Tiêu đề 4b.Joints.pdf ✅

JOINTS AND HOMEOSTASIS The joints of the skeletal system con- tribute to homeostasis by holding bones together in ways that allow for movement and flexibility. • Bones are too rigid to bend without being damaged. Fortunately, flexible connective tissues form joints that hold bones together while still permitting, in most cases, some degree of movement. A joint, also called an articulation (ar-tik-u ̄-LA ̄ -shun) or arthrosis (ar- THRO ̄ -sis), is a point of contact between two bones, between bone and cartilage, or between bone and teeth. When we say one bone articulates with another bone, we mean that the bones form a joint. You can appreciate the importance of joints if you have ever had a cast over your knee joint, which makes walking difficult, or a splint on your finger, which limits your ability to manipulate small objects. The scientific study of joints is termed arthrology (ar- THROL-o ̄-je ̄; arthr- joint; logy study of). The study of mo- tion of the human body is called kinesiology (ki-ne ̄-se ̄-OL-o ̄-je ̄; kinesi movement). 264 9 JOINTS 2568T_c09_264-300.qxd 1/22/08 2:55 AM Page 264 Team B venus:JWQY057:Ch09:
FIBROUS JOINTS 265 JOINT CLASSIFICATIONS OBJECTIVE • Describe the structural and functional classifications of joints. Joints are classified structurally, based on their anatomical characteristics, and functionally, based on the type of movement they permit. The structural classification of joints is based on two criteria: (1) the presence or absence of a space between the articulating bones, called a synovial cavity, and (2) the type of connective tissue that binds the bones together. Structurally, joints are clas- sified as one of the following types: • Fibrous joints (FI ̄-brus): There is no synovial cavity, and the bones are held together by dense irregular connective tissue that is rich in collagen fibers. • Cartilaginous joints (kar-ti-LAJ-i-nus): There is no synovial cavity and the bones are held together by cartilage. • Synovial joints (sı ̄-NO ̄ -ve ̄-al): The bones forming the joint have a synovial cavity and are united by the dense irregular connective tissue of an articular capsule, and often by accessory ligaments. The functional classification of joints relates to the degree of movement they permit. Functionally, joints are classified as one of the following types: • Synarthrosis (sin-ar-THRO ̄ -sis; syn- together): An im- movable joint. The plural is synarthroses. • Amphiarthrosis (am-fe ̄-ar-THRO ̄ -sis; amphi- on both sides): A slightly movable joint. The plural is amphi- arthroses. • Diarthrosis (dı ̄-ar-THRO ̄ -sis movable joint): A freely movable joint. The plural is diarthroses. All diarthroses are synovial joints. They have a variety of shapes and permit several different types of movements. The following sections present the joints of the body according to their structural classifications. As we examine the structure of each type of joint, we will also outline its functions. CHECKPOINT 1. On what basis are joints classified? FIBROUS JOINTS OBJECTIVE • Describe the structure and functions of the three types of fibrous joints. As previously noted, fibrous joints lack a synovial cavity, and the articulating bones are held very closely together by dense ir- regular connective tissue. Fibrous joints permit little or no move- ment. The three types of fibrous joints are sutures, syndesmoses, and interosseous membranes. Sutures A suture (SOO-chur; sutur- seam) is a fibrous joint com- posed of a thin layer of dense irregular connective tissue; sutures occur only between bones of the skull. An example is the coro- nal suture between the parietal and frontal bones (Figure 9.1a). The irregular, interlocking edges of sutures give them added strength and decrease their chance of fracturing. Because a suture is immovable, it is classified functionally as a synarthrosis. Some sutures that are present during childhood are replaced by bone in the adult. Such a suture is an example of a synostosis (sin-os-TO ̄ -sis; os- bone), or bony joint—a joint in which there is a complete fusion of two separate bones into one bone. For example, the frontal bone grows in halves that join together across a suture line. Usually they are completely fused by age 6 and the suture becomes obscure. If the suture persists beyond age 6, it is Figure 9.1 Fibrous joints. At a fibrous joint the bones are held together by dense irregular connective tissue. Outer compact bone Spongy bone Inner compact bone Coronal suture (a) Suture between skull bones FIGURE 9.1 CONTINUES 2568T_c09_264-300.qxd 1/22/08 4:31 PM Page 265 PINNACLE venus:JWQY057:Ch09:
Fibula Tibia Anterior tibiofibular ligament Syndesmosis between tooth and socket of alveolar process (gomphosis) Syndesmosis between tibia and fibula (b) Syndesmosis Root of tooth Periodontal ligament Socket of alveolar process (c) Interosseous membrane between tibia and fibula Fibula Tibia Interosseous membrane 266 CHAPTER 9 • JOINTS ? Functionally, why are sutures classified as synarthroses, and syndesmoses as amphiarthroses? FIGURE 9.1 CONTINUED called a metopic suture (me-TO ̄ -pik; metopon forehead). A syn- ostosis is also classified functionally as a synarthrosis. Syndesmoses A syndesmosis (sin-dez-MO ̄ -sis; syndesmo- band or ligament) is a fibrous joint in which there is a greater distance between the articulating surfaces and more dense irregular connective tissue than in a suture. The dense irregular connective tissue is typically arranged as a bundle (ligament) and the joint permits limited movement. One example of a syndesmosis is the distal tibiofibular joint, where the anterior tibiofibular ligament connects the tibia and fibula (Figure 9.1b, left). It permits slight movement (amphiarthrosis). Another example of a syndesmosis is called a gomphosis (gom-FO ̄ -sis; gompbo-bolt or nail) or dentoalveolar joint, in which a cone-shaped peg fits into a socket. The only examples of gomphoses in the human body are the articulations between the roots of the teeth and their sockets (alveoli) in the maxillae and mandible (Figure 9.1b, right). The dense irregular connective tissue between a tooth and its socket is the thin peri- odontal ligament (membrane). A gomphosis permits no movement (synarthrosis). Inflammation and degeneration of the gums, periodontal ligament, and bone is called periodontal disease. Interosseous Membranes The final category of fibrous joint is the interosseous membrane, a substantial sheet of dense irregular connective tissue that binds neighboring long bones and permits slight movement (am- phiarthrosis). There are two principal interosseous membrane joints in the human body. One occurs between the radius and ulna in the forearm (see Figure 8.6a, b on page 241) and the other occurs between the tibia and fibula in the leg (Figure 9.1c). CHECKPOINT 2. Which fibrous joints are synarthroses? Which are amphiarthroses? 2568T_c09_264-300.qxd 1/22/08 2:55 AM Page 266 Team B venus:JWQY057:Ch09: