Bio 6 a Slides Skeletal Muscle Con

7/27/2019 Bio 6 a Slides Skeletal Muscle Con

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Skeletal Muscle Contraction8th ed 50.5

7th ed 49.6


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In order to move all animals require muscle

activity in response to nervous system input.

Skeletal muscles responsible for voluntary

movement.


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Skeletal muscles are

attached to bones by

tendons and are

responsible for theirmovement.

Bicepscontracts

Tricepsrelaxes

Forearmflexes

Bicepsrelaxes

Tricepscontracts

Forearmextends

Tendons


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Organization of

skeletal muscles:

Have a hierarchy of

smaller and smaller

units

Bundles of long fibers

running parallel to the

length of the muscle

Each fiber is made of

longitudinally arranged

myofibrils

Bundle ofmuscle fibers

Single muscle fiber(cell)

Plasma membrane

Nuclei

Muscle

Myofibril

Dark band

Sarcomere

Z lineLightband


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Bundle of

muscle fibersSingle muscle fiber(cell)

Plasma membrane

Nuclei

Muscle

Myofibril

Dark band

Sarcomere

Z lineLightband

I band

TEM

A band I band

0.5 m

M line

Thick filaments(myosin)

SarcomereH zoneZ line

Thin filaments(actin)

Z line

Myofibrils are composed

ofthin and thick

filaments

Thin filaments are made

of two strands of

regulatory proteins and

two stands of actin

Thick filaments are made

of staggered myosin

molecules


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Skeletal muscles are striated arrangement offilaments create dark and light bands

Sarcomere

0.5 m

Z H

A I

Skeletal muscle tissue under a light

microscope

Skeletal muscle tissue under a electron

microscope


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Sarcomere repeating contractile unit of a muscle Thin filaments are attached at the Z line and proceed to the center of the

sarcomere

Thick filaments are attached at the M line in the center of the sarcomere

Area near edge of sarcomere with only thin filament is the I band

Region corresponding to the length of the thick filament is the A band Center of A band containing only thick filaments is called the H zone

At a relaxed state thin and thick filaments partially overlap

This overlapping arrangement is key to the contraction mechanism

This regular arrangement produces dark and light bands and hence makethe fibers look striated

Sarcomere

I band

TEM

A band I band0.5 m

M line

Thick filaments(myosin)

SarcomereH zoneZ line

Thin filaments(actin)

Z line


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Sliding-Filamentmodel of musclecontraction (focuson a singlesarcomere) During contractions

the thin and thickfilaments do notchange in length butincrease the overlap.

This shortens thelength of thesarcomere

LE 49-29

Sarcomere

0.5 m

ZH

A

Relaxed muscle fiber

I

Contracting muscle fiber

Fully contracted muscle fiber


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Myosin molecule (thick filament) has a globular

head and a long tail. Tail adheres to other tails.

Thin filaments are actin molecules along with

regulatory proteins

Thin filaments

Thick filament

Thin filament

Thickfilament

Myosin head


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Steps in muscle contraction:

Myosin head is bound to ATP in a low-

energy configuration

Thin filaments

Thick filament

Thin filament

Thickfilament

Myosin head (low-energyconfiguration)


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ATP is hydrolyzed to ADP and Pi and the

head is now in high-energy configuration

Thin filaments

Thick filament

Thin filament

Thickfilament


Cross-bridgebinding site

Myosin head (high-energy configuration)

Actin


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Head binds with

actin filament at

the myosin bindingsites; forms cross

bridge

Thin filaments

Thick filament

Thin filament

Thickfilament




Actin

Cross-bridge


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Thin filaments

Thick filament

Thin filament

Thickfilament




Actin

Cross-bridge

Myosin head (low-energy configuration)

Thin filament movestoward center of sacomere.

Head releases the ADP andPi and returns to low-energyconfiguration; Actin (thin)filament moves towards thecenter of the sarcomere


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Myosin binds to new ATP molecule andresumes the cycle

Thin filaments

Thick filament

Thin filament

Thickfilament

Myosin head (low-energy

configuration)


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Synapse: junction where oneneuron communicates withanother neuron or withmuscle/gland etc.

Synaptic terminal: A bulb atthe end of the axon in whichneurotransmitter moleculesare stored and released.

Synaptic cleft: narrow gapseparating synaptic knob of atransmitting neuron or itseffector cell.

Synaptic cleft

Synaptic terminal

of motor neuron


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Regulation of skeletal muscle contraction:

Synaptic terminal receives action potential and

releases neurotransmitter Acetylcholine (ACh) ACh binds to receptors in the muscle and triggers

action potential in the muscle fiber.

Action potential is propagated along the plasmamembrane and down the T-tubule.

PLASMA

MEMBRANET TUBULESynaptic cleft

Synaptic terminal

of motor neuron

ACh


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Ca2+

CYTOSOL

Ca2+

SR

PLASMA


Synaptic terminal

of motor neuron

ACh

Action potential

triggers Ca2+

release from

sarcoplasmic

reticulum


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Ca2+CYTOSOL

Myosin cross-bridges attach and detach, powered

by ATP pulling the actin filament towards center of

the sarcomere

When action potential ends Ca2+ is absorbed backinto the sarcoplasmic reticulum

Muscle contraction ends, muscle fibers relax


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Ca2+CYTOSOL

Ca2+

SR

PLASMA


Synaptic terminal

of motor neuron

ACh


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Ca+ and regulatory proteins and their role

in muscle fiber contraction

Actin filaments have regulatory proteins on them. Tropomyosin, trponin complex and Ca2+ regulate

muscle contraction

At rest tropomyosin covers the actin binding sites

preventing actin and myosin from interacting

Myosin-binding sites blocked.

Tropomyosin Ca2+-binding sites

Actin Troponin complex


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Myosin-binding sites exposed.

Myosin-binding site

Ca2+

When Ca2+ is released into the cytosol from the

sarcoplasmic reticulum it binds to troponin complex.

This changes the alignment of the troponin

That in turn shifts the position of the tropomyosin, exposingthe myosin binding sites on the actin filament

When Ca2+ concentration drops the binding sites are

covered and contraction stops.


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Myosin-binding sites blocked.

Myosin-binding sites exposed.

Tropomyosin Ca2+-binding sites

Actin Troponin complex

Myosin-binding site

Ca2+

Bio 6 a Slides Skeletal Muscle Con

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