The Structure of bone Haversian system Red marrow Haversian canal Bone, the building material of the ver- in spongy bone tebrate skeleton, is a special form of Osteoblasts connective tissue(see chapter 49). In bone, an organic extracellular matrix containing collagen fibers is impreg Lacunae nated with small, needle-shaped crys- tals of calcium phosphate in the form of hydroxyapatite crystals. Hydroxyap tite is brittle but rigid, giving bone bone great strength. Collagen, on the other hand. is flexible but weak. As a result bone is both strong and flexible. The collagen acts to spread the stress over many crystals, making bone more re- sistant to fracture than hydroxyapatite Bone is a dynamic, living tissue that is constantly reconstructed throughout the life of an individual New bone is formed by osteoblasts, which secrete the collagen-containing organic matrix in which calcium phos hate is later deposited. After the cal now known as osteocytes, are encased within spaces called lacunae in the cal- cified matrix. Yet another type of bone cells. called osteoclasts. act to dissolve bone and thereby aid in the remodeling of bone in response to Bone is constructed in thin concen tric layers, or lamellae, which are laid FIGURE 50.4 down around narrow channels called The organization of bone, shown at three levels of detail. Some parts of bone ar Haversian canals that run parallel to the dense and compact, giving the bone strength. Other parts are spongy, with a more open length of the bone. Haversian canals lattice; it is there that most blood cells are formed conta ain nerve fibers and blood vessels, which keep the osteocytes alive even though they are entombed in a calcified matrix. The con- by bone. At this point, only the articular cartilage at the centric lamellae of bone, with their entrapped osteocytes, ends of the bone remains that surround a haversian canal form the basic unit of bone The ends and interiors of long bones are composed of structure, called a Haversian system. n open lattice of bone called spongy bone. The spaces Bone formation occurs in two ways In flat bones, such within contain marrow, where most blood cells are formed as those of the skull, osteoblasts located in a web of dense (figure 50.4). Surrounding the spongy bone tissue are con- connective tissue produce bone within that tissue. In long centric layers of compact bone, where the bone is much bones, the bone is first"modeled"in cartilage. Calcifica- denser. Compact bone tissue gives bone the strength to tion then occurs, and bone is formed as the cartilage de- withstand mechanical stress rates. At the end of this process, cartilage remains only at the articular joint) surfaces of the bones and at the growth plates located in the necks of the long bones Bone consists of cells and an extracellular matrix that contains collagen fibers, which provide flexibility, and a child grows taller as the cartilage thickens in the calcium phosphate, which provides strength. Bone growth plates and then is partly replaced with bone. A contains blood vessels and nerves and is capable of person stops growing(usually by the late teenage years) rowth and remodeling hen the entire cartilage growth plate becomes replaced Chapter 50 Locomotion 1001
The Structure of Bone Bone, the building material of the vertebrate skeleton, is a special form of connective tissue (see chapter 49). In bone, an organic extracellular matrix containing collagen fibers is impregnated with small, needle-shaped crystals of calcium phosphate in the form of hydroxyapatite crystals. Hydroxyapatite is brittle but rigid, giving bone great strength. Collagen, on the other hand, is flexible but weak. As a result, bone is both strong and flexible. The collagen acts to spread the stress over many crystals, making bone more resistant to fracture than hydroxyapatite is by itself. Bone is a dynamic, living tissue that is constantly reconstructed throughout the life of an individual. New bone is formed by osteoblasts, which secrete the collagen-containing organic matrix in which calcium phosphate is later deposited. After the calcium phosphate is deposited, the cells, now known as osteocytes, are encased within spaces called lacunae in the calcified matrix. Yet another type of bone cells, called osteoclasts, act to dissolve bone and thereby aid in the remodeling of bone in response to physical stress. Bone is constructed in thin, concentric layers, or lamellae, which are laid down around narrow channels called Haversian canals that run parallel to the length of the bone. Haversian canals contain nerve fibers and blood vessels, which keep the osteocytes alive even though they are entombed in a calcified matrix. The concentric lamellae of bone, with their entrapped osteocytes, that surround a Haversian canal form the basic unit of bone structure, called a Haversian system. Bone formation occurs in two ways. In flat bones, such as those of the skull, osteoblasts located in a web of dense connective tissue produce bone within that tissue. In long bones, the bone is first “modeled” in cartilage. Calcification then occurs, and bone is formed as the cartilage degenerates. At the end of this process, cartilage remains only at the articular (joint) surfaces of the bones and at the growth plates located in the necks of the long bones. A child grows taller as the cartilage thickens in the growth plates and then is partly replaced with bone. A person stops growing (usually by the late teenage years) when the entire cartilage growth plate becomes replaced by bone. At this point, only the articular cartilage at the ends of the bone remains. The ends and interiors of long bones are composed of an open lattice of bone called spongy bone. The spaces within contain marrow, where most blood cells are formed (figure 50.4). Surrounding the spongy bone tissue are concentric layers of compact bone, where the bone is much denser. Compact bone tissue gives bone the strength to withstand mechanical stress. Bone consists of cells and an extracellular matrix that contains collagen fibers, which provide flexibility, and calcium phosphate, which provides strength. Bone contains blood vessels and nerves and is capable of growth and remodeling. Chapter 50 Locomotion 1001 Red marrow in spongy bone Capillary in Haversian canal Lamellae Compact bone Haversian system Osteoblasts found here Lacunae containing osteocytes Compact bone Spongy bone FIGURE 50.4 The organization of bone, shown at three levels of detail. Some parts of bone are dense and compact, giving the bone strength. Other parts are spongy, with a more open lattice; it is there that most blood cells are formed
50.2 Skeletal muscles contract to produce movements at ioints Types of Joints of the skeleton occur at joints, or articulations, where one three main cl asses The skeletal movements of the body are produced by con- traction and shortening of muscles. Skeletal muscles are 1. Immovable joints include the sutures that join the generally attached by tendons to bones, so when the mus- bones of the skull (figure 505a). In a fetus, the skul cles shorten the attached bones move. These movements bones are not fully formed, and there are open areas of dense connective tissue ("soft spots, " or fontanels) shift slightly as the fetus moves through the birth 一 Suture anal during childbirth. Later, bone replaces most of this connective tissue 2. Slightly movable joints include those in which the bones are bridged by cartilage. The vertebral bones of the spine are separated by pads of cartilage called intervertebral discs(figure 50.56). These cartilaginous joints allow some movement while acting as efficient shock absorbers 3. Freely movable joints include many types of joints and are also called synovial joints, because the articu- lating ends of the bones are located within a synovial capsule filled with a lubricating fluid. The ends of the bones are capped with cartilage, and the synovial cap- ule is strengthened by ligaments that hold the articu- Articular Synovial joints allow the bones to move in direc- tions dictated by the structure of the joint. For exam- pIe, a joint in the finger allows only a hingelike move- Body of ment, while the joint between the thigh bone(femur) and pelvis has a ball-and-socket structure that permits a variety of different movements(figure 505c) disk Joints confer flexibility to a rigid skeleton, allowing a range of motions determined by the type of joint. Pelvic girdle L Head of Articular (c) Freely movable joints FIGURE 50.5 Three types of joints. (a) Immovable joints include the sutures of the skull; (b) slightly movable joints include the cartilaginous joints between the vertebrae; and(c) freely movable joints are the synovial joints, such as a finger joint and or a hip joint 1002 Part XIlI Animal Form and function
Types of Joints The skeletal movements of the body are produced by contraction and shortening of muscles. Skeletal muscles are generally attached by tendons to bones, so when the muscles shorten, the attached bones move. These movements of the skeleton occur at joints, or articulations, where one bone meets another. There are three main classes of joints: 1. Immovable joints include the sutures that join the bones of the skull (figure 50.5a). In a fetus, the skull bones are not fully formed, and there are open areas of dense connective tissue (“soft spots,” or fontanels) between the bones. These areas allow the bones to shift slightly as the fetus moves through the birth canal during childbirth. Later, bone replaces most of this connective tissue. 2. Slightly movable joints include those in which the bones are bridged by cartilage. The vertebral bones of the spine are separated by pads of cartilage called intervertebral discs (figure 50.5b). These cartilaginous joints allow some movement while acting as efficient shock absorbers. 3. Freely movable joints include many types of joints and are also called synovial joints, because the articulating ends of the bones are located within a synovial capsule filled with a lubricating fluid. The ends of the bones are capped with cartilage, and the synovial capsule is strengthened by ligaments that hold the articulating bones in place. Synovial joints allow the bones to move in directions dictated by the structure of the joint. For example, a joint in the finger allows only a hingelike movement, while the joint between the thigh bone (femur) and pelvis has a ball-and-socket structure that permits a variety of different movements (figure 50.5c). Joints confer flexibility to a rigid skeleton, allowing a range of motions determined by the type of joint. 1002 Part XIII Animal Form and Function 50.2 Skeletal muscles contract to produce movements at joints. Fibrous connective tissue Bone (a) Immovable joint Suture (b) Slightly movable joints Body of vertebra Articular cartilage Intervertebral disk Synovial membrane Synovial fluid Fibrous capsule Articular cartilage Pelvic girdle Head of femur Femur (c) Freely movable joints Ligament FIGURE 50.5 Three types of joints. (a) Immovable joints include the sutures of the skull; (b) slightly movable joints include the cartilaginous joints between the vertebrae; and (c) freely movable joints are the synovial joints, such as a finger joint and or a hip joint
Actions of skeletal muscles Skeletal muscles produce movement of the skeleton when they contract. Usually, the two ends of a skeletal muscle are attached to different bones(although in some cases one or both ends may be connected to some other kind of structure, such as skin). The attachments to bone are Extensor Joint made by means of dense connective tissue straps called Flexor tendons. Tendons have elastic properties that allow "give- and-take"during muscle contraction. One attachment of the muscle, the origin, remains relatively stationary dur- ing a contraction. The other end of the muscle, the in- sertion, is attached to the bone that moves when the muscle contracts. For example, contraction of the biceps muscle in the upper arm causes the forearm(the insertion Extenso of the muscle)to move toward the shoulder(the origin of the Muscles that cause the same action at a joint are syner gists. For example, the various muscles of the quadriceps group in humans are synergists: they all act to extend the (a) knee joint. Muscles that produce opposing actions are an- tagonists. For example, muscles that flex a joint are antag (quadriceps onist to muscles that extend that joint(figure 506a). In hu mans,when the hamstring muscles contract, they cause flexion of the knee joint(figure 50.66). Therefore, the quadriceps and hamstrings are antagonists to each other. In general, the muscles that antagonize a given movement relaxed when that movement is performed. Thus, when hamstrings flex the knee joint, the quadriceps muscles relax Isotonic and isometric contractions In order for muscle fibers to shorten when they contract, IGURE 50.6 chey must generate a force that is greater than the opposing Flexor and extensor muscles of the leg. (a)Antagonistic forces that act to prevent movement of the muscle's inser muscles control the movement of an animal with an exoskeleton tion. When you lift a weight by contracting muscles in your such as the jumping of a grasshopper. When the smaller flexor greater than the force of gravity on the object you are lift- l a muscle contracts it pulls the lower leg in toward the upper ing. In this case, the muscle and all of its fibers shorten in leg and sends the insect into the air. (b) Similarly,antagonistic length. This type of contraction is referred to as isotonic muscles can act on an endoskeleton. In humans, the hamstrings, a contraction, because the force of contraction remains rela- group of three muscles, produce flexion of the knee joint, whereas tively constant throughout the shortening process(iso the quadriceps, a group of four muscles, produce extension. same; tonic= strength) Preceding an isotonic contraction, the muscle begins to contract but the tension is absorbed by the tendons Synergistic muscles have the same action, whereas and other elastic tissue associated with the muscle. The antagonistic muscles have opposite actions. Both muscle muscle does not change in length and so this is called roups are involved in locomotion. Isotonic contractions involve the shortening of muscle. while isometric isometric(literally, "same length")contraction. Isomet contractions do not alter the length of the muscle ric contractions occur as a phase of normal muscle con traction but also exist to provide tautness and stability to the bod Chapter 50 Locomotion 1003
Actions of Skeletal Muscles Skeletal muscles produce movement of the skeleton when they contract. Usually, the two ends of a skeletal muscle are attached to different bones (although in some cases, one or both ends may be connected to some other kind of structure, such as skin). The attachments to bone are made by means of dense connective tissue straps called tendons. Tendons have elastic properties that allow “giveand-take” during muscle contraction. One attachment of the muscle, the origin, remains relatively stationary during a contraction. The other end of the muscle, the insertion, is attached to the bone that moves when the muscle contracts. For example, contraction of the biceps muscle in the upper arm causes the forearm (the insertion of the muscle) to move toward the shoulder (the origin of the muscle). Muscles that cause the same action at a joint are synergists. For example, the various muscles of the quadriceps group in humans are synergists: they all act to extend the knee joint. Muscles that produce opposing actions are antagonists. For example, muscles that flex a joint are antagonist to muscles that extend that joint (figure 50.6a). In humans, when the hamstring muscles contract, they cause flexion of the knee joint (figure 50.6b). Therefore, the quadriceps and hamstrings are antagonists to each other. In general, the muscles that antagonize a given movement are relaxed when that movement is performed. Thus, when the hamstrings flex the knee joint, the quadriceps muscles relax. Isotonic and Isometric Contractions In order for muscle fibers to shorten when they contract, they must generate a force that is greater than the opposing forces that act to prevent movement of the muscle’s insertion. When you lift a weight by contracting muscles in your biceps, for example, the force produced by the muscle is greater than the force of gravity on the object you are lifting. In this case, the muscle and all of its fibers shorten in length. This type of contraction is referred to as isotonic contraction, because the force of contraction remains relatively constant throughout the shortening process (iso = same; tonic = strength). Preceding an isotonic contraction, the muscle begins to contract but the tension is absorbed by the tendons and other elastic tissue associated with the muscle. The muscle does not change in length and so this is called isometric (literally, “same length”) contraction. Isometric contractions occur as a phase of normal muscle contraction but also exist to provide tautness and stability to the body. Synergistic muscles have the same action, whereas antagonistic muscles have opposite actions. Both muscle groups are involved in locomotion. Isotonic contractions involve the shortening of muscle, while isometric contractions do not alter the length of the muscle. Chapter 50 Locomotion 1003 Extensor Exoskeleton Flexor Joint Flexor muscles contract Extensor muscles contract (b) (a) Flexors (hamstring) Extensors (quadriceps) FIGURE 50.6 Flexor and extensor muscles of the leg. (a) Antagonistic muscles control the movement of an animal with an exoskeleton, such as the jumping of a grasshopper. When the smaller flexor tibia muscle contracts it pulls the lower leg in toward the upper leg. Contraction of the extensor tibia muscles straightens out the leg and sends the insect into the air. (b) Similarly, antagonistic muscles can act on an endoskeleton. In humans, the hamstrings, a group of three muscles, produce flexion of the knee joint, whereas the quadriceps, a group of four muscles, produce extension
50.3 Muscle contraction powers animal locomotion The Sliding filament mechanism of bands called a bands. the thin filaments alone are found Contraction the light bands, or I bands Each I band in a myofibril is divided in half by a disc of accn sed in chauser 4o ears musee ther esclesese s, as micrographs. The thin filaments are anchored to these dle of 4 to 20 elongated structures called myofibrils. Each discs of proteins that form the Z lines. If you look at an myofibril, in turn, is composed of thick and thin myofila- electron micrograph of a myofibril (figure 50.8), you will ments(figure 50.7). The muscle fiber is striated (has cross- see that the structure of the myofibril repeats from Z lin striping) because its myofibrils are striated, with dark and to Z line. This repeating structure, called a sarcomere, is light bands. The banding pattern results from the organiza the smallest subunit of muscle contraction tion of the myofilaments within the myofibril. The thick The thin filaments stick partway into the stack of thick myofilaments are stacked together to produce the dark filaments on each side of an A band, but, in a resting Tendon Muscle fascicle muscle fibers Muscle fiber Myofilame FIGURE 50.7 The organization of skeletal muscle. Each muscle is composed of many fascicles, which are bundles of muscle cells, or fibers. Each fibe is composed of many myofibrils, which are each, in turn, composed of myofilaments. 100 Part XIlI Animal Form and Function
The Sliding Filament Mechanism of Contraction Each skeletal muscle contains numerous muscle fibers, as described in chapter 49. Each muscle fiber encloses a bundle of 4 to 20 elongated structures called myofibrils. Each myofibril, in turn, is composed of thick and thin myofilaments (figure 50.7). The muscle fiber is striated (has crossstriping) because its myofibrils are striated, with dark and light bands. The banding pattern results from the organization of the myofilaments within the myofibril. The thick myofilaments are stacked together to produce the dark bands, called A bands; the thin filaments alone are found in the light bands, or I bands. Each I band in a myofibril is divided in half by a disc of protein, called a Z line because of its appearance in electron micrographs. The thin filaments are anchored to these discs of proteins that form the Z lines. If you look at an electron micrograph of a myofibril (figure 50.8), you will see that the structure of the myofibril repeats from Z line to Z line. This repeating structure, called a sarcomere, is the smallest subunit of muscle contraction. The thin filaments stick partway into the stack of thick filaments on each side of an A band, but, in a resting 1004 Part XIII Animal Form and Function 50.3 Muscle contraction powers animal locomotion. Tendon Skeletal muscle Muscle fascicle (with many muscle fibers) Muscle fiber (cell) Myofilaments Myofibrils Plasma membrane Nuclei Striations FIGURE 50.7 The organization of skeletal muscle. Each muscle is composed of many fascicles, which are bundles of muscle cells, or fibers. Each fiber is composed of many myofibrils, which are each, in turn, composed of myofilaments
muscle, do not project all the way to the center of the a band. as a result the center of an a band(called an H band) is lighter than each side, with Its Inte tating thick and thi ments. This appearance of the sar- Myofibril comers changes when the muscle contracts A muscle contracts and shortens be- cause its myofibrils contract and shorten. When this occurs, the myofil aments do not shorten: instead. the thin filaments slide deeper into the a bands(figure 50.9). This makes the h FIGURE 50.8 bands narrower until, at maximal An electron micrograph of a skeletal muscle fiber. The Z lines that serve as the borders shortening, they disappear entirely. It f the res are clearly seen within each myofibril. The thick filaments comprise the a also makes the I bands narrower, be- bands; the thin filaments are within the I bands and stick partway into the A bands cause the dark a bands are brought overlapping with the thick filaments. There is no overlap of thick and thin filaments at the closer together. This is the sliding fil- central region of an A band, which is therefore lighter in appearance. This is the H band ament mechanism of contraction Thin filaments(acti Thick filaments(myosin FIGURE 50.9 Electron micrograph(a)and diagram(b)of the sliding filament mechanism of contraction. As the thin filaments slide deeper into the centers of the sarcomeres, the Z lines are brought closer together. (I)Relaxed muscle; (2) partially contracted muscle Chapter 50 Locomotion 1005
muscle, do not project all the way to the center of the A band. As a result, the center of an A band (called an H band) is lighter than each side, with its interdigitating thick and thin filaments. This appearance of the sarcomeres changes when the muscle contracts. A muscle contracts and shortens because its myofibrils contract and shorten. When this occurs, the myofilaments do not shorten; instead, the thin filaments slide deeper into the A bands (figure 50.9). This makes the H bands narrower until, at maximal shortening, they disappear entirely. It also makes the I bands narrower, because the dark A bands are brought closer together. This is the sliding filament mechanism of contraction. Chapter 50 Locomotion 1005 Myofibril Myofibril FIGURE 50.8 An electron micrograph of a skeletal muscle fiber. The Z lines that serve as the borders of the sarcomeres are clearly seen within each myofibril. The thick filaments comprise the A bands; the thin filaments are within the I bands and stick partway into the A bands, overlapping with the thick filaments. There is no overlap of thick and thin filaments at the central region of an A band, which is therefore lighter in appearance. This is the H band. 1 Z 2 Z Z H band H band I band I band (a) 1 2 (b) Z Z Z Z Z Z Thin filaments (actin) Thick filaments (myosin) Cross-bridges FIGURE 50.9 Electron micrograph (a) and diagram (b) of the sliding filament mechanism of contraction. As the thin filaments slide deeper into the centers of the sarcomeres, the Z lines are brought closer together. (1) Relaxed muscle; (2) partially contracted muscle