concept 49.5: animal skeletons function in support, protection, and movement

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Concept 49.5: Animal skeletons function in support, protection, and movement

• The various types of animal movements

– All result from muscles working against some type of skeleton


Types of Skeletons

• The three main functions of a skeleton are

– Support, protection, and movement

• The three main types of skeletons are

– Hydrostatic skeletons, exoskeletons, and endoskeletons


Hydrostatic Skeletons

• A hydrostatic skeleton

– Consists of fluid held under pressure in a closed body compartment

• This is the main type of skeleton

– In most cnidarians, flatworms, nematodes, and annelids


• Annelids use their hydrostatic skeleton for peristalsis

– A type of movement on land produced by rhythmic waves of muscle contractions

Figure 49.25a–c

(a) Body segments at the head and just in front of the rear are short and thick (longitudinal muscles contracted; circular muscles relaxed) and anchored to the ground by bristles. The other segments are thin and elongated (circular muscles contracted; longitudinal muscles relaxed.)

(b) The head has moved forward because circular muscles in the head segments have contracted. Segments behind the head and at the rear are now thick and anchored, thus preventing the worm from slipping backward.

(c) The head segments are thick again and anchored in their new positions. The rear segments have released their hold on the ground and have been pulled forward.

Longitudinalmuscle relaxed(extended)

Circularmusclecontracted

Circularmusclerelaxed

Longitudinalmusclecontracted

HeadBristles


Exoskeletons

• An exoskeleton is a hard encasement

– Deposited on the surface of an animal

• Exoskeletons

– Are found in most molluscs and arthropods


Endoskeletons

• An endoskeleton consists of hard supporting elements

– Such as bones, buried within the soft tissue of an animal

• Endoskeletons

– Are found in sponges, echinoderms, and chordates


• The mammalian skeleton is built from more than 200 bones

– Some fused together and others connected at joints by ligaments that allow freedom of movement


• The human skeleton

Figure 49.26

1 Ball-and-socket joints, where the humerus contactsthe shoulder girdle and where the femur contacts thepelvic girdle, enable us to rotate our arms andlegs and move them in several planes.

2 Hinge joints, such as between the humerus andthe head of the ulna, restrict movement to a singleplane.

3 Pivot joints allow us to rotate our forearm at theelbow and to move our head from side to side.

keyAxial skeletonAppendicularskeleton

Skull

Shouldergirdle

Clavicle

Scapula

Sternum

RibHumerus

Vertebra

RadiusUlnaPelvicgirdle

Carpals

Phalanges

Metacarpals

Femur

Patella

Tibia

Fibula

TarsalsMetatarsalsPhalanges

1

Examplesof joints

2

3

Head ofhumerus

Scapula

Humerus

Ulna

UlnaRadius


Physical Support on Land

• In addition to the skeleton

– Muscles and tendons help support large land vertebrates


• Concept 49.6: Muscles move skeletal parts by contracting

• The action of a muscle

– Is always to contract


• Skeletal muscles are attached to the skeleton in antagonistic pairs

– With each member of the pair working against each other

Figure 49.27

Human Grasshopper

Bicepscontracts

Tricepsrelaxes

Forearmflexes

Bicepsrelaxes

Tricepscontracts

Forearmextends

Extensormusclerelaxes

Flexormusclecontracts

Tibiaflexes

Extensormusclecontracts

Flexormusclerelaxes

Tibiaextends


Vertebrate Skeletal Muscle

• Vertebrate skeletal muscle

– Is characterized by a hierarchy of smaller and smaller units

Figure 49.28

Muscle

Bundle ofmuscle fibers

Single muscle fiber(cell)

Plasma membrane

Myofibril

Lightband Dark band

Z line

Sarcomere

TEM 0.5 mI band A band I band

M line

Thickfilaments(myosin)

Thinfilaments(actin)

H zoneSarcomere

Z lineZ line

Nuclei

animation


• A skeletal muscle consists of a bundle of long fibers

– Running parallel to the length of the muscle

• A muscle fiber (muscle cell)

– Is itself a bundle of smaller myofibrils arranged longitudinally


• The myofibrils are composed to two kinds of myofilaments

– Thin filaments, consisting of two strands of actin and one strand of regulatory protein

– Thick filaments, staggered arrays of myosin molecules


• Skeletal muscle is also called striated muscle

– Because the regular arrangement of the myofilaments creates a pattern of light and dark bands


• Each repeating unit is a sarcomere

– Bordered by Z lines

• The areas that contain the myofilments

– Are the I band, A band, and H zone


The Sliding-Filament Model of Muscle Contraction

• According to the sliding-filament model of muscle contraction

– The filaments slide past each other longitudinally, producing more overlap between the thin and thick filaments


• As a result of this sliding

– The I band and the H zone shrink

Figure 49.29a–c

(a) Relaxed muscle fiber. In a relaxed muscle fiber, the I bandsand H zone are relatively wide.

(b) Contracting muscle fiber. During contraction, the thick andthin filaments slide past each other, reducing the width of theI bands and H zone and shortening the sarcomere.

(c) Fully contracted muscle fiber. In a fully contracted musclefiber, the sarcomere is shorter still. The thin filaments overlap,eliminating the H zone. The I bands disappear as the ends ofthe thick filaments contact the Z lines.

0.5 m

Z HA

Sarcomere


• The sliding of filaments is based on

– The interaction between the actin and myosin molecules of the thick and thin filaments

• The “head” of a myosin molecule binds to an actin filament

– Forming a cross-bridge and pulling the thin filament toward the center of the sarcomere


• Myosin-actin interactions underlying muscle fiber contraction

Figure 49.30

Thick filament

Thin filaments

Thin filament

ATPATP

ADPADP

ADP

P i P i

P i

Cross-bridge

Myosin head (low-energy configuration)

Myosin head (high-energy configuration)

+

Myosin head (low-energy configuration)

Thin filament moves toward center of sarcomere.

Thick filament

ActinCross-bridge binding site

1 Starting here, the myosin head is bound to ATP and is in its low-energy confinguration.

2 The myosin head hydrolyzes ATP to ADP and inorganic phosphate ( I ) and is in its high-energy configuration.

P

1 The myosin head binds toactin, forming a cross-bridge.

3

4 Releasing ADP and ( i), myosinrelaxes to its low-energy configuration, sliding the thin filament.

P

5 Binding of a new mole-cule of ATP releases the myosin head from actin,and a new cycle begins.


The Role of Calcium and Regulatory Proteins

• A skeletal muscle fiber contracts

– Only when stimulated by a motor neuron


• When a muscle is at rest

– The myosin-binding sites on the thin filament are blocked by the regulatory protein tropomyosin

Figure 49.31a

ActinTropomyosin Ca2+-binding sites

Troponin complex

(a) Myosin-binding sites blocked


• For a muscle fiber to contract

– The myosin-binding sites must be uncovered

• This occurs when calcium ions (Ca2+)

– Bind to another set of regulatory proteins, the troponin complex

Figure 49.31b

Ca2+

Myosin-binding site

(b) Myosin-binding sites exposed


• The stimulus leading to the contraction of a skeletal muscle fiber

– Is an action potential in a motor neuron that makes a synapse with the muscle fiber

Figure 49.32

Motorneuron axon

Mitochondrion

Synapticterminal

T tubule

Sarcoplasmicreticulum

Myofibril

Plasma membraneof muscle fiber

Sarcomere

Ca2+ releasedfrom sarcoplasmicreticulum


• The synaptic terminal of the motor neuron

– Releases the neurotransmitter acetylcholine, depolarizing the muscle and causing it to produce an action potential


• Action potentials travel to the interior of the muscle fiber

– Along infoldings of the plasma membrane called transverse (T) tubules

• The action potential along the T tubules

– Causes the sarcoplasmic reticulum to release Ca2+

• The Ca2+ binds to the troponin-tropomyosin complex on the thin filaments

– Exposing the myosin-binding sites and allowing the cross-bridge cycle to proceed


ACh

Synapticterminalof motorneuron

Synaptic cleft T TUBULEPLASMA MEMBRANE

SR

ADP

CYTOSOL

Ca2

Ca2

P2

Cytosolic Ca2+ is removed by active transport into SR after action potential ends.

6

• Review of contraction in a skeletal muscle fiber

Figure 49.33

Acetylcholine (ACh) released by synaptic terminal diffuses across synapticcleft and binds to receptor proteins on muscle fiber’s plasma membrane, triggering an action potential in muscle fiber.

1

Action potential is propa-gated along plasmamembrane and downT tubules.

2

Action potentialtriggers Ca2+

release from sarco-plasmic reticulum(SR).

3

Myosin cross-bridges alternately attachto actin and detach, pulling actinfilaments toward center of sarcomere;ATP powers sliding of filaments.

5

Calcium ions bind to troponin;troponin changes shape,removing blocking actionof tropomyosin; myosin-bindingsites exposed.

4

Tropomyosin blockage of myosin-binding sites is restored; contractionends, and muscle fiber relaxes.

7


Neural Control of Muscle Tension

• Contraction of a whole muscle is graded

– Which means that we can voluntarily alter the extent and strength of its contraction


• There are two basic mechanisms by which the nervous system produces graded contractions of whole muscles

– By varying the number of fibers that contract

– By varying the rate at which muscle fibers are stimulated


• In a vertebrate skeletal muscle

– Each branched muscle fiber is innervated by only one motor neuron

• Each motor neuron

– May synapse with multiple muscle fibers

Figure 49.34

Spinal cord

Nerve

Motor neuroncell body

Motorunit 1

Motorunit 2

Motor neuronaxon

Muscle

Tendon

Synaptic terminals

Muscle fibers


• A motor unit

– Consists of a single motor neuron and all the muscle fibers it controls

• Recruitment of multiple motor neurons

– Results in stronger contractions


• A twitch

– Results from a single action potential in a motor neuron

• More rapidly delivered action potentials

– Produce a graded contraction by summation

Figure 49.35

Actionpotential Pair of

actionpotentials

Series of action potentials at

high frequency

Time

Ten

sion

Singletwitch

Summation of two twitches

Tetanus


• Tetanus is a state of smooth and sustained contraction

– Produced when motor neurons deliver a volley of action potentials


Types of Muscle Fibers

• Skeletal muscle fibers are classified as slow oxidative, fast oxidative, and fast glycolytic

– Based on their contraction speed and major pathway for producing ATP


• Types of skeletal muscles


Other Types of Muscle

• Cardiac muscle, found only in the heart

– Consists of striated cells that are electrically connected by intercalated discs

– Can generate action potentials without neural input


• In smooth muscle, found mainly in the walls of hollow organs

– The contractions are relatively slow and may be initiated by the muscles themselves

• In addition, contractions may be caused by

– Stimulation from neurons in the autonomic nervous system


• Concept 49.7: Locomotion requires energy to overcome friction and gravity

• Movement is a hallmark of all animals

– And usually necessary for finding food or evading predators

• Locomotion

– Is active travel from place to place


Swimming

• Overcoming friction

– Is a major problem for swimmers

• Overcoming gravity is less of a problem for swimmers

– Than for animals that move on land or fly


Locomotion on Land

• Walking, running, hopping, or crawling on land

– Requires an animal to support itself and move against gravity


• Diverse adaptations for traveling on land

– Have evolved in various vertebrates

Figure 49.36


Flying

• Flight requires that wings develop enough lift

– To overcome the downward force of gravity


CONCLUSIONFor animals of a given

body mass, swimming is the most energy-efficient and running the least energy-efficient mode of locomotion. In any mode, a small animal expends more energy per kilogram of body mass than a large animal.

FlyingRunning

Swimming

10–3 103 1061

10–1

10

102

1

Body mass(g)

En

erg

y co

st (

J/K

g/m

)CONCLUSION

This graph compares the energy cost, in joules per kilogram of body mass per meter traveled, for animals specialized for running, flying, and swimming (1 J = 0.24 cal). Notice that both axes are plotted on logarithmic scales.

RESULTS

Physiologists typically determine an animal’s rate of energy use during locomotion by measuring its oxygen consumption or carbon dioxide production while it swims in a water flume, runs on a treadmill, or flies in a wind tunnel. For example, the trained parakeet shown below is wearing a plastic face mask connected to a tube that collects the air the bird exhales as it flies.

EXPERIMENT

•The energy cost of locomotion

–Depends on the mode of locomotion and the environment

Figure 49.37

Comparing Costs of Locomotion

concept 49.5: animal skeletons function in support, protection, and movement

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