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Angela has taught college Microbiology and has a doctoral degree in Microbiology.
Reach down and put your hand on the back of your lower leg. What you should be feeling is the largest of your calf muscles, the gastrocnemius muscle. Now, start sliding your hand down your calf. The muscle should begin to narrow until all you can feel is a thin, cord-like structure that runs from the narrow point of your calf and seems to dead end on your heel bone, the calcaneus bone. You probably already know that this is your Achilles tendon. But what exactly is a tendon, anyway?
The simplest answer is that a tendon connects skeletal muscles to bones. As is generally the case in science, however, the story of the tendon requires a few more chapters than that.
Every structure in your body can be broken down into four basic types of tissues. Epithelial tissue covers surfaces and lines cavities. Muscle tissue generates force and movement. Nervous tissue detects bodily changes and relays messages. And connective tissue protects and supports organs and other tissues.
Tendons fall into the connective tissue category. A complete tendon is built by building up and combining multiple layers of connective tissue. Let's examine the building process, beginning at the microscopic level.
The primary building blocks of tendons are collagen fibers. These fibers are very strong, flexible, and resistant to damage from pulling stresses. Collagen fibers are usually arranged in parallel bundles, which helps multiply the strength of the individual fibers.
Now, do you remember that the function of a tendon is to connect muscle to bone? Well, the structure of the tendon and the muscle are literally connected and intertwined. Deep inside a muscle are individual muscle fibers. Collagen, in conjunction with other types of connective tissue, forms very thin sheaths that keep the individual muscle fibers separate from each other. This layer is called the endomysium. 'Endo-' translates to 'within', and '-mysium' translates to 'muscle'.
Groups of 10 to 100 muscles fibers securely wrapped in the endomysium sheets form fascicles. Collagen from the endomysium layers extends out and combines with a larger layer of collagen that covers each fascicle. This layer is called the perimysium, 'peri' meaning 'around.'
By combining the many individual muscle fascicles, you get an entire muscle, such as the gastrocnemius, or calf muscle, from the introduction. Surrounding each muscle is another collagen layer called the epimysium ('epi-' means 'upon'). This layer is also composed of lengths of collagen fibers from the layers beneath it, the perimysium and endomysium.
Now, we have one more layer to look at before we circle back to tendons. Often there is more than one muscle responsible for a specific movement. The muscle of your upper arm that bends your elbow is generally known as the biceps muscle. The bending of your elbow, however, requires two major muscles in your upper arm, the well-known biceps brachii, and the lesser-known brachialis.
Each of these muscles is wrapped in its own epimysium, but they are also held to each other by another layer of collagen called deep fascia. This layer holds the muscles together, allows for free movement of those muscles, and provides the blood supply. The collagen of the deep fascia is also connected to the collagen from the lower muscle layers.
Finally, we can get back to the tendon. Each of the four layers from above are composed primarily of collagen. Collagen from the deepest endomysium layer all the way up to the collagen of the deep fascia combine to form the tendon. So, you can imagine that where the cord-like structure of your Achilles tendon meets the calf muscle, it begins branching into the many collagen layers that infiltrate the muscle. When you flex or move your lower leg, you engage the Achilles tendon and the calf muscle, since they are inextricably linked. This ensures that the force of the muscle contraction is spread out throughout the entire length and depth of the muscle. It also ensures that no portion of the muscle is experiencing more stress than the others, protecting the muscle from tearing.
We have examined the connection between muscle and tendon, but for the body to move, the bones must also move. There is a crucial final connection between muscle, tendon, and bone. The cord-like bundle of tendon collagen extends out of the muscle and attaches to the layer of connective tissue that surrounds the bones, the periosteum. Each muscle now has a strong, flexible attachment to bone, allowing for motion with minimal damage to the muscle fibers.
There is one additional structural element that is found on specific tendons. In the wrists and ankles, many large and crucial tendons come together in a small space. These tendons are packed together tightly and are required to shift and move rhythmically during activities like walking and running. These tendons are encased in a layer of connective tissue called a tendon sheath. The sheaths contain a slippery film of synovial fluid that acts to smooth movement and reduce friction. Without this layer, the tendons would quickly be damaged by the high friction of movement.
The cord-like structure of the tendon is not the only conformation a tendon can take. The muscles of the skull are held together by tendons with the same basic building blocks (collagen), but they take a different shape. Go ahead and feel your head until you locate a dense tendon cord like the Achilles. Instead of forming a tight cord, the collagen fibers spread out into a flat, fan shape. This type of tendon is called an aponeurosis. Specifically, the top of your skull is covered by the galea aponeurotica.
We already stated that the primary function of a tendon is to connect muscles to bones. They also have another important function that is more sensory than structural. In the junction between the tendon and bone, a series of nerve cells wind around a layer of collagen fibers, creating a tendon organ. Flexing the muscle creates tension on the tendon. These nerve cells detect and measure this level of tension. If the tension gets too high and threatens to tear the muscle fibers, the tendon organ sends nervous impulses to the muscle, causing a reflexive relaxation of the muscle, thus protecting it from damage.
You almost certainly have heard of tendonitis. If I told you that the '-itis' means 'inflammation,' could you figure out what tendonitis is? If you said 'inflammation of a tendon,' you are correct! Excessive use or damage can result in inflammation of a tendon, causing stiff and painful joints. By learning about the structure of tendons and the links between muscles, tendons, and bones, you can easily see how an inflamed tendon could limit movement and cause pain.
Let's review. The main function of a tendon is to connect skeletal muscles to bones. Tendons are a type of connective tissue, and the primary building blocks of tendons are collagen fibers. These fibers build up to create a tendon through multiple layers, including the endomysium, the fascicles, the perimysium, the epimysium, and deep fascia. Small joints, like wrists and ankles, have a tendon sheath, which contains synovial fluid, and the muscles of your skull are connected via a type of tendon called an aponeurosis.
In addition to connecting skeletal muscles to bone, tendons can sense when muscles are under too much strain. The tendon organ will then induce a reflexive relaxation of the muscle to protect it. Finally, a common tendon injury is tendonitis, which means inflammation of the tendon.
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Back To CourseAnatomy & Physiology: Tutoring Solution
19 chapters | 326 lessons