The human body can carry out an incredible variety of movements, from typing a letter or playing a video game to riding a bike or flipping on a trampoline. Body movements, large or small, require the coordinated action of our muscles and our bones. With only one exception (the hyoid bone in the throat), every bone in the human body meets up with at least one other bone at junctions called joints. Our skeletons are rigid and offer great protection and support, but thanks to joints, they are also flexible and allow for a great range of motion. Without joints, we would be unable to bend and flex.
Joints can be classified by either their structure or their function. Functionally, joints are classified by how much motion they allow. Some joints permit very little movement, but are very strong and durable. Immovable joints and slightly movable joints are restricted mainly to the axial skeleton where protection and stability are key. Other joints provide a greater degree of motion, but do not provide as much strength. Freely movable joints are found on the appendicular skeleton and permit flexibility in the limbs.
Structurally, joints are classified as fibrous, cartilaginous, or synovial. Most fibrous joints are also called "fixed" or "immovable", because they do not move; these joints have no joint cavity and are connected via fibrous connective tissue. Cartilaginous joints are connected entirely by cartilage. Cartilaginous joints allow more movement between bones than a fibrous joint but less than the highly mobile synovial joint. Synovial means relating to a type of joint that is surrounded by a thick flexible membrane forming a sac into which is secreted a viscous fluid that lubricates the joint for easy, painless movement. These classifications are based on whether there is fibrous tissue, cartilage or a fluid filled cavity separating the bony ends of the joint. The skull bones are connected by fibrous joints called sutures. In fetal skulls the sutures are wide to allow slight movement during birth at the same time shielding your brain. The pubic symphysis, the piece of cartilage at the bottom of the pelvic bone, is actually a slightly moveable cartilaginous joint. While all of the types of joints play a role in movement and protection of the human frame, this activity will focus on synovial joints, freely moveable joints. It is these joints and the oily fluid-filled cavity that separates the bones that allow us to swing our arms or jump up and down.
In this activity, you will investigate the main types of synovial joints that are found in the human body. Each joint involves a unique interaction between bones and permits a different set of movements. By looking at an elbow joint of a cow, you will have a chance to see a joint in action and analyze important structure. In the next activity, you will explore the flexibility of joints found in your body as you measure the range of motion of your own joints.
Part I: Background Research Use the Internet to research the structure and function of synovial joints, the main type of joints that assist with movement. Describe at least three distinguishing features of these joints in the space below.
Feature #1: Articular Cartilage – the smooth, white tissue that covers the ends of bones where they come together to form joints; may be damaged by injury or normal wear and tear
Feature #2: Joint Cavity – Part of the joint that is filled with synovial fluid
Feature #3: Articular (fibrous) capsule – An envelope surrounding the synovial joint. Each joint capsule has two parts: an outer fibrous layer and an inner synovial layer.
Feature #4: Synovial Membrane – Also known as synovium or stratum synoviale. Specialized connective tissue that lines the inner surface of capsules of synovial joints. It makes direct contact with the synovial fluid lubricant which it is primarily responsible for maintaining.
Feature #5: Synovial Fluid – Also called synovia. A viscous fluid that reduces friction between the articular cartilage of the synovial joints during movement.
Research the six main types of synovial joints and fill in the table below.