Skeletal System Joints and Muscle Actions Explained

Act 6: Joint Classifications

Synovial Joint

In a synovial joint, the bones move and are lined by cartilage. Between the bones, a bursa is present, which secretes synovial fluid. The bones are wrapped in a tough, flexible tissue called the joint capsule, which prevents the bones from dislocating. Ligaments are found outside the joint capsule.

Examples include the wrist, shoulder, and hip.

Synarthrosis

These are stiff and motionless bone joints, such as those connecting the bones of the skull, known as sutures.

Amphiarthrosis

These are semi-mobile joints, mainly found in the coxal bones at the symphysis pubis and in the vertebrae. The bones are united by cartilage; between the vertebral bodies, there is a disc called fibrocartilage, which allows for slight movement.

Key Joint Components

• Ligament: Tissue that connects and holds two bones together at a joint.
• Cartilage: Connective tissue found in joints that provides support and aids movement.
• Meniscus: Crescent-shaped cartilage found between the femur and tibia; its function is to cushion movements.
• Synovial Fluid: Fluid that lubricates the cartilage covering the ends of the bones, allowing them to glide over one another.
• Synovial Membrane: A thin membrane lining the inner side of the fibrous capsule and the non-articulated bone surfaces; it secretes synovial fluid.
• Articular Surface: Smooth areas found on the epiphyses of long bones.
• Auricular Cartilage: Covers the entire articular surface of both bones to limit movement between them. This cartilage is white, smooth, and bright, facilitating surface movement.

Act 2: Muscle Actions

Muscle Roles

• Agonist: The muscle that supplies the main force to perform a particular movement. Example: Pectoralis Major.
• Antagonist: A muscle that has an action contrary to the agonist muscle. Example: Biceps antagonist (triceps), etc.

Types of Movement

• Flexion: Bending a muscle, decreasing the angle of the joint, which brings the bones closer together. For example, bending the arm.
• Extension: The opposite of flexion; the muscle extends, increasing the articulation angle.
• Abduction: Moving a bone away from the midline of the body (e.g., moving the arm laterally and upward).
• Adduction: The opposite of abduction; moving the bone toward the midline of the body (e.g., moving the arm back toward the body).