CHAPTER 6 PHYSIO PPT.pdf

CHAPTER

5

Cellular Movement and Muscles

PowerPoint® Lecture Slides prepared by Stephen Gehnrich, Salisbury University

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Cytoskeleton and Motor Proteins  All physiological processes depend on movement  Intracellular transport  Changes in cell shape  Cell motility  Animal locomotion

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Cytoskeleton and Motor Proteins  All movement is due to the same cellular “machinery”  Cytoskeleton  Protein-based intracellular network

 Motor proteins  Enzymes that use energy from ATP to move

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Use of Cytoskeleton for Movement  Cytoskeleton elements  Microtubules  Microfilaments

 Three ways to use the cytoskeleton for movement  Cytoskeleton “road” and motor protein carriers  To reorganize the cytoskeletal network  Motor proteins pull on the cytoskeletal “rope” Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Figure 5.1

Cytoskeleton and Motor Protein Diversity  Structural and functional diversity  Multiple isoforms of cytoskeletal and motor proteins  Various ways of organizing cytoskeletal elements  Alteration of cytoskeletal and motor protein function

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Microtubules

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Microtubules  Are tubelike polymers of the protein tubulin  Similar protein in diverse animal groups  Multiple isoforms

 Are anchored at both ends  Microtubule-organization center (MTOC) (–) near the nucleus  Attached to integral proteins (+) in the plasma membrane

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Microtubules

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Figure 5.2

Function of Microtubules  Motor proteins can transport subcellular components along microtubules  Motor proteins kinesin and dynein  For example, rapid change in skin color

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Movement of Pigment Granules

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Figure 5.3

Microtubules: Composition and Formation  Microtubules are polymers of the protein tubulin  Tubulin is a dimer of a-tubulin and b-tubulin  Tubulin forms spontaneously  For example, does not require an enzyme

 Polarity  The two ends of the microtubule are different  Minus (–) end  Plus (+) end

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Microtubule Assembly      

Activation of tubulin monomers by GTP Monomers join to form tubulin dimer Dimers form a single-stranded protofilament Many protofilaments form a sheet Sheet rolls up to form a tubule Dimers can be added or removed from the ends of the tubule  Asymmetrical growth  Growth is faster at + end

 Cell regulates rates of growth and shrinkage Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Microtubule Assembly

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Figure 5.4

Microtubule Growth and Shrinkage  Factors affecting growth/shrinkage are  Local concentrations of tubulin  High [tubulin] promotes growth

 Dynamic instability  GTP hydrolysis on b-tubulin causes disassembly

 Microtubule-associated proteins (MAPs)  Temperature  Low temperature causes disassembly

 Chemicals that disrupt the dynamics  For example, plant poisons such as taxol and colchicine

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Microtubule Dynamics

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Figure 5.5

Regulation by MAPs

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Figure 5.6

Pacific yew tree

Taxol

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Movement Along Microtubules  Motor proteins move along microtubules  Direction is determined by polarity and the type of motor protein  Kinesin move in (+) direction  Dynein moves in (–) direction

 Movement is fueled by hydrolysis of ATP  Rate of movement is determined by the ATPase domain of motor protein and regulatory proteins  Dynein is larger than kinesin and moves five times faster Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Vesicle Traffic in a Neuron

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Figure 5.7

Cilia and Flagella  Cilia  Numerous, wavelike motion

 Flagella  Single or in pairs, whiplike movement

 Composed of microtubules arranged into axoneme  Bundle of parallel microtubules  Nine pairs of microtubules around a central pair  “Nine-plus-two”

 Asymmetric activation of dynein causes movement

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Cilia and Flagella

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Figure 5.8

Cilia and Flagella

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Figure 5.8

Microtubules and Physiology

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Table 5.1

Microfilaments

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Microfilaments    

Polymers composed of the protein actin Found in all eukaryotic cells Often use the motor protein myosin Movement arises from  Actin polymerization  Sliding filaments using myosin

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Microfilament Structure and Growth  G-actin monomers polymerize to form a polymer called F-actin  Spontaneous growth  6–10 times faster at + end

 Treadmilling  Assembly and disassembly occur simultaneously and overall length is constant

 Capping proteins  Increase length by stabilizing – end and slowing disassembly

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Microfilament Structure and Growth

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Figure 5.9

Microfilament (Actin) Arrangement  Arrangement of microfilaments in the cell  Tangled neworks  Microfilaments linked by filamin protein

 Bundles  Cross-linked by fascin protein

 Networks and bundles of microfilaments are attached to cell membrane by dystrophin protein  Maintain cell shape  Can be used for movement

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Microfilament (Actin) Arrangement

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Figure 5.10

Movement by Actin Polymerization  Two types of amoeboid movement  Filapodia are rodlike extensions of cell membrane  Neural connections  Microvilli of digestive epithelia

 Lamellapodia are sheetlike extensions of cell membrane  Leukocytes  Macrophages

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Movement by Actin Polymerization  Filapodia

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Movement by Actin Polymerization  Filapodia

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Movement by Actin Polymerization  Lamellapodia

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Actin Polymerization and Fertilization

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Figure 5.11

Actin + Myosin = motor protein

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Myosin  Most actin-based movements involve the motor protein myosin  Sliding filament model

 Myosin is an ATPase  Converts energy from ATP to mechanical energy

 17 classes of myosin (I–XVII)  Multiple isoforms in each class  All isoforms have a similar structure  Head (ATPase activity)  Tail (can bind to subcellular components)  Neck (regulation of ATPase) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Myosin

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Figure 5.12

Sliding Filament Model  Analogous to pulling yourself along a rope  Actin – the rope  Myosin – your arm

 Alternating cycle of grasp, pull, and release  Your hand grasps the rope  Your muscle contracts to pull rope  Your hand releases, extends, and grabs further along the rope

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Sliding Filament Model  Two processes  Chemical reaction  Myosin binds to actin (cross-bridge)

 Structural change  Myosin bends (power stroke)

 Cross-bridge cycle  Formation of cross-bridge, power stroke, release, and extension

 Need ATP to release and reattach to actin  Absence of ATP causes rigor mortis  Myosin cannot release actin Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Sliding Filament Model

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Figure 5.13

Actino-Myosin Activity Two factors affect movement  Unitary displacement  Distance myosin steps during each cross-bridge cycle  Depends on  Myosin neck length  Location of binding sites on actin  Helical structure of actin

 Duty cycle  Cross-bridge time/cross-bridge cycle time  Typically ~0.5

 Use of multiple myosin dimers to maintain contact Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Myosin Activity

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Figure 5.14

Actin and Myosin Function

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Table 5.2

Muscle Structure and Regulation of Contraction

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Muscle Cells (Myocytes)  Myocytes (muscle cells)  Contractile cell unique to animals

 Contractile elements within myocytes  Thick filaments  Polymers of myosin  ~300 myosin II hexamers

 Thin filaments  Polymers of a-actin  Ends capped by tropomodulin and CapZ to stabilize  Proteins troponin and tropomyosin on outer surface

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Thick and Thin Filaments

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Figure 5.15

Muscle Cells  Two main types of muscle cells are based on the arrangement of actin and myosin  Striated (striped appearance)  Skeletal and cardiac muscle  Actin and myosin arranged in parallel

 Smooth (do not appear striped)  Actin and myosin are not arranged in any particular way

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Striated and Smooth Muscle

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Figure 5.16

Striated Muscle Types

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Table 5.3

Striated Muscle Cell Structure  Thick and thin filaments arranged into sarcomeres  Repeated in parallel and in series  Side-by-side across myocyte  Causes striated appearance

 End-to-end along myocyte

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Sarcomeres  Structural features of sarcomeres  Z-disk  Forms border of each sarcomere  Thin filaments are attached to the Z-disk and extend from it towards the middle of the sarcomere

 A-band  Middle region of sarcomere occupied by thick filaments

 I-band  Located on either side of Z-disk  Occupied by thin filament

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Sarcomeres

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Figure 5.17

Sarcomeres  Each thick filament is surrounded by six thin filaments  Three-dimensional organization of thin and thick filaments is maintained by other proteins  Nebulin  Along length of thin filament

 Titin  Keeps thick filament centered in sarcomere  Attaches thick filament to Z-disk

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Three-Dimensional Structure of Sarcomere

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Figure 5.18

Muscle Actinomyosin Activity is Unique  Myosin II cannot drift away from actin  Structure of sarcomere

 Duty cycle of myosin II is 0.05 (not 0.5)  Each head is attached for a short time  Does not impede other myosins from pulling the thin filament

 Unitary displacement is short  Small amount of filament sliding with each movement of the myosin head

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Contractile Force  Contractile force depends on overlap of thick and thin filaments  More overlap allows for more force  Amount of overlap depends on sarcomere length as measured by distance between Z-disks

 Maximal force occurs at optimal length  Decreased force is generated at shorter or longer lengths

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Length–Force Relationship

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Figure 5.19

Myofibril  In muscle cells, sarcomeres are arranged into myofibrils  Single, linear continuous stretch of interconnected sarcomeres (i.e., in series)  Extends the length of the muscle cell  Have parallel arrangement in the cell  More myofibrils in parallel can generate more force

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Myofibrils in Muscle Cells

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Figure 5.20

Contraction and Relaxation in Vertebrate Striated Muscle

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Regulation of Contraction Excitation-contraction coupling (EC coupling)  Depolarization of the muscle plasma membrane (sarcolemma)  Elevation of intracellular Ca2+  Contraction  Sliding filaments

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Ca2+ Allows Myosin to Bind to Actin  At rest, cytoplasmic [Ca2+] is low  Troponin-tropomyosin cover myosin binding sites on actin

 As cytoplasmic [Ca2+] increases  Ca2+ binds to TnC (calcium binding site on troponin)  Troponin-tropomyosin moves, exposing myosinbinding site on actin  Myosin binds to actin and cross-bridge cycle begins  Cycles continue as long as Ca2+ is present  Cell relaxes when the sarcolemma repolarizes and intracellular Ca2+ returns to resting levels Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Troponin and Tropomyosin

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Figure 5.21

Regulation of Contraction by Ca2+

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Figure 5.22

Ionic Events in Muscle Contraction

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Figure 5.23

Troponin–Tropomyosin Isoforms  Properties of isoforms affect contraction  For example, fTnC has a higher affinity for Ca2+ than s/cTnC  Muscle cells with the fTnC isoform respond to smaller increases in cytoplasmic [Ca2+]

 Isoforms differ in the affect of temperature and pH

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Myosin Isoforms  Properties of isoforms affect contraction  Multiple isoforms of myosin II in muscle  Isoforms can change over time

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Table 5.4

Excitation and EC coupling in Vertebrate Striated Muscle

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Excitation of Vertebrate Striated Muscle  Skeletal muscle and cardiac muscle differ in mechanism of excitation and EC coupling  Differences include  Initial cause of depolarization  Time course of the change in membrane potential (action potential)  Propagation of the action potential along the sarcolemma  Cellular origins of Ca2+

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Action Potentials  APs along sarcolemma signal contraction  Na+ enters cell when Na+ channels open  Depolarization

 Voltage-gated Ca2+ channel open  Increase in cytoplasmic [Ca2+]

 Na+ channels close  K+ leave cell when K+ channels open  Repolarization

 Reestablishment of ion gradients by Na+/K+ ATPase and Ca2+ ATPase

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Time Course of Depolarization

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Figure 5.24

Initial Cause of Depolarization  Myogenic (“beginning in the muscle”)  Spontaneous  For example, vertebrate heart

 Pacemaker cells  Cells that depolarize fastest  Unstable resting membrane potential

 Neurogenic (“beginning in the nerve”)  Excited by neurotransmitters from motor nerves  For example, vertebrate skeletal muscle

 Can have multiple (tonic) or single (twitch) innervation sites Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Neurogenic Muscle

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Figure 5.25

T-Tubules and Sarcoplasmic Reticulum  Transverse tubules (T-tubules)  Invaginations of sarcolemma  Enhance penetration of action potential into myocyte  More developed in larger, faster twitching muscles  Less developed in cardiac muscle

 Sarcoplasmic reticulum (SR)  Stores Ca2+ bound to protein sequestrin  Terminal cisternae increase storage

 T-tubules and terminal cisternae are adjacent to one another Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

T-Tubules and SR

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Figure 5.28

Ca2+ Channels and Transporters  Channels allow Ca2+ to enter cytoplasm  Ca2+ channels in cell membrane  Dihydropyridine receptor (DHPR)

 Ca2+ channels in the SR membrane  Ryanodine receptor (RyR)

 Transporters remove Ca2+ from cytoplasm  Ca2+ transporters in cell membrane  Ca2+ ATPase  Na+/Ca2+ exchanger (NaCaX)

 Ca2+ transporters in SR membrane  Ca2+ ATPase (SERCA) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

p229

Ca2+ Channels and Transporters

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Figure 5.27

Induction of Ca2+ Release From SR  AP along sarcolemma conducted down T-tubules  Depolarization opens DHPR  Ca2+ enters cell from extracellular fluid  In heart,  [Ca2+] causes RyR to open, allowing release of Ca2+ from SR  “Ca2+ induced Ca2+ release”  In skeletal muscle, change in DHPR shape causes RyR to open, allowing release of Ca2+ from SR  “Depolarization induced Ca2+ release”

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Ca2+ Induced Ca2+ Release

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Figure 5.29

Depolarization Induced Ca2+ Release

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Figure 5.30

Relaxation  Repolarization of sarcolemma  Remove Ca2+ from cytoplasm  Ca2+ ATPase in sarcolemma and SR  Na+/Ca2+ exchanger (NaCaX) in sarcolemma  Parvalbumin  Cytosolic Ca2+ binding protein buffers Ca2+

 Ca2+ dissociates from troponin  Tropomyosin blocks myosin binding sites  Myosin can no longer bind to actin

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Relaxation

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Figure 5.27

Summary of Striated Muscles

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Table 5.5

Smooth Muscle

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Smooth Muscle  Slow, prolonged contractions  Often found in the wall of “tubes” in the body  Blood vessels, intestine, airway, etc.

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Smooth Muscle  Key differences from skeletal muscle  No sarcomeres (no striations)  Thick and thin filaments are scattered in the cell  Attached to cell membrane at adhesion plaques

 No T-tubules and minimal SR  Often connected by gap junctions  Function as a single unit

 Different mechanism of EC coupling

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Smooth Muscle

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Figure 5.31

Control of Smooth Muscle Contraction  Regulated by nerves, hormones, and physical conditions (e.g., stretch)  At rest, the protein caldesmon is bound to actin and blocks myosin binding  Smooth muscle does not have troponin

 Stimulation of cell increases intracellular Ca2+  Ca2+ binds to calmodulin  Calmodulin binds caldesmon and removes it from actin  Cross-bridges form and contraction occurs  Calmodulin also causes phosphorylation of myosin  Increase in myosin ATPase activity

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Control of Smooth Muscle Contraction

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Figure 5.32

Muscle Diversity in Vertebrates and Invertebrates

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p235

Diversity of Muscle Fibers  Different protein isoforms affect EC coupling  Ion channels  Ion pumps  Ca2+-binding proteins  Speed of myosin ATPase

 Variation in other properties of muscle cells  Myoglobin content  Number of mitochondria

 Skeletal muscle cells can be classified as “fast,” “slow,” “white,” “red,” “oxidative,” “glycolytic” Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Changing Fiber Types  Developmental (from embryo to adult)  Increased proportion of fast muscle isoforms

 Physiological response  For example, exercise  Can change both cardiac and skeletal muscle

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Changing Fiber Types  Changes due to hormonal and nonhormonal mechanisms  For example, thyroid hormones repress expression of b-myosin II gene and induce a-myosin II gene  a-myosin II exhibits the fastest actino-myosin ATPase rates

 For example, direct stimulation of cell can alter gene expression

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Nonhormonal Mechanisms

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Figure 5.33

Trans-Differentiation of Muscle Cells  Trans-differentiation  Cells used for novel functions  For example, heater organs of billfish eye  Specialized muscle cells  Few myofibrils (little actin and myosin)  Abundant SR and mitochondria  Futile cycle of Ca2+ in and out of the SR  High rate of ATP synthesis and consumption

 Electric organs

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Heater Organ

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Figure 5.34

Invertebrate Muscles  Variation in contraction force due to graded excitatory postsynaptic potentials (EPSP)  Innervation by multiple neurons  EPSPs can summate to give stronger contraction  Some nerve signals can be inhibitory Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Figure 5.35

Asynchronous Insect Flight Muscles Wing beats: 250–1000 Hz  Fastest vertebrate contraction ≈ 100 Hz (toadfish)

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Asynchronous Insect Flight Muscles Asynchronous muscle contractions  Contraction is not synchronized to nerve stimulation  Stretch-activation  Sensitivity of the myofibril to Ca2+ changes during contraction/relaxation cycle  Intracellular [Ca2+] remains high  Contracted muscle is Ca2+ insensitive  Muscle relaxes  Stretched muscle is Ca2+ sensitive  Muscle contracts

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Asynchronous Insect Flight Muscles

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Figure 5.36

Mollusc (Bivalve) Catch Muscle  Muscle that holds shell closed  Capable of long duration contractions with little energy consumption  Protein twitchin may stablilize actin-myosin crossbridges  Cross-bridges do not continue to cycle  Phosphorylation/dephophorylation of twitchin regulates its function

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Mollusc (Bivalve) Catch Muscle

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Figure 5.37