Chapter 50: Locomotion

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50Locomotion

Concept Outline

50.1 A skeletal system supports movement in animals.

Types of Skeletons. There are three types of skeletalsystems found in animals: hydrostatic skeletons,exoskeletons, and endoskeletons. Hydrostatic skeletonsfunction by the movement of fluid in a body cavity.Exoskeletons are made of tough exterior coverings onwhich muscles attach to move the body. Endoskeletons arerigid internal bones or cartilage which move the body bythe contraction of muscles attached to the skeleton.The Structure of Bone. The human skeleton, anexample of an endoskeleton, is made of bone that containscells called osteocytes within a calcified matrix.

50.2 Skeletal muscles contract to produce movementsat joints.

Types of Joints. The joints where bones meet may beimmovable, slightly movable, or freely movable.Actions of Skeletal Muscles. Synergistic andantagonistic muscles act on the skeleton to move the body.

50.3 Muscle contraction powers animal locomotion.

The Sliding Filament Mechanism of Contraction.Thick and thin myofilaments slide past one another tocause muscle shortening.The Control of Muscle Contraction. Duringcontraction Ca++ moves aside a regulatory protein whichhad been preventing cross-bridges from attaching to thethin filaments. Nerves stimulate the release of Ca++ from itsstorage depot so that contraction can occur.Types of Muscle Fibers. Muscle fibers can becategorized as slow-twitch (slow to fatigue) or fast-twitch(fatigue quickly but can provide a fast source of power).Comparing Cardiac and Smooth Muscles. Cardiacmuscle cells are interconnected to form a single functioningunit. Smooth muscles lack the myofilament organizationfound in striated muscle but they still contract via thesliding filament mechanism.Modes of Animal Locomotion. Animals rarely move instraight lines. Their movements are adjusted both bymechanical feedback and by neural control. Musclesgenerate power for movement, and also act as springs,brakes, struts, and shock absorbers.

Plants and fungi move only by growing, or as the passivepassengers of wind and water. Of the three multicellu-

lar kingdoms, only animals explore their environment in anactive way, through locomotion. In this chapter we exam-ine how vertebrates use muscles connected to bones toachieve movement. The rattlesnake in figure 50.1 slithersacross the sand by a rhythmic contraction of the musclessheathing its body. Humans walk by contracting muscles intheir legs. Although our focus in this chapter will be onvertebrates, it is important to realize that essentially all ani-mals employ muscles. When a mosquito flies, its wings aremoved rapidly through the air by quickly contracting flightmuscles. When an earthworm burrows through the soil, itsmovement is driven by strong muscles pushing its bodypast the surrounding dirt.

FIGURE 50.1On the move. The movements made by this sidewinderrattlesnake are the result of strong muscle contractions acting onthe bones of the skeleton. Without muscles and some type ofskeletal system, complex locomotion as shown here would not bepossible.

There are three types of animal skeletons: hydrostaticskeleton, exoskeleton, and endoskeleton. Theendoskeletons found in vertebrates are composed ofbone or cartilage and are organized into axial andappendicular portions.

1000 Part XIII Animal Form and Function

Types of SkeletonsAnimal locomotion is accomplished through the force ofmuscles acting on a rigid skeletal system. There are threetypes of skeletal systems in the animal kingdom: hydraulicskeletons, exoskeletons, and endoskeletons.

Hydrostatic skeletons are primarily found in soft-bodied invertebrates such as earthworms and jellyfish. Inthis case, a fluid-filled cavity is encircled by muscle fibers.As the muscles contract, the fluid in the cavity moves andchanges the shape of the cavity. In an earthworm, for ex-ample, a wave of contractions of circular muscles beginsanteriorly and compresses each segment of the body, sothat the fluid pressure pushes it forward. Contractions oflongitudinal muscles then pull the rear of the body for-ward (figure 50.2).

Exoskeletons surround the body as a rigid hard casein most animals. Arthropods, such as crustaceans and in-sects, have exoskeletons made of the polysaccharide chitin(figure 50.3a). An exoskeleton offers great protection tointernal organs and resists bending. However, in order togrow, the animal must periodically molt. During molt-ing, the animal is particularly vulnerable to predation be-cause its old exoskeleton has been shed. Having an exo-skeleton also limits the size of the animal. An animalwith an exoskeleton cannot get too large because its ex-oskeleton would have to become thicker and heavier, inorder to prevent collapse, as the animal grew larger. If aninsect were the size of a human being, its exoskeletonwould have to be so thick and heavy it would be unableto move.

Endoskeletons, found in vertebrates and echino-derms, are rigid internal skeletons to which muscles areattached. Vertebrates have a flexible exterior that accom-modates the movements of their skeleton. The en-doskeleton of vertebrates is composed of cartilage orbone. Unlike chitin, bone is a cellular, living tissue capa-ble of growth, self-repair, and remodeling in response tophysical stresses.

The Vertebrate Skeleton

A vertebrate endoskeleton (figure 50.3b) is divided into anaxial and an appendicular skeleton. The axial skeleton’sbones form the axis of the body and support and protectthe organs of the head, neck, and chest. The appendicularskeleton’s bones include the bones of the limbs, and thepectoral and pelvic girdles that attach them to the axialskeleton.

The bones of the skeletal system support and protect thebody, and serve as levers for the forces produced by con-traction of skeletal muscles. Blood cells form within thebone marrow, and the calcified matrix of bones acts as areservoir for calcium and phosphate ions.


FIGURE 50.2Locomotion in earthworms. The hydrostatic skeleton of theearthworm uses muscles to move fluid within the segmented bodycavity changing the shape of the animal. When an earthworm’scircular muscles contract, the internal fluid presses on thelongitudinal muscles, which then stretch to elongate segments ofthe earthworms. A wave of contractions down the body of theearthworm produces forward movement.

Chitinous outercovering

Vertebral column

Pelvis

Femur

Tibia

FibulaUlna

Radius

Humerus

Skull Scapula Ribs

(a) Exoskeleton

(b) Endoskeleton

FIGURE 50.3Exoskeleton and endoskeleton. (a) The hard, tough outcoveringof an arthropod, such as this crab, is its exoskeleton. (b)Vertebrates, such as this cat, have endoskeletons. The axialskeleton is shown in the peach shade, the appendicular skeleton inthe yellow shade. Some of the major bones are labeled.

The Structure of BoneBone, the building material of the ver-tebrate skeleton, is a special form ofconnective tissue (see chapter 49). Inbone, an organic extracellular matrixcontaining collagen fibers is impreg-nated with small, needle-shaped crys-tals of calcium phosphate in the formof hydroxyapatite crystals. Hydroxyap-atite is brittle but rigid, giving bonegreat strength. Collagen, on the otherhand, is flexible but weak. As a result,bone is both strong and flexible. Thecollagen acts to spread the stress overmany crystals, making bone more re-sistant to fracture than hydroxyapatiteis by itself.

Bone is a dynamic, living tissuethat is constantly reconstructedthroughout the life of an individual.New bone is formed by osteoblasts,which secrete the collagen-containingorganic matrix in which calcium phos-phate is later deposited. After the cal-cium phosphate is deposited, the cells,now known as osteocytes, are encasedwithin spaces called lacunae in the cal-cified matrix. Yet another type ofbone cells, called osteoclasts, act todissolve bone and thereby aid in theremodeling of bone in response tophysical stress.

Bone is constructed in thin, concen-tric layers, or lamellae, which are laiddown around narrow channels calledHaversian canals that run parallel to thelength of the bone. Haversian canalscontain nerve fibers and blood vessels,which keep the osteocytes alive eventhough they are entombed in a calcified matrix. The con-centric lamellae of bone, with their entrapped osteocytes,that surround a Haversian canal form the basic unit of bonestructure, called a Haversian system.

Bone formation occurs in two ways. In flat bones, suchas those of the skull, osteoblasts located in a web of denseconnective tissue produce bone within that tissue. In longbones, the bone is first “modeled” in cartilage. Calcifica-tion then occurs, and bone is formed as the cartilage de-generates. At the end of this process, cartilage remainsonly at the articular (joint) surfaces of the bones and atthe growth plates located in the necks of the long bones.A child grows taller as the cartilage thickens in thegrowth plates and then is partly replaced with bone. Aperson 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 theends of the bone remains.

The ends and interiors of long bones are composed ofan open lattice of bone called spongy bone. The spaceswithin contain marrow, where most blood cells are formed(figure 50.4). Surrounding the spongy bone tissue are con-centric layers of compact bone, where the bone is muchdenser. Compact bone tissue gives bone the strength towithstand mechanical stress.

Bone consists of cells and an extracellular matrix thatcontains collagen fibers, which provide flexibility, andcalcium phosphate, which provides strength. Bonecontains blood vessels and nerves and is capable ofgrowth and remodeling.

Chapter 50 Locomotion 1001

Red marrowin spongy bone

Capillary inHaversian canal

Lamellae

Compactbone

Haversian system

Osteoblastsfound here

Lacunaecontainingosteocytes

Compactbone

Spongy bone

FIGURE 50.4The organization of bone, shown at three levels of detail. Some parts of bone aredense and compact, giving the bone strength. Other parts are spongy, with a more openlattice; it is there that most blood cells are formed.

Types of JointsThe skeletal movements of the body are produced by con-traction and shortening of muscles. Skeletal muscles aregenerally attached by tendons to bones, so when the mus-cles shorten, the attached bones move. These movements

of the skeleton occur at joints, or articulations, where onebone meets another. There are three main classes of joints:

1. Immovable joints include the sutures that join thebones of the skull (figure 50.5a). In a fetus, the skullbones are not fully formed, and there are open areasof dense connective tissue (“soft spots,” or fontanels)between the bones. These areas allow the bones toshift slightly as the fetus moves through the birthcanal during childbirth. Later, bone replaces most ofthis connective tissue.

2. Slightly movable joints include those in which thebones are bridged by cartilage. The vertebral bonesof the spine are separated by pads of cartilage calledintervertebral discs (figure 50.5b). These cartilaginousjoints allow some movement while acting as efficientshock absorbers.

3. Freely movable joints include many types of jointsand are also called synovial joints, because the articu-lating ends of the bones are located within a synovialcapsule filled with a lubricating fluid. The ends of thebones are capped with cartilage, and the synovial cap-sule is strengthened by ligaments that hold the articu-lating bones in place.

Synovial joints allow the bones to move in direc-tions dictated by the structure of the joint. For exam-ple, a joint in the finger allows only a hingelike move-ment, while the joint between the thigh bone (femur)and pelvis has a ball-and-socket structure that permitsa variety of different movements (figure 50.5c).

Joints confer flexibility to a rigid skeleton, allowing arange of motions determined by the type of joint.


50.2 Skeletal muscles contract to produce movements at joints.

Fibrousconnectivetissue

Bone

(a) Immovable joint

Suture

(b) Slightly movable joints

Body ofvertebra

Articularcartilage

Intervertebraldisk

Synovialmembrane

Synovialfluid

Fibrouscapsule

Articularcartilage

Pelvic girdle

Head offemur

Femur

(c) Freely movable joints

Ligament

FIGURE 50.5Three types of joints. (a) Immovable joints include the sutures of the skull; (b) slightly movable joints include the cartilaginous jointsbetween the vertebrae; and (c) freely movable joints are the synovial joints, such as a finger joint and or a hip joint.

Actions of Skeletal MusclesSkeletal muscles produce movement of the skeleton whenthey contract. Usually, the two ends of a skeletal muscleare attached to different bones (although in some cases,one or both ends may be connected to some other kind ofstructure, such as skin). The attachments to bone aremade by means of dense connective tissue straps calledtendons. Tendons have elastic properties that allow “give-and-take” during muscle contraction. One attachment ofthe 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 themuscle contracts. For example, contraction of the bicepsmuscle in the upper arm causes the forearm (the insertionof the muscle) to move toward the shoulder (the origin ofthe muscle).

Muscles that cause the same action at a joint are syner-gists. For example, the various muscles of the quadricepsgroup in humans are synergists: they all act to extend theknee joint. Muscles that produce opposing actions are an-tagonists. For example, muscles that flex a joint are antag-onist to muscles that extend that joint (figure 50.6a). In hu-mans, when the hamstring muscles contract, they causeflexion of the knee joint (figure 50.6b). Therefore, thequadriceps and hamstrings are antagonists to each other. Ingeneral, the muscles that antagonize a given movement arerelaxed when that movement is performed. Thus, when thehamstrings flex the knee joint, the quadriceps musclesrelax.

Isotonic and Isometric Contractions

In order for muscle fibers to shorten when they contract,they must generate a force that is greater than the opposingforces that act to prevent movement of the muscle’s inser-tion. When you lift a weight by contracting muscles in yourbiceps, for example, the force produced by the muscle isgreater than the force of gravity on the object you are lift-ing. In this case, the muscle and all of its fibers shorten inlength. This type of contraction is referred to as isotoniccontraction, because the force of contraction remains rela-tively constant throughout the shortening process (iso =same; tonic = strength).

Preceding an isotonic contraction, the muscle beginsto contract but the tension is absorbed by the tendonsand other elastic tissue associated with the muscle. Themuscle does not change in length and so this is calledisometric (literally, “same length”) contraction. Isomet-ric contractions occur as a phase of normal muscle con-traction but also exist to provide tautness and stability tothe body.

Synergistic muscles have the same action, whereasantagonistic muscles have opposite actions. Both musclegroups are involved in locomotion. Isotonic contractionsinvolve the shortening of muscle, while isometriccontractions do not alter the length of the muscle.


Extensor

FlexorExoskeletonJoint

Flexormusclescontract

Extensormusclescontract

(b)

(a)

Flexors(hamstring)

Extensors(quadriceps)

FIGURE 50.6Flexor and extensor muscles of the leg. (a) Antagonisticmuscles control the movement of an animal with an exoskeleton,such as the jumping of a grasshopper. When the smaller flexortibia muscle contracts it pulls the lower leg in toward the upperleg. Contraction of the extensor tibia muscles straightens out theleg and sends the insect into the air. (b) Similarly, antagonisticmuscles can act on an endoskeleton. In humans, the hamstrings, agroup of three muscles, produce flexion of the knee joint, whereasthe quadriceps, a group of four muscles, produce extension.

The Sliding Filament Mechanism ofContractionEach skeletal muscle contains numerous muscle fibers, asdescribed in chapter 49. Each muscle fiber encloses a bun-dle of 4 to 20 elongated structures called myofibrils. Eachmyofibril, in turn, is composed of thick and thin myofila-ments (figure 50.7). The muscle fiber is striated (has cross-striping) because its myofibrils are striated, with dark andlight bands. The banding pattern results from the organiza-tion of the myofilaments within the myofibril. The thickmyofilaments are stacked together to produce the dark

bands, called A bands; the thin filaments alone are found inthe light bands, or I bands.

Each I band in a myofibril is divided in half by a disc ofprotein, called a Z line because of its appearance in electronmicrographs. The thin filaments are anchored to thesediscs of proteins that form the Z lines. If you look at anelectron micrograph of a myofibril (figure 50.8), you willsee that the structure of the myofibril repeats from Z lineto Z line. This repeating structure, called a sarcomere, isthe smallest subunit of muscle contraction.

The thin filaments stick partway into the stack of thickfilaments on each side of an A band, but, in a resting



Tendon

Skeletal muscle

Muscle fascicle(with manymuscle fibers)

Muscle fiber(cell)

Myofilaments

Myofibrils

Plasmamembrane

Nuclei Striations

FIGURE 50.7The organization of skeletal muscle. Each muscle is composed of many fascicles, which are bundles of muscle cells, or fibers. Each fiberis composed of many myofibrils, which are each, in turn, composed of myofilaments.

muscle, do not project all the way tothe center of the A band. As a result,the center of an A band (called an Hband) is lighter than each side, withits interdigitating thick and thin fila-ments. This appearance of the sar-comeres changes when the musclecontracts.

A muscle contracts and shortens be-cause its myofibrils contract andshorten. When this occurs, the myofil-aments do not shorten; instead, thethin filaments slide deeper into the Abands (figure 50.9). This makes the Hbands narrower until, at maximalshortening, they disappear entirely. Italso makes the I bands narrower, be-cause the dark A bands are broughtcloser together. This is the sliding fil-ament mechanism of contraction.


Myofibril

Myofibril

FIGURE 50.8An electron micrograph of a skeletal muscle fiber. The Z lines that serve as the bordersof the sarcomeres are clearly seen within each myofibril. The thick filaments comprise the Abands; 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 thecentral region of an A band, which is therefore lighter in appearance. This is the H band.

1

Z

2

Z Z

Hband

Hband

I band

I band

(a)

1

2(b)

Z

Z Z Z

Z Z

Thin filaments (actin) Thick filaments (myosin)

Cross-bridges

FIGURE 50.9Electron micrograph (a) and diagram (b) of the sliding filament mechanism of contraction. As the thin filaments slide deeper intothe centers of the sarcomeres, the Z lines are brought closer together. (1) Relaxed muscle; (2) partially contracted muscle.

Electron micrographs reveal cross-bridges that extend from the thick tothe thin filaments, suggesting a mecha-nism that might cause the filaments toslide. To understand how this is accom-plished, we have to examine the thickand thin filaments at a molecular level.Biochemical studies show that eachthick filament is composed of manymyosin proteins packed together, andevery myosin molecule has a “head” re-gion that protrudes from the thick fila-ments (figure 50.10). These myosinheads form the cross-bridges seen inelectron micrographs. Biochemicalstudies also show that each thin filamentconsists primarily of many globularactin proteins twisted into a double helix (figure 50.11).Therefore, if we were able to see a sarcomere at a molecu-lar level, it would have the structure depicted in figure50.12a.

Before the myosin heads bind to the actin of the thinfilaments, they act as ATPase enzymes, splitting ATP intoADP and Pi. This activates the heads, “cocking” them sothat they can bind to actin and form cross-bridges. Once amyosin head binds to actin, it undergoes a conformational(shape) change, pulling the thin filament toward the cen-

ter of the sarcomere (figure 50.12b) in a power stroke. Atthe end of the power stroke, the myosin head binds to anew molecule of ATP. This allows the head to detachfrom actin and continue the cross-bridge cycle (figure50.13), which repeats as long as the muscle is stimulatedto contract.

In death, the cell can no longer produce ATP andtherefore the cross-bridges cannot be broken—thiscauses the muscle stiffness of death, or rigor mortis. A liv-ing cell, however, always has enough ATP to allow the


Myosin head

Myosin molecule

(a)

(b) Thick filament

Myosin head

FIGURE 50.10Thick filaments are composed of myosin. (a) Each myosin molecule consists of two polypeptide chains wrapped around each other; atthe end of each chain is a globular region referred to as the “head.” (b) Thick filaments consist of myosin molecules combined into bundlesfrom which the heads protrude at regular intervals.

Actin molecules

Thin filament

FIGURE 50.11Thin filaments are composed of globular actin proteins. Two rows of actin proteinsare twisted together in a helix to produce the thin filaments.

myosin heads to detach from actin. How, then, is thecross-bridge cycle arrested so that the muscle can relax?The regulation of muscle contraction and relaxation re-quires additional factors that we will discuss in the nextsection.

Thick and thin filaments are arranged to formsarcomeres within the myofibrils. Myosin proteinscomprise the thick filaments, and the heads of themyosin form cross-bridges with the actin proteins ofthe thin filaments. ATP provides the energy for thecross-bridge cycle and muscle contraction.


Z lineThin filaments (actin)

(a)

(b)

Thick filament (myosin)

Cross-bridges

FIGURE 50.12The interaction of thick and thin filaments in striated muscle sarcomeres. The heads on the two ends of the thick filaments areoriented in opposite directions (a), so that the cross-bridges pull the thin filaments and the Z lines on each side of the sarcomere towardthe center. (b) This sliding of the filaments produces muscle contraction.

Thin filament(actin)

Myosinhead

(a)

(b)

ATP

(d)

(c)

Thick filament(myosin)

Cross-bridge

ADPPi

FIGURE 50.13The cross-bridge cycle in musclecontraction. (a) With ADP and Piattached to the myosin head, (b) thehead is in a conformation that canbind to actin and form a cross-bridge. (c) Binding causes the myosinhead to assume a more bentconformation, moving the thinfilament along the thick filament (tothe left in this diagram) and releasingADP and Pi. (d) Binding of ATP tothe head detaches the cross-bridge;cleavage of ATP into ADP and Piputs the head into its originalconformation, allowing the cycle tobegin again.

The Control of Muscle ContractionThe Role of Ca++ in Contraction

When a muscle is relaxed, its myosin heads are “cocked”and ready, through the splitting of ATP, but are unable tobind to actin. This is because the attachment sites for themyosin heads on the actin are physically blocked by an-other protein, known as tropomyosin, in the thin fila-ments. Cross-bridges therefore cannot form in the relaxedmuscle, and the filaments cannot slide.

In order to contract a muscle, the tropomyosin must bemoved out of the way so that the myosin heads can bind toactin. This requires the function of troponin, a regulatoryprotein that binds to the tropomyosin. The troponin andtropomyosin form a complex that is regulated by the cal-cium ion (Ca++) concentration of the muscle cell cytoplasm.

When the Ca++ concentration of the muscle cell cyto-plasm is low, tropomyosin inhibits cross-bridge formationand the muscle is relaxed (figure 50.14). When the Ca++

concentration is raised, Ca++ binds to troponin. This causesthe troponin-tropomyosin complex to be shifted away fromthe attachment sites for the myosin heads on the actin.Cross-bridges can thus form, undergo power strokes, andproduce muscle contraction.

Where does the Ca++ come from? Muscle fibers storeCa++ in a modified endoplasmic reticulum called a sar-coplasmic reticulum, or SR (figure 50.15). When a musclefiber is stimulated to contract, an electrical impulse travelsinto the muscle fiber down invaginations called the trans-verse tubules (T tubules). This triggers the release ofCa++ from the SR. Ca++ then diffuses into the myofibrils,where it binds to troponin and causes contraction. Thecontraction of muscles is regulated by nerve activity, and sonerves must influence the distribution of Ca++ in the musclefiber.


Myosinhead

Myosin

Troponin

Tropomyosin

Binding sites forcross-bridges blocked

Binding sites forcross-bridges exposed

Actin

(a)

(b)

Ca++

Ca++

Ca++

Ca++

FIGURE 50.14How calcium controls striated muscle contraction. (a) Whenthe muscle is at rest, a long filament of the protein tropomyosinblocks the myosin-binding sites on the actin molecule. Becausemyosin is unable to form cross-bridges with actin at these sites,muscle contraction cannot occur. (b) When Ca++ binds to anotherprotein, troponin, the Ca++-troponin complex displacestropomyosin and exposes the myosin-binding sites on actin,permitting cross-bridges to form and contraction to occur.

Nucleus Mitochondrion

Myofibril

Sarcolemma

Z line Sarcoplasmicreticulum

Transverse tubule (T tubules)

FIGURE 50.15The relationship between themyofibrils, transverse tubules,and sarcoplasmic reticulum.Impulses travel down the axon of amotor neuron that synapses with amuscle fiber. The impulses areconducted along the transverse tubules and stimulatethe release of Ca++ from thesarcoplasmic reticulum into thecytoplasm. Ca++ diffuses toward themyofibrils and causes contraction.

Nerves Stimulate Contraction

Muscles are stimulated to contract by motor neurons. Theparticular motor neurons that stimulate skeletal muscles, asopposed to cardiac and smooth muscles, are called somaticmotor neurons. The axon (see figure 49.12) of a somaticmotor neuron extends from the neuron cell body andbranches to make functional connections, or synapses, with anumber of muscle fibers. (Synapses are discussed in moredetail in chapter 54.) One axon can stimulate many musclefibers, and in some animals a muscle fiber may be inner-vated by more than one motor neuron. However, in hu-mans each muscle fiber only has a single synapse with abranch of one axon.

When a somatic motor neuron produces electrochemi-cal impulses, it stimulates contraction of the muscle fibersit innervates (makes synapses with) through the followingevents:

1. The motor neuron, at its synapse with the musclefibers, releases a chemical known as a neurotransmit-ter. The specific neurotransmitter released by so-matic motor neurons is acetylcholine (ACh). AChacts on the muscle fiber membrane to stimulate themuscle fiber to produce its own electrochemicalimpulses.

2. The impulses spread along the membrane of themuscle fiber and are carried into the muscle fibersthrough the T tubules.

3. The T tubules conduct the impulses toward the sar-coplasmic reticulum, which then release Ca++. As de-scribed earlier, the Ca++ binds to troponin, which ex-poses the cross-bridge binding sites on the actinmyofilaments, stimulating muscle contraction.

When impulses from the nerve stop, the nerve stops re-leasing ACh. This stops the production of impulses in themuscle fiber. When the T tubules no longer produce im-pulses, Ca++ is brought back into the SR by active trans-port. Troponin is no longer bound to Ca++, so tropomyosinreturns to its inhibitory position, allowing the muscle torelax.

The involvement of Ca++ in muscle contraction is called,excitation-contraction coupling because it is the releaseof Ca++ that links the excitation of the muscle fiber by themotor neuron to the contraction of the muscle.

Motor Units and Recruitment

A single muscle fiber responds in an all-or-none fashionto stimulation. The response of an entire muscle dependsupon the number of individual fibers involved. The set ofmuscle fibers innervated by all axonal branches of a givenmotor neuron is defined as a motor unit (figure 50.16).Every time the motor neuron produces impulses, all mus-cle fibers in that motor unit contract together. The divi-sion of the muscle into motor units allows the muscle’sstrength of contraction to be finely graded, a requirement

for coordinated movements of the skeleton. Muscles thatrequire a finer degree of control have smaller motor units(fewer muscle fibers per neuron) than muscles that re-quire less precise control but must exert more force. Forexample, there are only a few muscle fibers per motorneuron in the muscles that move the eyes, while there areseveral hundred per motor neuron in the large muscles ofthe legs.

Most muscles contain motor units in a variety of sizes,which can be selectively activated by the nervous system.The weakest contractions of a muscle involve the activa-tion of a few small motor units. If a slightly stronger con-traction is necessary, additional small motor units are alsoactivated. The initial increments to the total force gener-ated by the muscle are therefore relatively small. As evergreater forces are required, more and larger motor unitsare brought into action, and the force increments becomelarger. The nervous system’s use of increased numbers andsizes of motor units to produce a stronger contraction istermed recruitment.

The cross-bridges are prevented from binding to actinby tropomyosin in a relaxed muscle. In order for amuscle to contract, Ca++ must be released from thesarcoplasmic reticulum, where it is stored, so that it canbind to troponin and cause the tropomyosin to shift itsposition in the thin filaments. Muscle contraction isstimulated by neurons. Varying sizes and numbers ofmotor units are used to produce different types ofmuscle contractions.


Musclefiber

Motor unit

(a) Tapping toe (b) Running

FIGURE 50.16The number and size of motor units. (a) Weak, precise musclecontractions use smaller and fewer motor units. (b) Larger andstronger movements require additional motor units that arelarger.

Types of Muscle FibersMuscle Fiber Twitches

An isolated skeletal muscle can be studied by stimulating itartificially with electric shocks. If a muscle is stimulatedwith a single electric shock, it will quickly contract andrelax in a response called a twitch. Increasing the stimulusvoltage increases the strength of the twitch up to a maxi-mum. If a second electric shock is delivered immediatelyafter the first, it will produce a second twitch that may par-tially “ride piggyback” on the first. This cumulative re-sponse is called summation (figure 50.17).

If the stimulator is set to deliver an increasing frequencyof electric shocks automatically, the relaxation time be-tween successive twitches will get shorter and shorter, asthe strength of contraction increases. Finally, at a particularfrequency of stimulation, there is no visible relaxation be-tween successive twitches. Contraction is smooth and sus-tained, as it is during normal muscle contraction in thebody. This smooth, sustained contraction is called tetanus.(The term tetanus should not be confused with the diseaseof the same name, which is accompanied by a painful stateof muscle contracture, or tetany.)

Skeletal muscle fibers can be divided on the basis oftheir contraction speed into slow-twitch, or type I,fibers, and fast-twitch, or type II, fibers. The musclesthat move the eyes, for example, have a high proportionof fast-twitch fibers and reach maximum tension in about7.3 milliseconds; the soleus muscle in the leg, by con-trast, has a high proportion of slow-twitch fibers and re-quires about 100 milliseconds to reach maximum tension(figure 50.18).

Muscles like the soleus must be able to sustain a con-traction for a long period of time without fatigue. Theresistance to fatigue demonstrated by these muscles isaided by other characteristics of slow-twitch (type I)fibers that endow them with a high capacity for aerobicrespiration. Slow-twitch fibers have a rich capillary sup-ply, numerous mitochondria and aerobic respiratory en-zymes, and a high concentration of myoglobin pigment.Myoglobin is a red pigment, similar to the hemoglobin inred blood cells, but its higher affinity for oxygen im-proves the delivery of oxygen to the slow-twitch fibers.Because of their high myoglobin content, slow-twitchfibers are also called red fibers.

The thicker, fast-twitch (type II) fibers have fewer capil-laries and mitochondria than slow-twitch fibers and not asmuch myoglobin; hence, these fibers are also called whitefibers. Fast-twitch fibers are adapted to respire anaerobi-cally by using a large store of glycogen and high concentra-tions of glycolytic enzymes. Fast-twitch fibers are adaptedfor the rapid generation of power and can grow thicker andstronger in response to weight training. The “dark meat”and “white meat” found in meat such as chicken and turkeyconsists of muscles with primarily red and white fibers,respectively.

In addition to the type I (slow-twitch) and type II (fast-twitch) fibers, human muscles also have an intermediateform of fibers that are fast-twitch but also have a high ox-idative capacity, and so are more resistant to fatigue. En-durance training increases the proportion of these fibers inmuscles.


Twitches

Incompletetetanus

• • • • • •

Completetetanus

Summation

Am

plitu

de o

fm

uscl

e co

ntra

ctio

ns

Stimuli

Time

FIGURE 50.17Muscle twitches summate to produce a sustained, tetanized contraction. This pattern is produced when the muscle is stimulatedelectrically or naturally by neurons. Tetanus, a smooth, sustained contraction, is the normal type of muscle contraction in the body.

Muscle Metabolism during Rest and Exercise

Skeletal muscles at rest obtain most of their energy fromthe aerobic respiration of fatty acids. During exercise, mus-cle glycogen and blood glucose are also used as energysources. The energy obtained by cell respiration is used tomake ATP, which is needed for (1) the movement of thecross-bridges during muscle contraction and (2) the pump-ing of Ca++ into the sarcoplasmic reticulum for muscle re-laxation. ATP can be obtained by skeletal muscles quicklyby combining ADP with phosphate derived from creatinephosphate. This compound was produced previously in theresting muscle by combining creatine with phosphate de-rived from the ATP generated in cell respiration.

Skeletal muscles respire anaerobically for the first 45 to90 seconds of moderate-to-heavy exercise, because the car-diopulmonary system requires this amount of time to suffi-ciently increase the oxygen supply to the exercising mus-cles. If exercise is moderate, aerobic respiration contributesthe major portion of the skeletal muscle energy require-ments following the first 2 minutes of exercise.

Whether exercise is light, moderate, or intense for agiven person depends upon that person’s maximal capacityfor aerobic exercise. The maximum rate of oxygen con-sumption in the body (by aerobic respiration) is called themaximal oxygen uptake, or the aerobic capacity. The inten-sity of exercise can also be defined by the lactate threshold.This is the percentage of the maximal oxygen uptake atwhich a significant rise in blood lactate levels occurs as aresult of anaerobic respiration. For average, healthy people,for example, a significant amount of blood lactate appearswhen exercise is performed at about 50 to 70% of the maxi-mal oxygen uptake.

Muscle Fatigue and Physical Training

Muscle fatigue refers to the use-dependant decrease in theability of a muscle to generate force. The reasons for fa-

tigue are not entirely understood. In most cases, however,muscle fatigue is correlated with the production of lacticacid by the exercising muscles. Lactic acid is produced bythe anaerobic respiration of glucose, and glucose is ob-tained from muscle glycogen and from the blood. Lactateproduction and muscle fatigue are therefore also related tothe depletion of muscle glycogen.

Because the depletion of muscle glycogen places a limiton exercise, any adaptation that spares muscle glycogen willimprove physical endurance. Trained athletes have an in-creased proportion of energy derived from the aerobic res-piration of fatty acids, resulting in a slower depletion oftheir muscle glycogen reserve. The greater the level ofphysical training, the higher the proportion of energy de-rived from the aerobic respiration of fatty acids. Becausethe aerobic capacity of endurance-trained athletes is higherthan that of untrained people, athletes can perform moreexercise before lactic acid production and glycogen deple-tion cause muscle fatigue.

Endurance training does not increase muscle size.Muscle enlargement is produced only by frequent periodsof high-intensity exercise in which muscles work againsthigh resistance, as in weight lifting. As a result of resis-tance training, type II (fast-twitch) muscle fibers becomethicker as a result of the increased size and number oftheir myofibrils. Weight training, therefore, causes skele-tal muscles to grow by hypertrophy (increased cell size)rather than by cell division and an increased number ofcells.

Muscles contract through summation of thecontractions of their fibers, producing tension that mayresult in shortening of the muscle. Slow-twitch skeletalmuscle fibers are adapted for aerobic respiration andare slower to fatigue than fast-twitch fibers, which aremore adapted for the rapid generation of power.


Time (msec)

Con

trac

tion

stre

ngth

Eye muscle(lateral rectus)

Calf muscle(gastrocnemius)

Deep muscle of leg(soleus)

FIGURE 50.18Skeletal muscles have differentproportions of fast-twitch and slow-twitch fibers. The muscles that move theeye contain mostly fast-twitch fibers,whereas the deep muscle of the leg (thesoleus) contains mostly slow-twitch fibers.The calf muscle (gastrocnemius) isintermediate in its composition.

Comparing Cardiac andSmooth MusclesCardiac and smooth muscle are similarin that both are found within internalorgans and both are generally notunder conscious control. Cardiac mus-cle, however, is like skeletal muscle inthat it is striated and contracts bymeans of a sliding filament mecha-nism. Smooth muscle (as its name im-plies) is not striated. Smooth muscledoes contain actin and myosin fila-ments, but they are arranged less reg-ularly within the cell.

Cardiac Muscle

Cardiac muscle in the vertebrateheart is composed of striated musclecells that are arranged differently from the fibers in askeletal muscle. Instead of the long, multinucleate cellsthat form skeletal muscle, cardiac muscle is composed ofshorter, branched cells, each with its own nucleus, thatinterconnect with one another at intercalated discs (fig-ure 50.19). Intercalated discs are regions where the mem-branes of two cells fuse together, and the fused mem-branes are pierced by gap junctions (chapter 7). The gapjunctions permit the diffusion of ions, and thus thespread of electric excitation, from one cell to the next.The mass of interconnected cardiac muscle cells forms asingle, functioning unit called a myocardium. Electric im-pulses begin spontaneously in a specific region of the my-ocardium known as the pacemaker. These impulses are notinitiated by impulses in motor neurons, as they are inskeletal muscle, but rather are produced by the cardiacmuscle cells themselves. From the pacemaker, the im-pulses spread throughout the myocardium via gap junc-tions, causing contraction.

The heart has two myocardia, one that receives bloodfrom the body and one that ejects blood into the body. Be-cause all of the cells in a myocardium are stimulated as aunit, cardiac muscle cannot produce summated contrac-tions or tetanus. This would interfere with the alternationbetween contraction and relaxation that is necessary forpumping.

Smooth Muscle

Smooth muscle surrounds hollow internal organs, includ-ing the stomach, intestines, bladder, and uterus, as well asall blood vessels except capillaries. Smooth muscle cells arelong and spindle-shaped, and each contains a single nu-cleus. They also contain actin and myosin, but these con-tractile proteins are not organized into sarcomeres. Parallelarrangements of thick and thin filaments cross diagonally

from one side of the cell to the other.The thick filaments are attached eitherto structures called dense bodies, thefunctional equivalents of Z lines, or tothe plasma membrane. Most smoothmuscle cells have 10 to 15 thin fila-ments per thick filament, compared to3 per thick filament in striated musclefibers.

Smooth muscle cells do not have asarcoplasmic reticulum; during a con-traction, Ca++ enters from the extracel-lular fluid. In the cytoplasm, Ca++

binds to calmodulin, a protein that isstructurally similar to troponin. TheCa++-calmodulin complex activates anenzyme that phosphorylates (adds aphosphate group to) the myosin heads.Unlike the case with striated muscles,this phosphorylation is required for the

myosin heads to form cross-bridges with actin.This mechanism allows gradations in the strength of

contraction in a smooth muscle cell, increasing contractionstrength as more Ca++ enters the cytoplasm. Heart patientssometimes take drugs that block Ca++ entry into smoothmuscle cells, reducing the cells’ ability to contract. Thistreatment causes vascular smooth muscle to relax, dilatingthe blood vessels and reducing the amount of work theheart must do to pump blood through them.

In some smooth muscle tissues, the cells contract onlywhen they are stimulated by the nervous system. Thesemuscles line the walls of many blood vessels and make upthe iris of the eye. Other smooth muscle tissues, like thosein the wall of the gut, contains cells that produce electricimpulses spontaneously. These impulses spread to adjoin-ing cells through gap junctions, leading to a slow, steadycontraction of the tissue.

Neither skeletal nor cardiac muscle can be greatlystretched because if the thick and thin filaments no longeroverlay in the sarcomere, cross-bridges cannot form. Un-like these striated muscles, smooth muscle can contracteven when it is greatly stretched. If one considers the de-gree to which some internal organs may be stretched—auterus during pregnancy, for example—it is no wonder thatthese organs contain smooth muscle instead of striatedmuscle.

Cardiac muscle cells interconnect physically andelectrically to form a single, functioning unit called amyocardium, which produces its own impulses at apacemaker region. Smooth muscles lack theorganization of myofilaments into sarcomeres and lacksarcoplasmic reticulum but contraction still occurs asmyofilaments slide past one another by use of cross-bridges.


Intercalateddisks

FIGURE 50.19Cardiac muscle. Cells are organized intolong branching chains that interconnect,forming a lattice; neighboring cells arelinked by structures called intercalateddiscs.

Modes of Animal LocomotionAnimals are unique among multicellular organisms in theirability to actively move from one place to another. Loco-motion requires both a propulsive mechanism and a controlmechanism. Animals employ a wide variety of propulsivemechanisms, most involving contracting muscles to gener-ate the necessary force. The quantity, quality, and positionof contractions are initiated and coordinated by the ner-vous system. In large animals, active locomotion is almostalways produced by appendages that oscillate—appendicularlocomotion—or by bodies that undulate, pulse, or undergoperistaltic waves—axial locomotion.

While animal locomotion occurs in many differentforms, the general principles remain much the same in allgroups. The physical restraints to movement—gravity andfrictional drag—are the same in every environment, differ-ing only in degree. You can conveniently divide the envi-ronments through which animals move into three types,each involving its own forms of locomotion: water, land,and air.

Locomotion in Water

Many aquatic and marine invertebrates move along thebottom using the same form of locomotion employed byterrestrial animals moving over the land surface. Flatwormsemploy ciliary activity to brush themselves along, round-worms a peristaltic slither, leeches a contract-anchor-extend creeping. Crabs walk using limbs to pull themselvesalong; mollusks use a muscular foot, while starfish useunique tube feet to do the same thing.

Moving directly through the water, or swimming, pre-sents quite a different challenge. Water’s buoyancy reducesthe influence of gravity. The primary force retarding for-ward movement is frictional drag, so body shape is impor-tant in reducing the friction and turbulence produced byswimming through the water.

Some marine invertebrates swim using hydraulic propul-sion. Scallops clap their shells together forcefully, whilesquids and octopuses squirt water like a marine jet. Allaquatic and marine vertebrates, however, swim.

Swimming involves using the body or its appendagesto push against the water. An eel swims by sinuous undu-lations of its whole body (figure 50.20a). The undulatingbody waves of eel-like swimming are created by waves ofmuscle contraction alternating between the left and rightaxial musculature. As each body segment in turn pushesagainst the water, the moving wave forces the eelforward.

Fish, reptiles, and aquatic amphibians swim in a waysimilar to eels, but only undulate the posterior (back) por-tion of the body (figure 50.20b) and sometimes only thecaudal (rear) fin. This allows considerable specialization ofthe front end of the body, while sacrificing little propulsiveforce.

Whales also swim using undulating body waves, but un-like any of the fishes, the waves pass from top to bottomand not from side to side. The body musculature of eelsand fish is highly segmental; that is, a muscle segment al-ternates with each vertebra. This arrangement permits thesmooth passage of undulatory waves along the body.Whales are unable to produce lateral undulations becausemammals do not have this arrangement.

Many tetrapod vertebrates swim, usually with appendic-ular locomotion. Most birds that swim, like ducks andgeese, propel themselves through the water by pushingagainst it with their hind legs, which typically have webbedfeet. Frogs, turtles, and most marine mammals also swimwith their hind legs and have webbed feet. Tetrapod verte-brates that swim with their forelegs usually have theselimbs modified as flippers, and pull themselves through thewater. These include sea turtles, penguins, and fur seals. Afew principally terrestrial tetrapod vertebrates, like polarbears and platypuses, swim with walking forelimbs notmodified for swimming.


Eel

Trout

ThrustReactiveforce

Lateralforce

Push

90˚

Trout

Reactiveforce

Push

90˚Lateral

force

Thrust

FIGURE 50.20Movements of swimming fishes. (a) An eel pushes against thewater with its whole body, (b) a trout only with its posterior half.

(a)

(b)

Locomotion on Land

The three great groups of terrestrial animals—mollusks,arthropods, and vertebrates—each move over land in dif-ferent ways.

Mollusk locomotion is far less efficient than that of theother groups. Snails, slugs, and other terrestrial molluskssecrete a path of mucus that they glide along, pushing witha muscular foot.

Only vertebrates and arthropods (insects, spiders, andcrustaceans) have developed a means of rapid surface loco-motion. In both groups, the body is raised above theground and moved forward by pushing against the groundwith a series of jointed appendages, the legs.

Because legs must provide support as well as propulsion,it is important that the sequence of their movements notshove the body’s center of gravity outside of the legs’ zoneof support. If they do, the animal loses its balance and falls.It is the necessity to maintain stability that determines thesequence of leg movements, which are similar in verte-brates and arthropods.

The apparent differences in the walking gaits of thesetwo groups reflects the differences in leg number. Verte-brates are tetrapods (four limbs), while all arthropods havesix or more limbs. Although having many legs increases sta-bility during locomotion, they also appear to reduce themaximum speed that can be attained.

The basic walking pattern of all tetrapod vertebrates isleft hind leg (LH), left foreleg (LF), right hindleg (RH),right foreleg (RF), and then the same sequence again andagain. Unlike insects, vertebrates can begin to walk withany of the four legs, and not just the posterior pair. Both

arthropods and vertebrates achieve faster gaits by overlap-ping the leg movements of the left and right sides. For ex-ample, a horse can convert a walk to a trot, by moving di-agonally opposite legs simultaneously.

The highest running speeds of tetrapod vertebrates,such as the gallop of a horse, are obtained with asymmetricgaits. When galloping, a horse is never supported by morethan two legs, and occasionally is supported by none. Thisreduces friction against the ground to an absolute mini-mum, increasing speed. With their larger number of legs,arthropods cannot have these speedy asymmetric gaits, be-cause the movements of the legs would interfere with eachother.

Not all animals walk or run on land. Many insects, likegrasshoppers, leap using strong rear legs to propel them-selves through the air. Vertebrates such as kangaroos, rab-bits, and frogs are also effective leapers (figure 50.21).

Many invertebrates use peristaltic motion to slide overthe surface. Among vertebrates, this form of locomotion isexhibited by snakes and caecilians (legless amphibians).Most snakes employ serpentine locomotion, in which thebody is thrown into a series of sinuous curves. The move-ments superficially resemble those of eel-like swimming,but the similarity is more apparent than real. Propulsion isnot by a wave of contraction undulating the body, but by asimultaneous lateral thrust in all segments of the body incontact with the ground. To go forward, it is necessary thatthe strongest muscular thrust push against the ground op-posite the direction of movement. Because of this, thrusttends to occur at the anterior (outside) end of the inward-curving side of the loop of the snake’s body.


FIGURE 50.21Animals that hop or leap use their rear legs to propel themselves through the air. The powerful leg muscles of this frog allow it toexplode from a crouched position to a takeoff in about 100 milliseconds.

Locomotion in Air

Flight has evolved among the animals four times: insects,pterosaurs (extinct flying reptiles), birds, and bats. In allfour groups, active flying takes place in much the same way.Propulsion is achieved by pushing down against the airwith wings. This provides enough lift to keep insects in theair. Vertebrates, being larger, need greater lift, obtaining itwith wings that are convex in cross section. Because airmust travel farther over the top surface, it moves faster,creating lift over the wing.

In birds and most insects, the raising and lowering of thewings is achieved by the alternate contraction of extensormuscles (elevators) and flexor muscles (depressors). Fourinsect orders (containing flies, mosquitoes, wasps, bees, andbeetles), however, beat their wings at frequencies from 100to more than 1000 times per second, faster than nerves cancarry successive impulses! In these insects, the flight mus-cles are not attached to the wings at all but rather to thestiff wall of the thorax, which is distorted in and out bytheir contraction. The reason that these muscles can beatso fast is that the contraction of one set stretches the other,triggering its contraction in turn without waiting for thearrival of a nerve impulse.

Among vertebrates (figure 50.22), flight first evolvedsome 200 million years ago among flying reptiles calledpterosaurs. A very successful and diverse group, pterosaursranged in size from individuals no bigger than sparrows topterodons the size of a fighter plane. For much of this time,they shared the skies with birds, which most paleontologistsbelieve evolved from feathered dinosaurs about 150 millionyears ago. How did they share their ecological world for 100million years without competition driving one or the otherfrom the skies? No one knows for sure. Perhaps these earlybirds were night fliers, while pterosaurs flew by day.

Such an arrangement for sharing resources is not as un-likely as it might at first appear. Bats, flying mammals whichevolved after the pterosaurs disappeared with the dinosaurs,are night fliers. By flying at night bats are able to shop in astore with few other customers and a wealth of food: night-flying insects. It has proven to be a very successful approach.One-quarter of all mammal species are bats.

Locomotion in larger animals is almost always producedby appendages that push against the surroundings insome fashion, or by shoving the entire body forward byan undulation.


Eastern bluebirdPterosaur(extinct)

Samoanflying fox(fruitbat)

FIGURE 50.22Flight has evolved three times among the vertebrates. These three very different vertebrates all have lightened bones and forelimbstransformed into wings.


Chapter 50 Summary Questions Media Resources


• There are three types of skeleton: hydrostaticskeletons, exoskeletons, and endoskeletons.

• Bone is formed by the secretion of an organic matrixby osteoblasts; this organic matrix becomes calcified.

1. What are the two majorcomponents of the extracellularmatrix in bone? What structuralproperties does each componenthave? How do the twocomponents combine to makebone resistant to fracture?

• Freely movable joints surround the articulating boneswith a synovial capsule filled with a lubricating fluid.

• Skeletal muscles can work together as synergists, oroppose each other as antagonists.

2. What are the three types ofjoints in a vertebrate skeleton?Give an example of where eachtype is found in the body.3. What is the difference

between a skeletal muscle’sorigin and its insertion?

50.2 Skeletal muscles contract to produce movements at joints.

• A muscle fiber contains numerous myofibrils, whichconsist of thick filaments composed of myosin andthin filaments of actin.

• There are small cross-bridges of myosin that extendout toward the actin; the cross-bridges are activatedby the hydrolysis of ATP so that it can bind to actinand undergo a power stroke that causes the sliding ofthe myofilaments.

• When Ca++ binds to troponin, the tropomyosin shiftsposition in the thin filament, allowing the cross-bridges to bind to actin and undergo a power stroke.

• The release of Ca++ from the sarcoplasmic reticulumis stimulated by impulses in the muscle fiberproduced by neural stimulation.

• Slow-twitch fibers are adapted for aerobic respirationand are resistant to fatigue; fast-twitch fibers can pro-vide power quickly but produce lactic acid and fatiguequickly.

• Cardiac muscle cells have gap junctions that permitthe spread of electric impulses from one cell to thenext.

• Cardiac and smooth muscles are involuntary and reg-ulated by autonomic nerves; the contractions are au-tomatically produced in cardiac muscle and somesmooth muscles.

• Animals have adapted modes of locomotion to threedifferent environments: water, land, and air.

4. Of what proteins are thickand thin filaments composed? 5. Describe the steps involved

in the cross-bridge cycle. Whatfunctions does ATP perform inthe cycle?6. Describe the steps involved

in excitation-contractioncoupling. What functions doacetylcholine and Ca++ performin this process?7. How does a somatic motor

neuron stimulate a muscle fiberto contract? 8. What is the difference

between a muscle twitch andtetanus? 9. Why can’t a myocardium

produce a sustained contraction?10. How does smooth musclediffer from skeletal muscle interms of thick and thin filamentorganization, the role of Ca++ incontraction, and the effect ofstretching on the muscle’s abilityto contract?11. What do all modes oflocomotion have in common?


www.mhhe.com/raven6e www.biocourse.com

• On Science Article:Running improperly

• Bioethics case study:Sports and fitness

• On Science Article:Climbing the walls

• Straited musclecontraction

• Muscle contractionaction potential

• Detailed straitedmuscle

• Actin-myosincrossbridges

• Activity: Musclecontraction

• Muscle cell function• Body musculature• Head and neck

muscles• Trunk muscles• Upper limb muscles• Lower limb muscles• Muscle characteristics

• Walking

Chapter 50: Locomotion

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Transcript of Chapter 50: Locomotion