Extension: The Role of the Skeleton

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1 Extension: The Role of the Skeleton Jellyfish, sea anemones, and various kinds of worms accomplish movement by muscle alone. Animals capable of more rapid movements have muscles linked to a structural framework. Insects and crustaceans rely on external skeletons (exoskeletons) for support. Vertebrates have cartilaginous or bony internal skeletons (endoskeletons) that are surrounded by muscles. Both types of skeleton provide protection for delicate internal organs and serve as shock absorbers. Exoskeletons of insects and crustaceans cover all of the internal organs and the muscles; however, the endoskeletons of vertebrates only cover some of the internal organs. The vertebrate skeleton also produces blood cells and stores fat in the bone marrow. Found in the centre of certain bones, the marrow is a hub of activity. One of the most important functions of the skeleton locomotion is possible because the skeleton works in combination with the muscles. Locomotion occurs because muscles move bones like levers. Muscle and bone enable you to lift heavy objects, run, jump, and write. Skeletal Structure Your skeleton contains 206 bones that vary in shape and structure. The vast majority of the bones are contained in the hands and feet. Not surprisingly, these two areas are capable of the greatest diversity of movement. There are two types of bone: compact bone, which is composed of hard, dense materials, and spongy bone, which is composed of lighter, less dense materials. The long bones are composed of dense compact tissue, while the ribs are largely composed of spongy tissue. The bone is covered by periosteum, a thin covering that contains blood vessels, nerves, and bone-forming cells (Figure 1). bone marrow the tissue located in the central cavity of the long bones that produces blood cells and stores fat compact bone dense bone spongy bone porous bone periosteum the thin tissue that covers bone Figure 1 The structure of bone The human skeleton has two major divisions: the axial skeleton and the appendicular skeleton (Figure 2, next page). The axial skeleton includes the 28 bones of the skull, the tiny bones of the middle ear, the vertebral column, and the 12 pairs of ribs, which join with the sternum. The upper segment of the appendicular skeleton includes the arms, hands, and pectoral girdle. The lower segment of the appendicular system includes the feet, legs, and pelvic girdle. axial skeleton the central supporting part of the skeleton consisting of the skull, vertebral column, ribs, and sternum appendicular skeleton the bones of the upper and lower limbs and their supporting structures 1

2 Unit 20 D Human Systems Figure 2 The human skeleton with the axial skeleton shown in yellow and the appendicular skeleton shown in white Nelson Biology Alberta Thomson Nelson

3 Bones and Cartilage The human skeleton is like a modern high-rise building in many ways. Like a building, the skeleton contains many structural parts that perform specific functions. In a building, steel posts and beams are arranged and fastened together in a pattern that will support the weight of the building and everything in it. In a skeleton, bones are shaped and attached to one another to perform particular functions. The thin, flat bones of the skull (Figure 3) are curved and fused to one another to form a strong, hard, protective covering for the brain. The short, cylindrical bones of the vertebral column are stacked on top of one another, providing support for the upper body and a hollow, protective casing for the spinal cord (Figure 4). Figure 3 Human skull Figure 4 Short, hollow, cylindrical bones called vertebrae are stacked on top of one another to form the vertebral column. There are spongy intervertebral discs in between the vertebrae. Like a building, the human skeleton is made of many different materials. Bones are covered in a tough membrane called the periosteum. Below the periosteum is a layer of compact bone containing living bone cells called osteocytes. The osteocytes are set in a hard, dense mixture, or matrix, made of minerals such as calcium phosphate and a network of protein fibres called collagen. In many of the long bones, such as the femur, or thigh bone, a tissue called spongy bone lies below the compact bone layer (see Figure 1). Spongy bone is more porous and less dense than compact bone. Bone marrow is a soft spongy material found in the spongy bone and the hollow interior of the long bones of the body. Bone marrow contains immature cells called stem cells that can develop into a variety of mature cells, including red and white blood cells. Some parts of the skeleton are made of a semisolid, flexible connective tissue called cartilage. Cartilage is a supporting material containing living cells called chondrocytes and a flexible matrix composed of collagen fibres and polysaccharides. The skeleton of a developing embryo begins as a framework of cartilage. As the baby grows, the cartilage is replaced by compact bone and spongy bone in a process called ossification. Ossification continues into the late teens and early twenties. During this time, a diet containing adequate amounts of minerals and protein is very important. osteocyte a bone cell collagen the connective protein fibres found in bone matrix cartilage semisolid, flexible connective tissue chondrocyte a cartilage cell ossification the process in which bone tissue is created 3

4 Bone Development Bone tissue either forms in cartilage or in connective tissue. Figure 5 shows the anatomy of a long bone from a mammal. In cartilage, bone-forming cells called osteoblasts begin to form bone tissue inside the shaft of cartilage. As calcium and phosphorus salts are deposited in the cartilage during ossification, cartilage cells begin to die and canals are formed within the structure. The canals, known as Haversian canals, provide pathways for nerves and blood vessels. Because bone is living tissue, a supply of oxygen and other nutrients is essential. Eventually, the canals grow together, forming the bone marrow. osteoblasts boneforming cells Haversian canals small canals located in bone tissue that are occupied by blood vessels and nerves Figure 5 Anatomy of the long bone of a mammal Long bones continue to grow as the calcified cartilage is replaced by bony tissue, beginning at the middle of the bone and proceeding toward the ends. The only remaining cartilage is found on the ends of bones that form joints and near the ends of the long bones where it forms the epiphyseal plates. The epiphyseal plates are often referred to as the growth plates. New cartilage cells develop in the epiphyseal plates, but eventually they are overtaken by the boneproducing cells. Growth in the length of a bone stops once the epiphyseal plates become ossified. Bone formation in connective tissue follows much the same plan. Osteoblasts form splinter-like fragments called spicules, which eventually become joined to form spongy bone. Should the matrix continue to thicken, compact bone is formed. epiphyseal plates areas of cartilage near the ends of long bones; often called growth plates 4

5 Although most people think of bone as being dead, bone is one of the most dynamic tissues in your body. Bone is continually being remodelled by osteoclasts, which dissolve bone, and osteoblasts, which prepare new structures. In much the same way as buildings within a city s core are torn down and replaced with newer structures, your bones are continually being refurbished. The balance between destruction and restoration is a critical one. A rapid loss of bone mass is known as osteoporosis, which commonly affects the vertebral column and hips. Especially prevalent in older women, this decay of bone can lead to a partial collapse of the vertebral column and broken hips. In some extreme cases, the ribs may even come to rest on the pelvic girdle. Although the cause of the disease is not known, many researchers believe that it is linked to decreasing levels of female sex hormones, calcium deficiency, high protein consumption, and physical inactivity. Joints The point at which bones come together is a joint. Some joints, like those of the skull, are not capable of movement. In adults, these fibrous or immovable joints have no gaps between the bones (Figure 6(a)).However, as the fetus descends through the birth canal, the bones of the cranium shift slightly to permit passage. A small area called the soft spot (fontanelle) is later replaced by bone, and the small spaces between skull bones become fused. Other joints, like those of the vertebral column, are capable of restricted movement. Referred to as cartilaginous joints because of small bands of cartilage between the bones, these joints can act as shock absorbers in a fall (Figure 6(b)). The cartilaginous disks found between the vertebrae compress upon impact. Still other joints, like those found in your arms and legs, are freely movable. In a movable joint, the joining surfaces of the bones are covered by a slippery, smooth cartilage that promotes frictionless movement of bones. Many of the joining surfaces are covered by a fluid-secreting membrane called the synovial membrane; hence, these joints are sometimes called synovial joints (Figure 6(c)). The synovial membrane acts like a capsule, lubricating the joint and thereby helping to reduce friction. osteoclasts cells that dissolve bone Figure 6 Joints can be (a) immovable, (b) slightly movable, or (c) freely movable. Figure 7 illustrates the knee an example of a synovial joint. Movable joints can be classified according to the type of movement the joint accommodates, as shown in Figure 8 (next page). Figure 7 Lubricating synovial fluid is surrounded by a synovial membrane in the knee joint. 5

6 Figure 8 Types of movable joints Joints as Fulcrums Your limbs work much like levers. Muscles provide the force to move the levers. The fixed point around which a lever moves is the fulcrum. In most situations, the joint acts as the fulcrum (Figure 9). Figure 9 (a) First-class levers apply a downward force to move a mass upward. Muscles at the back of the neck pull downward to move the head upward. The bones that attach the vertebral column to the skull act as the fulcrum. (b) With second-class levers, the mass being moved is positioned between the fulcrum and the force applied. The calf muscle pulls the leg upward. (c) Third-class levers apply a force between the mass being moved and the fulcrum. The elbow acts as a fulcrum. The load rests on the hand and the biceps muscle pulls the arm upward. Tendons and Ligaments Ligaments are bands of connective tissue that join bone to bone. They add support and strength to joints. Ligaments are found in your hands, ankles, wrists, feet, and, most obviously, in your knees. Seven ligaments hold the knee together (Figure 10). A severe blow to the knee can often sprain or tear the ligaments that hold the femur to the tibia and fibula. Ligaments are flexible, but they cannot stretch very much. Figure 10 The knee is held together by seven ligaments. ligaments bands of connective tissue that join bones 6

7 Tendons join muscle to bone and are remarkably strong. Tendons smaller than the diameter of a pencil are capable of supporting a load of several thousand kilograms. Wide muscles often taper to narrow tendons, which attach to the surfaces of bones. Bursae (singular: bursa) are small fluid-filled sacs located between the bone and tendon that reduce the rubbing of the tendon on bone. An inflammation of the bursae is known as bursitis. Baseball pitchers are particularly susceptible to this condition (Figure 11). tendons bands of connective tissue that join muscle to bone bursae sacs of fluid found between tendon and bone Skeletal Injuries A bone fracture is a broken bone. In a simple fracture (closed fracture), the broken bone does not break through the skin. A compound fracture (open fracture) occurs when a broken bone sticks out of the skin. Children s bones are more flexible than adults bones. When a force is applied to a child s bone, it will bend and crack but may not break all the way through. This type of fracture is known as a greenstick fracture. The term greenstick refers to a young branch of a plant that is flexible and resists breaking when you bend it. Greenstick fractures are difficult to diagnose because they may show up on an X-ray image as a bend in a bone without a clear break. Figure 15 shows the common types of bone fractures. SUMMARY The Role of the Skeleton The axial skeleton is the central supporting part of the skeleton. The appendicular skeleton contains the appendages and their supporting structures. Joints are points at which bones meet. There are three types of joints: fibrous (immovable), cartilaginous, and synovial. Ligaments are bands of connective tissue that join bones to each other. Tendons are bands of connective tissue that join muscle to bone. Figure 11 Three common types of bone fracture Extension Questions 1. Differentiate between an axial and appendicular skeleton. 2. Explain the functions of osteoblasts and osteoclasts in maintaining bone growth and structure. 3. Briefly outline how long bones grow. 4. List three types of joints and provide an example of each. 5. Differentiate between tendons and ligaments. 7

8 6. Baseball pitchers often have difficulty with their shoulders. Bursitis, or inflammation of the bursae, in the shoulder can be painful and may prevent them from continuing to play. Explain why treatment with antiinflammatory drugs and painkillers requires caution. 7. Space flight causes bone to be broken down. Osteoclasts remain active, while osteoblasts reduce their activity. (a) Speculate why osteoblast activity decreases during space flight. (b) (c) (d) Identify one possible harmful effect of increasing blood calcium. It is believed that the balance between osteoblasts and osteoclasts is restored within about 2 weeks of returning to Earth; however, the effects of long-term weightlessness are not well understood. How might this be studied on Earth? Suggest a possible solution to forestall rapid bone loss for astronauts living in a space station. 8