Porth's Essentials of Pathophysiology, 4e

1075

Structure and Function of the Skeletal System

C h a p t e r 4 2

The nerve supply to joints is provided by the same nerve trunks that supply the muscles that move the joints. These nerve trunks also supply the skin over the joints. As a rule, all the joints of an extremity are innervated by the same peripheral nerves as they travel down an extremity. This helps to explain the referral of pain from one joint to another. For example, hip pain may be perceived as pain in the knee. The synovial membrane is innervated only by auto- nomic nerve fibers that control blood flow. It is relatively free of pain fibers, as evidenced by the fact that surgical procedures on the joint are often done under local anes- thesia. The joint capsule and the ligaments have pain receptors; these receptors are more easily stimulated by stretching and twisting than are other joint structures. Pain arising from the capsule tends to be diffuse and poorly localized. The tendons and ligaments of the joint capsule are sensitive to position and movement, particularly stretch- ing and twisting. These structures are supplied by the large sensory nerve fibers that form proprioceptor end- ings for the deep tendon reflex (see Chapter 36). The proprioceptors function reflexively to adjust the tension of the muscles that support the joint and are particu- larly important in maintaining muscular support for the joint. For example, when a weight is lifted, there is a proprioceptor-mediated reflex contraction and relax- ation of appropriate muscle groups to support the joint and protect the joint capsule and other joint structures. Loss of proprioception and reflex control of muscular support leads to destructive changes in the joint. Bursae andTendon Sheaths In some synovial joints, the synovial membrane forms a closed sac that is not part of the joint. These sacs, called bursae , contain synovial fluid. Their purpose is to prevent friction on a tendon. Bursae occur in areas such as the knee where pressure is exerted because of close approximation of joint structures (Fig. 42-8).

Ligaments are fibrous thickenings of the articular capsule that join one bone to its articulating mate. They vary in size and shape depending on their specific role. Although most ligaments are considered inelastic, they are pliable enough to permit movement at the joints. However, ligaments tear rather than stretch when exposed to excess stress. Torn ligaments are extremely painful and accompanied by local swelling. Tendons , which attach skeletal muscles to bone, are relatively inextensible because of their richness in collagen fibers. The intercellular bands of collagen fibers aggregate into bundles that are enveloped by loose connective tissue, blood vessels, and nerves. Tendons that may rub against bone or other friction-generating surfaces are enclosed in double-layered sheaths. An outer connective tissue tube is attached to the structures surrounding the tendon, and an inner sheath encloses the tendon and is attached to it. The space between the inner and outer sheath is filled with a fluid similar to synovial fluid. Overuse can result in tendonitis or inflammation of the tendon. Joint Vasculature and Innervation All the tissues of synovial joints, except the surfaces of the articulating cartilage, receive nourishment either directly or indirectly from blood vessels. The articulating areas are nourished indirectly by the synovial fluid that is distributed over the surface of the articular cartilage. The blood supply to a joint arises from blood vessels that enter the subchondral bone at or near the attach- ment of the joint capsule and form an arterial circle around the joint. The synovial membrane has a rich blood supply, and constituents of plasma diffuse rapidly between these vessels and the joint cavity. Because many of the capillaries are near the surface of the synovium, blood may escape into the synovial fluid after relatively minor injuries. Healing and repair of the synovial mem- brane usually are rapid and complete. This is important because synovial tissue is injured in many surgical pro- cedures that involve the joint.

Suprapatellar bursa

Femur

Synovial membrane

Prepatellar bursa

Patella

Anterior cruciate ligament

Patellar ligament

Tibia

FIGURE 42-8. Sagittal section of knee joint, showing prepatellar and suprapatellar bursa.