Chpt.50.Motor.mechanisms & Skeletal Support.2014

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Biology Eighth Edition

Neil Campbell and Jane Reece

Chapter 50

Motor Mechanisms & Skeletal Support

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

Skeletal Muscle

• 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


Skeletal Muscle Fibers• A skeletal muscle consists of a bundle of long fibers

– Running parallel to the length of the muscle

• A muscle fiber

– 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

•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 myofilaments 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

(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-Filament Model


Skeletal Muscle Contraction

• 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


“Dark Meat or Light?”

• Dark or Red fibers have many mitochondria and more of the pigment myoglobin storing extra oxygen; slow contracting but better for endurance (e.g., breast muscles of migratory birds)

• Light or White fibers have fewer mitochondria, less myoglobin but contract more quickly and powerfully; downside: they fatigue more quickly


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


Other Types of Muscle• In smooth muscle, found mainly in the walls of hollow

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

• Contractions may also be caused by stimulation from neurons in the autonomic nervous system


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 (external hard parts), and endoskeletons (internal hard parts)


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


Exoskeletons

• An exoskeleton is a hard encasement

– Deposited on the surface of an animal

• Exoskeletons

– Are found in most molluscs and all 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 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

SternumRibHumerus

Vertebra

RadiusUlnaPelvicgirdleCarpals

Phalanges

Metacarpals

FemurPatella

Tibia

Fibula

TarsalsMetatarsalsPhalanges

1

Examplesof joints

23

Head ofhumerus

Scapula

Humerus

Ulna

UlnaRadius


Structural Organization of Bone

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Chpt.50.Motor.mechanisms & Skeletal Support.2014

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