and its tendon to serve as a "dynamic ligament" to resist forces and support the joint. This indirect support by muscles is essential at the shoulder joint, for example, where the ligaments are relatively weak.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 9.4: Synovial Joints