Chapter IV - All about Muscles
Joint, Muscle, Tendon & Ligament
Structurally, joints are classified as:
- Cartilaginous - bones are connected by cartilage.
- Synovial - there is a space (synovial cavity) between the articulating bones.
- bones are connected by fibrous connective tissue.
In fibrous joints bones are joined by tight and inflexible layers of dense connective tissue, consisting mainly of collagen fibers. In adults, these are not designed to allow any movement; however, in children, fibrous joints have not solidified and are movable. Examples of fibrous joints are:
- Cranial sutures
- Gomphoses, the joints between the roots of the teeth and their sockets (or alveoli) in maxilla and mandible.
- joining the bones of the cranium.
In cartilaginous joints (also known as synchondroses) bones are connected entirely by cartilage. In comparison to synovial joints, cartilaginous joints allow only slight movement. Examples of cartilaginous joints are the pubic symphysis, the joints between the ribs and the sternum, and the cartilage connecting the growth regions of immature long bones. Another example is in the spinal column - the cartilaginous region between adjacent vertebrae.
The term "Synovial joint" and "Diarthrosis joint" are often used interchangeably, although the first term refers to the structure and the second one to the function. For more details, see "Diarthrosis joints" below.
Functionally, they can be classified as:
- Synarthrosis - permit no movement.
- Amphiarthrosis - permit little movement.
- Diarthrosis - permit a variety of movements (e.g. Flexion, adduction, pronation). Only synovial joints are diarthrosis.
The synovium is a membrane that covers all the non-cartilaginous surfaces within the articular capsule. It secretes synovial fluid into the joint, which nourishes and lubricates the articular cartilage. The synovium is separated from the capsule by a layer of cellular tissue that contains blood vessels and nerves.
Synovial joints can be further grouped by their shape, which controls the movement they allow:
- Gliding joints, such as in the carpals of the wrist. These joints allow a wide variety of movement, but not much distance.
- Hinge joints, such as the elbow (between the humerus and the ulna). These joints act like a door hinge, allowing flexion and extension in just one plane.
- Pivot joints, such as the elbow (between the radius and the ulna). This is where one bone rotates about another.
- Condyloid (ellipsoid) joints, such as the knee. When the knee is extended there is no rotation, when it is flexed some rotation is possible. A condyloid joint is where two bones fit together with an odd shape (e.g. an ellipse), and one bone is concave, the other convex. Some classifications make a distinction between condyloid and ellipsoid joints.
- Saddle joints, such as at the thumb (between the metacarpal and carpal). Saddle joints, which resemble a saddle, permit the same movements as the condyloid joints.
- Ball and socket joints, such as the hip joint. These allow a wide arrange of movement.
Muscles are needed to maintain the posture, help in movement, and generate heat. Muscle is a contractile form of tissue. It is one of the four major tissue types, the other three being epithelium, connective tissue and nervous tissue. Muscle contraction is used to move parts of the body, as well as to move substances within the body.
There are three general types of muscles:
· Cardiac muscle is a specialized kind of muscle found only within the heart. It cannot get tired and is "involuntary".
- Skeletal muscle or "voluntary muscle" is anchored by tendons to bone and is used to affect skeletal movement such as locomotion. It gets tired.
or "involuntary muscle" is found within structures such as the intestines, throat and blood vessels. It does not fatigue.
Cardiac and skeletal muscle are "striated" in that they contain sarcomeres and are packed into highly regular arrangements of bundles; smooth muscle has neither.
Muscle is composed of muscle cells (sometimes known as "muscle fibers"). Within the cells are myofibrils; myofibrils contain sarcomeres, which are composed of actin and myosin. Individual muscle cells are lined with endomysium. Muscle cells are bound together by perimysium into bundles called fascicles; the bundles are then grouped together to form muscle, which is lined by epimysium. Muscle spindles are distributed throughout the muscles and provide feedback sensory information to the central nervous system. There are approximately 650 skeletal muscles in the human body.
Symptoms of muscle disease may include weakness or spasticity/rigidity, myoclonus (twitching) and myalgia (muscle pain). Diagnostic procedures that may reveal muscular disorders include testing creatine kinase levels in the blood and electromyography (measuring electrical activity in muscles). Some muscle diseases are
- Neuromuscular diseases are those that affect the muscles and/or their nervous control.
- Myasthenia gravis
- Tetanus and botulism are bacterial infections in which bacterial toxins cause increased or decreased muscle tone, respectively.
- The myopathies are all diseases affecting the muscle itself, rather than its nervous control.
- Muscular dystrophy is a large group of diseases, many of them hereditary, where the muscle integrity is disrupted. It leads to progressive loss of strength, high dependence and decreased life span.
Inflammatory muscle disorders:
- Polymyalgia rheumatica
- Polymyositis, dermatomyositis and inclusion body myositis are autoimmune conditions in which the muscle is affected.
- Rhabdomyolysis is the breakdown of muscular tissue due to any cause. While it may not lead to any muscular symptoms at all, the myoglobin thus released may cause acute renal failure.
A tendon (or sinew) is a tough band of fibrous connective tissue that connects muscle to bone. They are similar to ligaments except that join one bone to another. Tendons are composed mainly of water, type-I collagen and cells called tenocytes.
The origin of a tendon is where it joins to a muscle. Collagen fibers from within the muscle organ are continuous with those of the tendon. A tendon inserts into bone at an enthesis where the collagen fibers are mineralized and integrated into bone tissue.
The Achilles tendon is a particularly large tendon connecting the heel to the muscles of the calf. It is so named because the mythic hero Achilles was said to have been killed due to an injury at this spot.
Sinew was also widely used in the medieval times as a form of ancient elastic. refers to swelling of a tendon.
A ligament is a short band of tough fibrous connective tissue composed mainly of long, stringy collagen fibers. Ligaments connect bones to other bones to form a joint. (They do not connect muscles to bones; that is the function of tendons.) Some ligaments limit the mobility of articulations, or prevent certain movements altogether.
Capsular ligaments are part of the articular capsule that surrounds synovial joints. They act as mechanical reinforcements. Extra-capsular ligaments join bones together and provide joint stability.
Ligaments are slightly elastic; under tension, they gradually lengthen.
The study of ligaments is called desmology.
List of major ligaments
- Anterior cruciate ligament (ACL)
- Lateral collateral ligament (LCL)
- Posterior cruciate ligament (PCL)
- Medial collateral ligament (MCL)
- Volar radiocarpal ligament
- Dorsal radiocarpal ligament
- Ulnar collateral ligament
- Radial collateral ligament
- Periodontal ligament
- Suspensory ligament
- Cricothyroid ligament
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Chapter IV - All about Muscles