Skeletal Systems

Human Anatomy & Physiology Premium Course

Human Anatomy and Physiology Study Course

Get Instant Access

Muscles can only contract and relax. To create significant movement, they must have something to pull on. In some cases, muscles pull on each other—consider the trunk of the elephant or the arms of an octopus. In most cases, however, skeletal systems provide rigid supports against which muscles can pull, creating directed movements. In this section, we'll examine the three types of skeletal systems found in animals: hydrostatic skeletons, exoskeletons, and endoskeletons.

A hydrostatic skeleton consists of fluid in a muscular cavity

The simplest type of skeleton is the hydrostatic skeleton of cnidarians, annelids, and many other soft-bodied invertebrates. As we saw in Chapter 32, a hydrostatic skeleton consists of a volume of fluid enclosed in a body cavity surrounded by muscle. When muscles oriented in a certain direction contract, the fluid-filled body cavity bulges out in the opposite direction.

The sea anemone, a cnidarian (see Figure 32.6a), has a hydrostatic skeleton. Its body cavity is filled with seawater. To extend its body and its tentacles, the anemone closes its mouth and constricts muscle fibers that are arranged in circles around its body. Contraction of these circular muscles puts pressure on the water in the body cavity, and that pressure forces the body and tentacles to extend. The anemone retracts its tentacles and body by contracting muscle fibers that are arranged longitudinally (lengthwise) in the body wall and along the tentacles.

An earthworm uses its hydrostatic skeleton to crawl. The earthworm's body cavity is divided into many separate segments, each of which contains a compartment filled with extracellular fluid. The body wall surrounding each segment has two muscle layers: a circular layer and a longitudinal layer. If the circular muscles in a segment contract, the compartment in that segment narrows and elongates. If the longitudinal muscles in a segment contract, the compartment shortens and bulges outward. Alternating contractions of the earthworm's circular and longitudinal muscles create waves of narrowing and widening, lengthening and shortening, that travel down the body. Bulging, shortened segments serve as anchors as long, narrow segments project forward and longitudinal contractions pull other segments forward. Bristles help the widest parts of the body to hold firm against the substratum (Figure 47.11).

Another type of locomotion made possible by hydrostatic skeletons is the jet propulsion used by squids and octopuses. Muscles surrounding a water-filled cavity in these cephalo-pods contract, forcefully expelling water from the animal's body. As the water shoots out under pressure, the animal is propelled in the opposite direction.

Exoskeletons are rigid outer structures

An exoskeleton is a hardened outer surface to which muscles can be attached. Contractions of the muscles cause jointed seg-

ments of the exoskeleton to move relative to each other. The simplest example of an exoskeleton is the shell of a mollusk. Some marine mollusks, such as clams and snails, have shells composed of protein strengthened by crystals of calcium carbonate (a rock-hard material). These shells can be massive, affording significant protection against predators. The shells of land snails generally lack the hard mineral component and are much lighter. Molluscan shells can grow as the animal grows, and growth rings are usually apparent on the shells.

The most complex exoskeletons are found among the arthropods. An exoskeleton, or cuticle, covers all the outer surfaces of the arthropod's body and all its appendages. It is made up of plates secreted by a layer of cells just below the exoskeleton. The cuticle contains stiffening materials everywhere except at the joints, where flexibility must be retained. Muscles attached to the inner surfaces of the arthropod ex-oskeleton move its parts around the joints (see Figure 33.8 ).

The layers of the cuticle include an outer, thin, waxy epicuticle that protects the body from drying out, and a thicker, inner endocuticle that forms most of the structure. The endocuticle is a tough, pliable material found only in arthropods. It consists of a complex of protein and chit in, a nitrogen-containing polysaccharide. In marine crustaceans, the endocuticle is further toughened by insoluble calcium salts. The thickness of the cuticle varies among species, but it can be thick enough to form a protective armor.

An exoskeleton protects all the soft tissues of the animal, but is itself subject to damage by abrasion and crushing. The greatest drawback of the arthropod exoskeleton is that it cannot grow. Therefore, if the animal is to become larger, it must molt, shedding its exoskeleton and forming a new, larger one. A molting animal is vulnerable because the new exoskeleton takes time to harden. The animal's body is temporarily unprotected, and without a firm exoskeleton against which its muscles can exert maximum tension, it is unable to move rapidly. Soft-shelled crabs, a gourmet delicacy, are crabs caught when they are molting.

Vertebrate endoskeletons provide supports for muscles

The endoskeleton of vertebrates is an internal scaffolding. Muscles are attached to it and pull against it. Endoskeletons are composed of rodlike, platelike, and tubelike bones connected to one another at a variety of joints that allow a wide range of movements. An advantage of en-

doskeletons over exoskeletons is that they can grow. Because bones are inside the body, the body can enlarge without shedding its skeleton.

The human skeleton consists of 206 bones, some of which are shown in Figure 47.12. It can be divided into an axial skeleton, which includes the skull, vertebral column, and ribs, and an appendicular skeleton, which includes the pectoral girdle, the pelvic girdle, and the bones of the arms, legs, hands, and feet.

Two kinds of connective tissue cells produce large amounts of extracellular matrix material to create the vertebrate endoskeleton. The matrix material produced by cartilage cells is a rubbery mixture of polysaccharides and proteins—mainly fibrous collagen. Collagen fibers run in all directions like reinforcing cords through the gel-like matrix and give it the well-known strength and resiliency of "gristle." This matrix, called cartilage, is found in parts of the en-doskeleton where both stiffness and resiliency are required,

Sternum

Vertebral column

Pelvic girdle

Pelvic Girdle Interior Veiw

Cranium

Maxilla

Mandible

Clavicle Pectorial

Scapula

| | Axial skeleton Q Apendicular skeleton Q Cartilage

Cranium

Maxilla

Mandible r Skull

Sternum

Vertebral column

Pelvic girdle

Clavicle Pectorial

Scapula girdle

Humerus

Radius Ulna

Carpal bones Metacarpal bones Phalanges

Femur

Patella

Fibula Tibia

Tarsal bones Metatarsal bones Phalanges Calcaneus

| | Axial skeleton Q Apendicular skeleton Q Cartilage

47.12 The Human Endoskeleton Cartilage and bone make up the internal skeleton of a human being.

such as on the surfaces of joints, where bones move against one another. Cartilage is also the supportive tissue in stiff but flexible structures such as the larynx (voice box), the nose, and the ear pinnae. Sharks and rays are called cartilaginous fishes because their skeletons are composed entirely of cartilage. In all other vertebrates, cartilage is the principal component of the embryonic skeleton, but during development most of it is gradually replaced by bone.

Bone consists mostly of extracellular matrix material that contains crystals of insoluble calcium phosphate, which give bone its rigidity and hardness, as well as collagen fibers. The skeleton serves as a reservoir of calcium for the rest of the body and is in dynamic equilibrium with soluble calcium in the extracellular fluids of the body. This equilibrium is under the control of calcitonin and parathyroid hormone (see Figure 42.9). If too much calcium is taken from the skeleton, the bones are seriously weakened.

The living cells of bone—called osteoblasts, osteocytes, and osteoclasts—are responsible for the dynamic remodeling of bone that is constantly under way (Figure 47.13). Os-teoblasts lay down new matrix material on bone surfaces. These cells gradually become surrounded by matrix and eventually become enclosed within the bone, at which point they cease laying down matrix, but continue to exist within small lacunae (cavities) in the bone. In this state they are called osteocytes. In spite of the vast amounts of matrix between them, osteocytes remain in contact with one another through long cellular extensions that run through tiny channels in the bone. Communication between osteocytes is important in controlling the activities of the cells that are laying down or removing bone.

Small blood vessel

Newly deposited bone matrix

Small blood vessel

Newly deposited bone matrix

Cell Plasma Membrane Blood Vessels

47.13 Renovating Bone Bones are constantly being remodeled by osteoblasts, which lay down bone,and osteoclasts, which resorb bone.

QpS;

Osteocytes are osteoblasts that become trapped by their own handiwork.

47.13 Renovating Bone Bones are constantly being remodeled by osteoblasts, which lay down bone,and osteoclasts, which resorb bone.

The cells resorb bone are the osteoclasts. They are derived from the same cell lineage that produces the white blood cells. Osteoclasts erode bone, forming cavities and tunnels. Osteoblasts follow osteoclasts, depositing new bone. Thus the interplay of osteoblasts and osteoclasts constantly replaces and remodels the bones.

How the activities of the bone cells are coordinated is not understood, but stress placed on bones somehow provides them with information. A remarkable finding in studies of astronauts who spent long periods in zero gravity was that their bones decalcified. Conversely, certain bones of athletes thicken during training. Both thickening and thinning of bones are experienced by anyone who has had a leg in a cast for a long time. The bones of the uninjured leg carry the person's weight and thicken, while the bones of the inactive leg in the cast thin. The jawbones of people who lose their teeth experience less compressional force during chewing and become considerably reduced.

Bones develop from connective tissues

Bones are divided into two types on the basis of how they develop. Membranous bone forms on a scaffold of connective tissue membrane. Cartilage bone forms first as a cartilaginous structure resembling the future mature bone, then gradually hardens (ossifies) to become bone. The outer bones of the skull are membranous bones; the bones of the limbs are cartilage bones.

Cartilage bones can grow throughout the ossification process. The long bones of the legs and arms, for example, ossify first at the centers and later at each end (Figure 47.14). Growth can continue until these areas of ossification join. The membranous bones forming the skull cap grow until their edges meet. The soft spot on the top of a baby's head is the point at which the skull bones have not yet joined.

The structure of bone may be compact (solid and hard) or cancellous (having numerous internal cavities that make it appear spongy, even though it is rigid). The architecture of a specific bone depends on its position and function, but most bones have both compact and cancellous regions. The shafts of the long bones of the limbs, for example, are cylinders of compact bone surrounding central cavities that contain the bone marrow, where the cellular elements of the blood are made. The ends of the long bones are cancellous (see Figure 47.14 ). Can-cellous bone is lightweight because of its numerous cavities, but it is also strong because its internal meshwork constitutes a support system. It can withstand considerable forces of compression. The rigid, tubelike shaft of compact bone can withstand compression and bending forces. Architects and nature alike use hollow tubes as lightweight structural elements.

Most of the compact bone in mammals is called Haversian bone because it is composed of structural units called Haver-

Cartilage

Primary ossification center

Blood vessel

At the center of the tube is a canal containing blood vessels and nerves.

Secondary ossification center

At the center of the tube is a canal containing blood vessels and nerves.

Cartilage

Primary ossification center

Blood vessel

Secondary ossification center

Primary Ossification Center

Glue line

47.15 Most Compact Bone Is Composed of Haversian Systems

A micrograph of a section of a long bone shows Haversian systems with their central canals. Glue lines separate Haversian systems.

Haversian System

Articular cartilage

47.14 The Growth of Long Bones In the long bones of human limbs, ossification occurs first at the centers and later at each end.

Articular cartilage

47.14 The Growth of Long Bones In the long bones of human limbs, ossification occurs first at the centers and later at each end.

sian systems (Figure 47.15). Each Haversian system is a set of thin, concentric bony cylinders, between which are the osteocytes in their lacunae. Through the center of each Haversian system runs a narrow canal containing blood vessels and nerves. Adjacent Haversian systems are separated by boundaries called glue lines. Haversian bone is resistant to fracturing because cracks tend to stop at glue lines.

Bones that have a common joint can work as a lever

Muscles and bones work together around joints, where two or more bones come together. Since muscles can only contract and relax, they create movement around joints by working in antagonistic pairs: When one contracts, the other relaxes. When both contract, the joint becomes rigid. With respect to a particular joint,

Bone Haversian System And How Works

47.16 Joints, Ligaments, and Tendons A side view of the knee shows the interactions of muscle, bone, cartilage, ligaments, and tendons at this crucial and vulnerable human joint.

Glue line

47.15 Most Compact Bone Is Composed of Haversian Systems

A micrograph of a section of a long bone shows Haversian systems with their central canals. Glue lines separate Haversian systems.

such as the knee, we can refer to the muscle that bends, or flexes, the joint as the flexor and the muscle that straightens, or extends, the joint as the extensor. The bones that meet at the joint are held together by ligaments, which are flexible bands of connective tissue. Other straps of connective tissue, called tendons, attach the muscles to the bones (Figure 47.16). In many kinds of joints, only the tendon spans the joint, sometimes moving over the surfaces of the bones like a rope over a pulley. The tendon of the quadriceps muscle traveling over the knee joint is what is tapped to elicit the knee-jerk reflex (see Figure 46.3). The human skeleton has a wide variety of joints with different ranges of movement (Figure 47.17).

Flexor muscle

Femur

Fibula-

47.16 Joints, Ligaments, and Tendons A side view of the knee shows the interactions of muscle, bone, cartilage, ligaments, and tendons at this crucial and vulnerable human joint.

Flexor muscle

Leg Muscles Joints Tendons Kick Ball

Femur

Fibula-

Tibia

Tibia

Knee Joint Almost Bone Bone

At the shoulders and hips are ball-and-socket joints that allow movement in almost any direction.

A pivot joint where the two bones of the forearm meet at the elbow allows the smaller bone to rotate when the wrist is twisted from side to side.

Several kinds of joints permit some rotation, but not in all directions.

The knee joint is a simple hinge that has almost no rotational movement and can flex in one direction only.

The joint in the ankle allows rotational movement in only one plane.

At the shoulders and hips are ball-and-socket joints that allow movement in almost any direction.

A pivot joint where the two bones of the forearm meet at the elbow allows the smaller bone to rotate when the wrist is twisted from side to side.

Several kinds of joints permit some rotation, but not in all directions.

The knee joint is a simple hinge that has almost no rotational movement and can flex in one direction only.

The joint in the ankle allows rotational movement in only one plane.

nematocysts. Nematocysts, found in cni-darians such as jellyfishes, are cellular structures that are fired like miniature missiles to capture prey and repel predators. They are concentrated in huge numbers on the outer surface of the tentacles. Each nematocyst consists of a slender thread coiled tightly within a capsule, armed with a spinelike trigger projecting to the outside (see Figure 32.7). When potential prey brushes the trigger, the nematocyst fires, turning the thread inside out and exposing little spines along its base. The thread either entangles or penetrates the body of the victim, and a poison may be simultaneously released around the point of contact. The Portuguese man-of-war has tentacles that can be several meters long. These animals can capture, subdue, and devour full-grown mackerel, and the poison of their nematocysts is so potent that it can kill a human who becomes entangled in the tentacles.

Plane joint

47.17 Types of Joints The designs of joints are similar to mechanical counterparts and enable a variety of movements.

Plane joint

47.17 Types of Joints The designs of joints are similar to mechanical counterparts and enable a variety of movements.

Bones can be thought of as a system of levers that are moved around joints by the muscles. A lever has a power arm and a load arm that work around a fulcrum (pivot). The length ratio of the two arms determines whether a particular lever can exert a lot of force over a short distance or is better at translating force into large or fast movements. Compare the jaw joint and the knee joint, for example (Figure 47.18). The power arm of the jaw is long relative to the load arm, allowing the jaw to apply great force over a small distance, as when you crack a nut with your teeth. The power arm of the lower leg, on the other hand, is short relative to the load arm, so you can run fast, jump high, and deliver swift kicks, but you cannot apply nearly the force with a leg that you can with your jaws.

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment