Muscular System

Cilia and flagella, however, are composed of different proteins, and thus are exceptionsto the rule.

Animal Movement

Movement is an important characteristic of animals.

Animal movement occurs in many forms in animal tissues, ranging from barelydiscernible streaming of cytoplasm to extensive movements of powerful striatedmuscles.

Most animal movement depends on a single fundamental mechanism: contractileproteins, which can change their form to elongate or contract.

This contractile machinery is always composed of ultrafine fibrils— fine filaments,striated fibrils, or tubular fibrils (microtubules) arranged to contract when powered byATP.

By far the most important protein contractile system is the actomyosin system,composed of two proteins, actin and myosin.

Three principal kinds of animal movement: ameboid, ciliary, and muscular.

-a form of movement especially characteristic of amebas and other unicellular forms.

Ameboid Movement

- it is also found in many wandering cells of metazoans, such as white blood cells,embryonic mesenchyme, and numerous other mobile cells that move through thetissue spaces.

Ameboid cells change their shape bysending out and withdrawingpseudopodia (false feet) from anypoint on the cell surface.

Beneath the plasmalemma lies anongranular layer, the gel-likeectoplasm, which encloses the moreliquid endoplasm.

Cilia are minute, hairlike, motile processes that extend from the surfaces of the cells ofmany animals.

Ciliary and Flagellar Movement

Particularly distinctive feature of ciliate protistans.

Cilia are found in all major groups of animals.

Cilia perform many roles either inmoving small organisms such asunicellular ciliates and flagellatesthrough their aquatic environmentor in propelling fluids and materialsacross epithelial surfaces of largeranimals.

Cilia are of remarkably uniform diameter(0.2 to 0.5 m) wherever they are found.

Each cilium contains a peripheral circle ofnine double microtubules arranged aroundtwo single microtubules in the center.

Exceptions to the 9 + 2 arrangement havebeen noted:for example: sperm tails of flatworms havebut one central microtubule, and spermtails of a mayfly have no centralmicrotubule.

Each microtubule is composed of a spiral arrayof protein subunits called tubulin.

The microtubule doublets around theperiphery are connected to each other and tothe central pair of microtubules by a complexsystem of connective elements.

Extending from each doublet is a pair of armscomposed of the protein dynein.

Dynein arms act as cross bridges between thedoublets, operate to produce a sliding forcebetween the microtubules.

A flagellum is a whiplike structure longer than a cilium and usually present singly or insmall numbers at one end of a cell.

They are found in members of flagellateprotistans, in animal spermatozoa, and insponges.

The main difference between a cilium and aflagellum is in their beating pattern rather thanin their structure, since both look alikeinternally.

A flagellum beats symmetrically with snakelike undulations so that water is propelledparallel to the long axis of the flagellum.

A cilium, in contrast, beats asymmetrically with a fast power stroke in one directionfollowed by a slow recovery during which the cilium bends as it returns to its originalposition. Water is propelled parallel to the ciliated surface.

Microtubules behave as “sliding filaments”that move past one another much like thesliding filaments of vertebrate skeletal muscle.

During ciliary flexion, the dynein arms link toadjacent microtubules, then swivel and releasein repeated cycles.

During the recovery stroke microtubules onthe opposite side slide outward to bring thecilium back to its starting position.

The Mechanism of Ciliary and Flagellar Locomotion

the dynein arms anchored along the A tubule of the lower doublet attach to binding sites on the B tubule of the upper doublet.

the dynein molecules undergo aconformational change, or power stroke, thatcauses the lower doublet to slide toward thebasal end of the upper doublet.

the dynein arms have detached from the B tubule of the upper doublet

the arms have reattachedto the upper doublet so that another cycle can begin.

Contractile tissue is most highly developed in muscle cells called fibers. Although musclefibers themselves can do work only by contraction and cannot actively lengthen, they canbe arranged in so many different configurations and combinations that almost anymovement is possible.

Muscular Movement

Vertebrate muscle is broadly classified on the basis of the appearance of muscle cells(fibers).

Types of Vertebrate Muscle

Skeletal muscle appears transversely striped (striated), with alternating dark and lightbands.

Cardiac muscle also possesses striations like skeletal muscle but is uninucleated and withbranching cells.

Smooth (visceral) muscle which lacks the characteristic alternating bands of the striatedtype.

Antagonistic Muscle Action

• Muscles are either contracted or relaxed

• When contracted the muscle exerts a pulling force, causing it to shorten

• Since muscles can only pull (not push), they work in pairs called antagonistic muscles

• The muscle that bends the joint is called the flexor muscle

• The muscle that straightens the joint is called the extensor muscle

Elbow Joint• The best known example of antagonistic muscles are the bicep &

triceps muscles

Elbow joint flexedFlexor muscles contractedExtensor muscles relaxed

Elbow joint extendedExtensor muscles contracted

Flexor muscles relaxed

biceps

triceps

Section through arm

Flexormuscles

Extensor muscles

HumerusBone

Skeletal muscle is typically organizedinto sturdy, compact bundles or bands.

They are packed into bundles called fascicles (fasciculus, small bundle). Most skeletalmuscles taper (thinner) at their ends, where they connect to bones by tendons.

Attached to skeletal elements.

Responsible for movements of thetrunk, appendages, eyes, mouthparts,and other structure.

Skeletal muscle fibers are extremely long, cylindrical, multinucleate cells that may reachfrom one end of the muscle to the other.

In most fishes, amphibians, and to some extent lizards and snakes, there is a segmentedorganization of muscles alternating with the vertebrae.

The skeletal muscles of othervertebrates, by splitting, fusion,and shifting, have developed intospecialized muscles best suited formanipulating jointed appendagesthat have evolved for locomotionon land.

Skeletal muscle contracts powerfully and quickly but fatigues more rapidly than doessmooth muscle.

Skeletal muscle is a voluntary muscle because it is stimulated by motor fibers and isunder conscious control.

Smooth (Visceral) muscle lacks thestriations.

Cells are long, tapering strands, eachcontaining a single nucleus.

Smooth muscle cells are organized intosheets of muscle circling the walls ofthe alimentary canal, blood vessels,respiratory passages, and genitalducts.

Smooth muscle is typically slow acting and can maintain prolonged contractions with verylittle energy expenditure.

It is under the control of the autonomic nervous system; its contractions are involuntaryand unconscious.

Cardiac muscle, the seemingly tireless muscle of the vertebrate heart, combines certaincharacteristics of both skeletal and smooth muscle.

It is fast acting and striated, but contraction is under involuntary autonomic control.

Actually the autonomic nerves serving the heart can only speed up or slow down the rateof contraction; the heartbeat originates within specialized cardiac muscle, and the heartcontinues to beat even after all autonomic nerves are severed.

Cardiac muscle is composed of closelyopposed, but separate, uninucleatedcell fibers (syncytium) andintercalated discs.

Smooth and striated muscles are also characteristic of invertebrate animals, but there aremany variations of both types and even instances in structural and functional features.

Types of Invertebrate Muscle

Striated muscle appears in invertebrate groups as diverse as cnidarians and arthropods.

The thickest muscle fibers known,approximately 3 mm in diameter and 6 cmlong, are those of giant barnacles and ofAlaska king crabs living along the Pacificcoast of North America.

Bivalve molluscan muscles contain fibers of two types.

One kind is striated muscle that can contract rapidly, enabling the bivalve to snap shut itsvalves when disturbed. Scallops use these “fast” muscle fibers to swim in their awkwardmanner.

The second muscle type is smooth muscle, capable of slow, long-lasting contractions.Using these fibers, a bivalve can keep its valves tightly shut for hours or even days.

Such adductor muscles use little metabolic energy and receive remarkably few nerveimpulses to maintain the activated state.

The wings of some small flies operate at frequencies greater than 1000 beats per second.The so-called fibrillar muscle.

It has very limited extensibility; that is, the wing leverage (control) system is arranged sothat the muscles shorten only slightly during each downbeat of the wings.

Furthermore, muscles and wings operate as a rapidly oscillating (back & forth) system inan elastic thorax.

Each cell, or fiber, is a multinucleated tubecontaining numerous myofibrils, packedtogether and invested by the cell membrane,the sarcolemma.

Structure of Striated Muscle

The myofibril contains two types ofmyofilaments: thick filaments composed ofthe protein myosin, and thin filaments,composed of the protein actin.

These are the actual contractile proteins of themuscle.

Thin filaments are held together by a dense structure called the Z line.

The functional unit of the myofibril, the sarcomere, extends between successive Z lines.

•Sarcomere = the basic contractile unit

A (anisotropic) – dark bandI (isotropic) – light band

Each myosin molecule is composed of two polypeptide chains, each having a club-shapedhead.

Each thick filament is made up of myosin molecules packed together in an elongatebundle.

Lined up as they are in a bundle to form a thick filament, the double heads of eachmyosin molecule face outward from the center of the filament.

These heads act as molecular cross bridges that interact with the thin filaments duringcontraction.

Thin filaments are more complex because they are composed of three different proteins.

The backbone of the thin filament is a double strand of the protein actin, twisted into adouble helix.

Surrounding the actin filament are two thin strands of another protein, tropomyosin, thatlie near the grooves between the actin strands. Each tropomyosin strand is itself a doublehelix.

The third protein of the thin filament is troponin, a complex of three globular proteinslocated at intervals along the filament. Troponin is a calcium-dependent switch that actsas the control point in the contraction process.

The Sarcomere

Thick filaments(myosin)

Thin filaments(actin)

Mline

Zline

Zline

proteins in the Z line

justthin

filament

overlap zone- both

thick & thinfilaments

justthick

filament

myosinbare zone

- nocross bridges

proteins in the M line

In the 1950s the English physiologists A. F. Huxley and H. E. Huxley independentlyproposed the sliding filament model to explain striatedmuscle contraction.

Sliding Filament Model of Muscle Contraction

Thick and thin filaments become linked together by molecular cross bridges, which act aslevers to pull the filaments past each other.

Sliding Filament Model of Muscle Contraction

During contraction, cross bridges on the thick filaments swing rapidly back and forth,alternately attaching to and releasing from special receptor sites on the thin filaments,and drawing thin filaments past thick filaments.

As contraction continues, the Z lines are pulled closer together. Thus the sarcomereshortens. Because all sarcomere units shorten together, the muscle contracts. Relaxationis a passive process. When cross bridges between the thick and thin filaments release, thesarcomeres are free to lengthen.

Sarcomere shortens when muscle contracts

• Shortening of the sarcomeres in a myofibril produces the shortening of the myofibril

Mechanism of muscle contraction

•The above micrographs show that the sarcomere gets shorter when the muscle contracts

•The light (I) bands become shorter

•The dark bands (A) bands stay the same length

Relaxedmuscle

Contractedmuscle

relaxed sarcomere

contracted sarcomere

The Sliding Filament Theory

• So, when the muscle contracts, sarcomeres become smaller

• However the filaments do not change in length.

• Instead they slide past each other (overlap)

• So actin filaments slide between myosin filaments

• and the zone of overlap is larger

Muscle contracts in response to nerve stimulation.

Control of Contraction

If the nerve supply to a muscle is severed, the muscle atrophies (weaken).

Skeletal muscle fibers are innervated by motor neurons whose cell bodies are located inthe spinal cord.

Each cell body gives rise to a motor axon that leaves the spinal cord to travel by way of aperipheral nerve trunk to a muscle where it branches repeatedly into many terminalbranches.

Each terminal branch innervates a single muscle fiber.

Depending on the type of muscle, a single motor axon may innervate as few as three orfour muscle fibers (where very precise control is needed, such as the muscles that controleye movement) or as many as 2000 muscle fibers (where precise control is not required,such as large leg muscles).

The motor neuron and all muscle fibers it innervates is called a motor unit.

Control of Contraction

The motor unit is the functional unit of skeletal muscle.

When a motor neuron fires, the action potential passes to all fibers of the motor unit andeach is stimulated to contract simultaneously.

Total force exerted by a muscle depends on the number of motor units activated.

Precise control of movement is achieved by varying the number of motor units activatedat any one time.

A smooth and steady increase in muscle tension is produced by increasing the number ofmotor units brought into play; this is called motor unit recruitment.

The place where a motor axonterminates on a muscle fiber iscalled themyoneural junction.

The Myoneural Junction

At the junction is a tiny gap, orsynaptic cleft, that thinlyseparates a nerve fiber andmuscle fiber.

In the vicinity of thejunction, the neuron stores achemical, acetylcholine, inminute vesicles known assynaptic vesicles.

Acetylcholine is released whena nerve impulse reaches asynapse.

This substance is a chemicalmediator that diffuses acrossthe narrow junction and acts onthe muscle fiber membrane togenerate an electricaldepolarization.

The depolarization spreadsrapidly through the musclefiber, causing it to contract.

Thus the synapse is a specialchemical bridge that couplestogether the electricalactivities of nerve andmuscle fibers.

Built into vertebrate skeletal muscle is anelaborate conduction system that serves tocarry the depolarization from the myoneuraljunction to the densely packed filamentswithin the fiber.

Along the surface of the sarcolemma arenumerous invaginations that project as asystem of tubules into the muscle fiber. Thisis called the T-system.

The T-system is continuous with thesarcoplasmic reticulum, a system offluid-filled channels that runs parallel tothe myofilaments. The system is ideallyarranged for speeding the electricaldepolarization from the myoneuraljunction to the myofilaments within thefiber.

Step 1: An action potential spreads along the sarcolemma and is conducted inward to thesarcoplasmic reticulum by way of T tubules (T-tubule system). Calcium ions released fromthe sarcoplasmic reticulum diffuse rapidly into the myofibrils and bind to troponinmolecules on the actin molecule. Troponin molecules are moved away from the activesites.

Excitation-contraction coupling in vertebrate skeletal muscle

Excitation-contraction coupling in vertebrate skeletal muscle

Step 2:Myosin cross bridges bind to the exposed active sites.

Excitation-contraction coupling in vertebrate skeletal muscle

Step 3: Using the energy stored in ATP, the myosin head swings toward the centerof the sarcomere. ADP and a phosphate group are released.

Excitation-contraction coupling in vertebrate skeletal muscle

Step 4: The myosin head binds another ATP molecule; this frees themyosin head from the active site on actin.

Excitation-contraction coupling in vertebrate skeletal muscle

Step 5: The myosin head splits ATP, retaining the energy released as well asthe ADP and the phosphate group. The cycle can now be repeated as long ascalcium is present to open active sites on the actin molecules.

Muscle contraction requires large amounts of energy. ATP is the immediate source ofenergy, but the amount present will sustain contraction for only a second or two.

Muscle cells immediately call on the second level of energy reserve, creatine phosphate.

Creatine phosphate is a high-energy phosphate compound that stores bond energyduring periods of rest.

As ADP is produced during contraction, creatine phosphate releases its stored bondenergy to convert ADP to ATP.

This reaction can be summarized as:Creatine phosphate ADP → ATP Creatine

Within a few seconds—perhaps as long as 30 seconds depending on the rapidity ofmuscle contraction—the reserves of creatine phosphate are depleted.

The contracting muscle now must be fueled from its third and largest store of energy, glycogen.

Glycogen is stored in both liver and muscle. Muscle has by far the larger store—some3/4s of all the glycogen in the body is stored in muscle.

As a supply of energy for contraction, glycogen has three important advantages: it isrelatively abundant, it can be mobilized quickly, and it can provide energy under anoxicconditions.

As soon as the muscle’s store of creatine phosphate declines, enzymes break downglycogen, converting it into glucose-6- phosphate, the first stage of glycolysis that leadsinto mitochondrial respiration and the generation of ATP.

Overview of the process

Overview of the process

The muscle fiber is stimulated.

Overview of the process

The muscle fiber is stimulated.

Ca2+ ions are released.

“End-on” view of thick & thin filaments, showing the effect of calcium ions after release from the S.R.

Overview of the process

The muscle fiber is stimulated.

Ca2+ ions are released.

Overview of the process

The muscle fiber is stimulated.

Ca2+ ions are released.

Thin filaments move to middle of sarcomere.

Calcium attaches to troponin; they roll away, exposing the active site on actin.

Myosin cross-bridges attach to active site on actin.

After attachment, the cross-bridges pivot, pulling the thin filaments.

A fresh ATP replaces the ADP+Pi, allowing myosin and actin to detach.

Energy from the splitting of the fresh ATP allows repositioning of the myosin head.

This leads back to Step 1, which continues the

cycle as long as calcium ions are

attached to troponin.

Overview of the process

The muscle fiber is stimulated.

Ca2+ ions are released.

Thin filaments move to middle of sarcomere.

Overview of the process

The muscle fiber is stimulated.

Ca2+ ions are released.

Thin filaments move to middle of sarcomere.

Muscle fiber contracts.

Overview of the process

The muscle fiber is stimulated.

Ca2+ ions are released.

Thin filaments move to middle of sarcomere.

Muscle fiber contracts.

Muscle tension increases.

Repetition of the cycle

•One ATP molecule is split by each cross bridge in each cycle.

•This takes only a few milliseconds

•During a contraction 1000’s of cross bridges in each sarcomere go through this cycle.

•However the cross bridges are all out of synch, so there are always many cross bridges attached at any one time to maintain force.

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