Hooking Up: Joints
Each time you lean forward, pick up a cup of coffee, raise it to your lips and put it back down, your bones, joints, muscles and other tissues are all working in perfect synchronicity to make this effort possible.
There are 68 joints in the human body, and each joint is comprised of several elements. Among them are:
- Bones. Well yeah, but more precisely, the articular cartilage at the end of the bones. This cartilage prevents the bone ends from being damaged by contact with each other. Cartilage itself can be harmed by infection, injury, disease or simple wear and tear. This damage may lead to pain, inflammation and stiffness, a condition known as arthritis.
- Skeletal muscle. Skeletal muscle attaches to bone and appears striped when viewed closely, earning the name "striated muscle." Unlike your cardiac muscle or the muscle in the walls of your stomach, skeletal muscle can be voluntarily moved, and lie at rest when not consciously activated. These muscles connect to bones through tendons.
- Tendons. When skeletal muscle contracts or lengthens, it pulls on bone through an attached tendon, a tough, flexible tissue.
- Ligaments. These tissues are pretty similar to tendons, except they connect bone to bone, ensuring that bones that meet together to form a joint will stay in place.
- Synovial membrane. This layer of connective tissue exists around each joint, providing it with protection and producing synovia, a fluid that lubricates the joint and nourishes the cartilage.
- Bursa. Similar to the synovial membrane, the bursa is a small sac that provides a lubricant to ease the movement of muscle against muscle or muscle against bone.
Not every joint moves. The skull, for example, consists of several bone plates that join together, but the fibrous tissue connecting these plates doesn't allow for movement.