1. Bone is one of the principal connective tissues in a vertebrate.

2. Bones are connected to one another by flexible ligaments.

3. Muscles are wrapped in thin sheets of connective tissue called fascia.

4. The fasciae converge into tendons, which connect the muscles to the skeleton.

5. Cartilage forms several distinctive structures such as the nose and ears in humans.

1. Vertebrates have an endoskeleton made of internal bones and cartilage, to which muscles are attached by tendons (Figure 44.2).

2. Most invertebrates, such as molluscs and arthropods, have an exoskeleton made of external shells or plates, with the muscles attached on the inside (Figure 44.2).

3. Inward folds of cuticle in arthropods make large surfaces for muscle attachment, and are often extended into neighboring segments in long, internal plates called apodemes. Apodemes function much as tendons do in vertebrates.

1. A lever rests on a fulcrum; a force applied to one point moves a load (resistance) at a second point.

2. The distance between each force and the fulcrum is an arm.

3. The force times the length of its arm is called a torque.

1. The strict definition of force is a mass times an acceleration and is measured in units called newtons. For this section of this text, it is sufficient to use units of weight, which is mass under the acceleration due to gravity on the earth.

2. Humans have arms and legs with leverage systems that work with a mechanical disadvantage but move the ends of these limbs very rapidly.

3. Human limbs are not suitable for lifting extremely heavy objects, and when this action became important, humans invented machines with large mechanical advantages.

1. Long fibers of collagen and crisscrossing reticular fibers give the tissue strength.

2. Fibers of the rubbery protein elastin provide resilience.

3. Cartilage has an extremely rubbery matrix.

4. In bone, the matrix is supplemented by the addition of calcium phosphate crystals.

5. In other types of connective tissue, the matrix remains soft and jellylike.

1. Dense fibrous connective tissue, the material of tendons, ligaments, and fascia, is mostly made of densely packed, parallel collagen fibers.

2. Elastic connective tissue, which has relatively little collagen and much more elastin, forms layers.

3. Loose connective tissue, is packed beneath the skin and around organs, protecting them from mechanical shocks.

4. Adipose tissue has relatively few fibers, and is mostly composed of adipose cells specialized for storing fat.

1. Fibroblasts arise from mesenchyme, a rather loose embryonic connective tissue.

2. When fibroblasts mature, they secrete the collagen and other proteins of the matrix.

3. Connective tissues are also home to leucocytes (white blood cells) that fight infections.

1. Bone is formed by the ossification of softer tissue–by the deposition of hard calcium phosphate crystals–and it develops as either endochondral bone from cartilage or as dermal bone from skin.

2. The most primitive vertebrates were jawless armored fishes, and the tough dermal plates of their armor became key elements of the skull, even though they were lost from most of the body.

3. The remnants of these dermal plates are laid down in an embryo, and before they have all fused, one can see that the skull is made of many separate plates, some endochondral and some dermal (Figure 44.5).

1. Diarthroses, found mostly in the appendicular skeleton, have a fluid-filled cavity between the ends of the bones, which are covered with cartilage (Figure 44.6).

2. The cavity found in diarthroses is encased in a fibrous capsule and lined by a synovial membrane, which secretes synovial fluid to lubricate movements of the bones.

3. Muscles and bones sometimes rub against one another as they move, and such places are usually protected by bursas, membranous sacs filled with synovial fluid.

1. Synarthroses are joints that allow essentially no movement, such as the sutures between skull bones.

2. Amphiarthroses are joints which are slightly moveable with discs of cartilage, as between vertebrae of the spinal column.

3. Diarthroses are freely movable joints, as between parts of the limbs.

1. Vertebrates resolve the paradox by forming bones that continually reshape themselves internally, so they can keep growing while providing support.

2. The process requires a balance between osteocytes, the cells that secrete bone, and osteoclasts, the cells that remove bone.

1. Fibroblasts deposit the long collagen fibers of the basic ground substance.

2. Some fibroblasts then change into osteoblasts, which start to lay down calcium phosphate crystals along fibrous strands.

3. A completely calcified fibrous strand is called a trabecula.

4. As calcification proceeds, the trabeculae fuse into networks and create spongy bone surrounding the marrow spaces.

5. Figure 44.7 shows the development of a long bone.

1. By secreting a mineral structure around themselves, osteoblasts become trapped within spaces (lacunae) in the bone, with their cellular extensions reaching out into fine radiating canals.

2. A cross section shows that the bone is organized into a series of osteons, each made of concentric layers around a central canal (Figure 44.9).

3. Compact bone is incredibly strong, yet light in weight.

1. Three major types of muscle have been identified and are shown in Figure 44.10).

2. Smooth muscle is relatively simple, made of cells shaped like spindles (cylinders with tapered heads). Smooth muscle is found in blood vessels and the digestive tract.

3. Skeletal muscle, which moves the skeleton and parts of the skin, consists of long muscle fibers with a repeating pattern of bands (striations) that characterize it as striated muscle.

4. Cardiac muscle forms the bulk of the vertebrate heart and is also made of long, striated fibers, fused with one another in a branching pattern.

1. A muscle can only contract, so once it has pulled a bone in one direction, an opposing muscle must pull the bone in the opposite direction, simultaneously stretching the first muscle.

2. Antagonistic muscles in the upper arm raise and extend the forearm, and another set of antagonistic muscles rotate the forearm back and forth around the elbow joint (Figure 44.11).

3. Working in coordination, a system of muscles achieves what one alone could not achieve.

1. As a muscle contracts, the tension it develops is transmitted through the strong collagen fibers of the tendons.

2. The tendons and other connective tissue create an elastic buffer that stretches as tension develops and recoils as it subsides.

3. If all the muscles that move the finger had to be located in the hand itself, it would be huge and unwieldy; the muscles are actually located in the forearm, with only narrow tendons running through the hand (Figure 44.12).

1. The cellular subunit of a muscle is a muscle fiber, a long cell with many nuclei called a syncytium.

2. Each fiber contains many myofibrils, which are bundles of contractile proteins called myofilaments.

3. Each myofibril is divided lengthwise into repeating structural units called sarcomeres.

4. A distinctive endoplasmic reticulum, called sarcoplasmic reticulum (SR), surrounds the myofibrils.

1. The actin filaments are held in this structure by their attachment on the Z lines, which are hexagonally symmetrical plates of the protein a-actinin.

2. Each sarcomere, the unit between two Z lines, can be divided into several bands on the basis of its appearance under polarized light.

3. Light I bands at the ends of the sarcomere contain actin filaments alone.

4. The light H zone in the middle contains myosin filaments alone.

5. The dark A bend is where actin and myosin filaments overlap.

1. When a muscle contracts, the two types of filaments slide past each other.

2. The globular myosin heads extending from each heavy filament attach to the actin filaments and pull on them.

3. Since the ends of the myosin filaments pull in opposite directions, toward the center of the sarcomere, the myosin band pulls the actin bands and Z lines closer together, contracting the whole muscle fiber (Figure 44.16).

1. The control is exerted through two other proteins attached to actin: the calcium-binding protein troponin and the inhibitory protein tropomyosin, which prevents contraction by blocking the sites on actin to which the myosin heads attach (Figure 44.17).

2. These two proteins interact with each other in a stereospecific complex that is sensitive to the Ca2+ concentration in the cytosol.

3. This system depends on the properties and interactions of four proteins–actin, myosin, troponin, and tropomyosin–which interact with each other stereospecifically and change their shapes in response to Ca2+ and ATP.

1. Ca2+ ions enter the cytoplasm through voltage-gated Ca2+ channels in the plasma membrane, rather than from a sarcoplasmic reticulum.

2. Smooth muscle fibers have no troponin or tropomyosin.

3. Ca2+ binds to the regulatory protein calmodulin, which is related to troponin, and calmodulin then activates a protein kinase.

4. The kinase phosphorylates the myosin heads, allowing them to bind to actin and initiate contraction.

1. A single motor neuron and all the muscle fibers it controls constitute a motor unit (Figure 44.18), the elementary unit of activity.

2. The fibers of each motor unit contract in an all-or-none action.

3. A muscle as a whole contracts with increasing strength as more and more units are recruited to contract at one time.

1. Each ending of the motor neuron's axon contains vesicles of acetylcholine, and nerve impulses arriving at a neuromuscular synapse stimulate the release of this neurotransmitter into the synaptic cleft.

2. Acetylcholine binds to receptors in the motor endplate, the plasma membrane of the muscle fiber within the synapse; these receptors open ligand-gated channels for Na+ ions, which flow into the motor endplate and increase the potential locally, creating an endplate potential (EPP).

3. The EPP then initiates an action potential that spreads lengthwise along the plasma membrane of the muscle fiber, just as an action potential is conducted along an axon.

4. This action potential activates the release of Ca2+ ions to produce muscle contraction by penetrating deep into the muscle fiber through transverse tubules.

1. T tubules are deep invaginations of the plasma membrane that run through the muscle fiber close to the A lines of each sarcomere, so the lumen of a T tubule is an extension of the extracellular space around the fiber.

2. An action potential initiated at a motor endplate propagates along the membrane of the muscle fiber and runs down the T tubules deep into the fiber's interior.

3. Though the reticulum membranes are in intimate contact with the T tubules at each end of a sarcomere, one membrane system does not open into the other.

1. Since the SR rapidly pumps Ca2+ ions back out of the cytosol, using its Ca2+-linked ATPases, the contractile apparatus quickly stops working and the tension in the muscle subsides.

2. Acetylcholinesterase on the motor endplate has been breaking down remaining molecules of acetylcholine, sharpening and defining each nervous signal.

3. If a series of nerve impulses arrive rapidly at a muscle fiber, they initiate contractions that build on one another and create a sustained state of contraction called tetanus.

1. Continued neural stimulation then builds tension in the muscle to higher and higher levels until the cross-bridges begin to slip and the contraction strength levels off.

2. Further stimulation eventually produces fatigue, as the number of cross-bridges is reduced with a corresponding reduction in tension.

3. The muscle fatigue we commonly feel after working a muscle hard involves the ability of the SR to accumulate Ca2+ ions, the ability of the muscle fiber to develop an action potential, and overall changes in the fiber's metabolism, which varies with fiber type.

1. Tonic fibers produce a steady contraction.

2. Phasic fibers produce twitches that can build into tetanus.

3. Fibers also differ in their speed, energy source, and color.

4. Highly oxidative muscle is also highly vascularized, with a rich blood supply, and the combination of reddish myoglobin and vascularization makes these fibers red; a glycolytic fiber has few mitochondria, little myoglobin, and is less vascularized, so it is white.

5. Fibers differ in their ability to resist fatigue.

1. Tonic fibers contract slowly, without twitching.

2. Oxidative slow phasic fibers (red, or type I) contract with slow, all-or-nothing twitches.

3. Oxidative fast phasic fibers (intermediate, or type IIA) contract quickly because of their rapid myosin-ATPases and are specialized for rapid, repetitive movement.

4. Glycolytic fast phasic fibers (white, or type IIB) are powerful fibers that contract quickly because of their rapid myosin-ATPases, but they use glycolytic metabolism and fatigue quite rapidly.

1. Cuticle is based on chitin, a tough polysaccharide comparable to the mucopolysaccharides of vertebrate connective tissue.

2. Chitin comprises from about 25 percent to 75 percent of the organic matter in cuticle, the remainder being protein.

3. Chitin by itself is quite soft; cuticle remains soft in some places, but it is toughened, like bone, by the deposition of calcium carbonate.

1. While both animal groups have striated muscle, most invertebrate muscles are tonic, rather than phasic.

2. Instead of contracting in an all-or-none twitch, each muscle fiber responds with a graded contraction whose force increases with greater stimulation.

3. In arthropods, each fiber is innervated by both excitatory and inhibitory neurons (Figure 44.23), and each neuron makes several synapses on a fiber.

4. The contraction of a vertebrate skeletal muscle is regulated by the number of muscle fibers recruited to contract simultaneously.

5. The contraction of an invertebrate muscle is regulated by a balance of graded excitatory and inhibitory stimuli to the same muscle fibers.

1. An insect's wing may vibrate 300 to 1,000 times per second, far too fast for each muscle contraction to be stimulated by a nerve impulse.

2. Impulses in excitatory motor neurons initiate beating, and the subsequent stretching of each flight muscle stimulates it to contract again and to maintain beating until being inhibited (Figure 44.24).