In this video, we're going to talk about the 3rd type of fibrous joint in our lesson, which is the syndesmosis. In the plural form, they are called syndesmoses. These syndesmoses are fibrous joints connecting bones via bundles or ligaments that are made of dense irregular connective tissue. This type of fibrous connective tissue is characterized by densely packed protein fibers in the extracellular matrix with an irregular or random protein fiber arrangement, which allows them to resist forces in multiple directions. Now, the greater bone separation and the longer fibers of these syndesmoses actually allow for more mobility than the other two types of fibrous joints, the sutures and the gomphoses. Recall from our previous lesson videos that whereas the sutures and gomphoses were both classified as synarthroses, since they were still or stationary joints that essentially allowed for no movement and are generally considered immovable joints.
Whereas the syndesmoses, because of their greater bone separation and longer fibers allowing for more mobility, they are always going to be classified as amphiarthroses. Amphiarthroses allow for a little movement, or an average amount of movement, if you will. Notice the symbol for amphiarthroses has one movement arrow going around it, which can hopefully help remind you that these syndesmoses are amphiarthroses that have a little or an average amount of movement. Interosseous membranes are also closely associated with these syndesmoses.
These very broad sheets of dense connective tissue, either dense regular or dense irregular connective tissue, are found between certain bones. Let's take a look at our image below where we can start to piece some of these ideas together. Notice here in the middle we have a person and we're focusing on specific regions of their skeleton. On the left-hand side, we're focusing on the person's lower leg, and you can see the tibia here and the fibula over here. Zooming in on the distal portion of the tibia and fibula, we can see that there is a syndesmosis here connecting the tibia and fibula at the distal end.
This syndesmosis is a ligament made of dense irregular connective tissue. You'll also notice that in between the tibia and fibula is this broad sheet of dense connective tissue, the interosseous membrane. On the right-hand side, we're focusing on this person's forearm and you can see the ulna here and the radius over here. Notice that between the radius and the ulna is another interosseous membrane. However, the protein fibers in the interosseous membrane between the ulna and the radius are longer and broader fibers.
Whereas, the fibers in the interosseous membrane between the tibia and fibula are shorter fibers, and so it's not quite as broad. The shorter fibers between the tibia and fibula are not going to accommodate as much movement, allowing for less movement. Whereas the longer fibers that we can see here between the radius and the ulna are going to allow and accommodate more movement. This is partially why our forearms are able to have more movement and accommodate more movement than our lower leg.
This concludes our brief lesson on syndesmosis, and we'll be able to get some practice applying these concepts and learn more about other types of joints as we move forward in our course. I'll see you all in our next video.