CHAPTER 6 PHYSIO PPT.pdf

(1)

PowerPoint

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_{Lecture Slides prepared by}

Stephen Gehnrich, Salisbury University

5

C H A P T E R

Cellular Movement

and Muscles

(2)

Cytoskeleton and Motor Proteins



All physiological processes depend on movement



Intracellular transport



Changes in cell shape



Cell motility

(3)

Cytoskeleton and Motor Proteins



All movement is due to the same cellular

“machinery”



Cytoskeleton



Protein-based intracellular network



Motor proteins

(4)

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”

(5)

Cytoskeleton and Motor Protein Diversity



Structural and functional diversity



Multiple isoforms of cytoskeletal and motor proteins



Various ways of organizing cytoskeletal elements

(6)

(7)

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

(8)

(9)

Function of Microtubules



Motor proteins can transport subcellular

components along microtubules



Motor proteins kinesin and dynein

(10)

(11)

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

(12)

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

(13)

Microtubule Assembly

(14)

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

(15)

Microtubule Dynamics

(16)

(17)

Pacific yew tree

(18)

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

(19)

Vesicle Traffic in a Neuron

(20)

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”

(21)

Cilia and Flagella

(22)

(23)

Microtubules and Physiology

(24)

(25)

Microfilaments



Polymers composed of the protein actin



Found in all eukaryotic cells



Often use the motor protein myosin



Movement arises from



Actin polymerization

(26)

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

(27)

Microfilament Structure and Growth

(28)

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

(29)

Microfilament (Actin) Arrangement

(30)

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

(31)

Movement by Actin Polymerization

(32)

Movement by Actin Polymerization

(33)

Movement by Actin Polymerization

(34)

(35)

(36)

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)

(37)

Myosin

(38)

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

(39)

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

(40)

(41)

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

(42)

(43)

Actin and Myosin Function

(44)

Muscle Structure

(45)

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

(46)

(47)

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)

(48)

(49)

Striated Muscle Types

(50)

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

(51)

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

(52)

(53)

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

(54)

(55)

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

(56)

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

(57)

Length–Force Relationship

(58)

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

(59)

Myofibrils in Muscle Cells

(60)

Contraction and Relaxation

in Vertebrate Striated Muscle

(61)

Regulation of Contraction

Excitation-contraction coupling (EC coupling)



Depolarization of the muscle plasma membrane

(sarcolemma)



Elevation of intracellular Ca

2+



Contraction

(62)

Ca

2+

Allows Myosin to Bind to Actin



At rest, cytoplasmic [Ca

2+

] is low



Troponin-tropomyosin cover myosin binding sites on

actin



As cytoplasmic [Ca

2+

] increases



Ca

2+

binds to TnC (calcium binding site on troponin)



Troponin-tropomyosin moves, exposing

myosin-binding site on actin



Myosin binds to actin and cross-bridge cycle begins



Cycles continue as long as Ca

2+

is present

(63)

Troponin and Tropomyosin

(64)

(65)

Ionic Events in Muscle Contraction

(66)

Troponin–Tropomyosin Isoforms



Properties of isoforms affect contraction



For example, fTnC has a higher affinity for Ca

2+

than

s/cTnC



Muscle cells with the fTnC isoform respond to smaller

increases in cytoplasmic [Ca

2+

]

(67)

Myosin Isoforms



Properties of isoforms affect contraction



Multiple isoforms of myosin II in muscle



Isoforms can change over time

(68)

Excitation and EC coupling

in Vertebrate Striated Muscle

(69)

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

(70)

Action Potentials



APs along sarcolemma signal contraction



Na

+

enters cell when Na

+

channels open



Depolarization



Voltage-gated Ca

2+

channel open



Increase in cytoplasmic [Ca

2+

_]



Na

+

channels close



K

+

leave cell when K

+

channels open



Repolarization



Reestablishment of ion gradients by Na

+

/K

+

ATPase

and Ca

2+

ATPase

(71)

Time Course of Depolarization

(72)

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

(73)

Neurogenic Muscle

(74)

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 Ca

2+

bound to protein sequestrin



Terminal cisternae increase storage



T-tubules and terminal cisternae are adjacent to one

(75)

T-Tubules and SR

(76)

Ca

2+

Channels and Transporters



Channels allow Ca

2+

to enter cytoplasm



Ca

2+

channels in cell membrane



Dihydropyridine receptor (DHPR)



Ca

2+

channels in the SR membrane



Ryanodine receptor (RyR)



Transporters remove Ca

2+

from cytoplasm



Ca

2+

transporters in cell membrane



Ca

2+

_ATPase



Na

+

_/Ca

2+

_{exchanger (NaCaX)}

(77)

Ca

2+

Channels and Transporters

(78)

Induction of Ca

2+

Release From SR



AP along sarcolemma conducted down T-tubules



Depolarization opens DHPR



Ca

2+

enters cell from extracellular fluid



In heart,



[Ca

2+

_{] causes RyR to open, allowing release of}

Ca

2+

from SR



“Ca

2+

_{induced Ca}

2+

_release”



In skeletal muscle, change in DHPR shape causes RyR to

open, allowing release of Ca

2+

_{from SR}

(79)

Ca

2+

Induced Ca

2+

Release

(80)

(81)

Relaxation



Repolarization of sarcolemma



Remove Ca

2+

from cytoplasm



Ca

2+

ATPase in sarcolemma and SR



Na

+

/Ca

2+

exchanger (NaCaX) in sarcolemma



Parvalbumin



Cytosolic Ca

2+

_{binding protein buffers Ca}

2+



Ca

2+

dissociates from troponin



Tropomyosin blocks myosin binding sites

(82)

(83)

Summary of Striated Muscles

(84)

(85)

Smooth Muscle



Slow, prolonged contractions



Often found in the wall of “tubes” in the body

(86)

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

(87)

Smooth Muscle

(88)

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 Ca

2+



Ca

2+

binds to calmodulin



Calmodulin binds caldesmon and removes it from actin



Cross-bridges form and contraction occurs



Calmodulin also causes phosphorylation of myosin

(89)

Control of Smooth Muscle Contraction

(90)

Muscle Diversity

(91)

Diversity of Muscle Fibers



Different protein isoforms affect EC coupling



Ion channels



Ion pumps



Ca

2+

-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,”

(92)

Changing Fiber Types



Developmental (from embryo to adult)



Increased proportion of fast muscle isoforms



Physiological response



For example, exercise

(93)

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

(94)

(95)

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 Ca

2+

in and out of the SR



High rate of ATP synthesis and consumption

(96)

(97)

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

(98)

Asynchronous Insect Flight Muscles

Wing beats: 250–1000 Hz

(99)

Asynchronous Insect Flight Muscles

Asynchronous muscle contractions



Contraction is not synchronized to nerve stimulation



Stretch-activation



Sensitivity of the myofibril to Ca

2+

changes during

contraction/relaxation cycle



Intracellular [Ca

2+

] remains high



Contracted muscle is Ca

2+

insensitive



Muscle relaxes



Stretched muscle is Ca

2+

sensitive

(100)

(101)

Mollusc (Bivalve) Catch Muscle



Muscle that holds shell closed



Capable of long duration contractions with little

energy consumption



Protein twitchin may stablilize actin-myosin

cross-bridges



Cross-bridges do not continue to cycle



Phosphorylation/dephophorylation of twitchin regulates

its function

(102)