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CHAPTER 20

COORDINATION

20.2 The Mechanism of

Muscular Contraction

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PREVIOUS LESSON

20.1 Nervous System

(a)Describe the organization of the nervous system.

(b)Explain the general role of the sympathetic and

parasympathetic nervous system.

(c) Explain the generation of action potential,

transmission and characteristic of nerve impulse

along an axon.

(d)Describe the structure of synapse and explain the

mechanism of synaptic transmission across synapses.(e)Compare the transmission of impulse at the synapse

and along the axon.

(f) Explain the mechanism of action of drugs on the

nervous system.

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PREVIOUS LECTURE

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OBJECTIVES

(a) Describe the structure of neuromuscular junction

and explain impulse transmission at the junction

(b) Describe the structure of sarcomere

(c) Describe the mechanism of muscular contraction

based on sliding filament theory

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PRIOR KNOWLEDGE :

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Neuromuscular Junction

A specialized form of

synapse

Found between the

nerve terminals of a

motor neuron and the

muscle fibres

STRUCTURE

Objective:

(a)Describe the structure of neuromuscular junction and explain impulse transmission at thejunction

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Motor end-plate A region on each muscle fibre, where the axon of the

motor neuron divides and form non-myelinated branches

on the membrane cell surface

Postsynaptic membrane

The muscle cell membrane

Has many deep folds called junctional foldsSarcolemma

Objective:


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Objective:


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Objective:


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Objective:


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Skeletal Muscle

STRUCTURE

Muscle fiber

Myofibrils

Actin(thin filament)

Myosin

(thick filament)

Sarcoplasm

Sarcoplasmicreticulum

cytoplasm

Internal membranes

Objective:


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Structure of Skeletal Muscle

Sarcolemma

Muscle cell membrane

Myofibrils

Thread-like strands within muscle fibers

Actin (thin filament)

Troponin

Tropomyosin

Myosin (thick filament)

Accessory proteins

Objective:


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Transverse tubules (T tubules)

Invagination of sarcolemma

Extend transversely and surround each myofibril at A and

I bands

Sarcoplasmic reticulum

Made up of smooth ER

Longitudinal interconnected tubules between the

myofibrils


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Terminal cisternae

Enlargement of the sarcoplasmic reticulum adjacent to

the T tubule

Store calcium ions

Terminal

cisternae

Objective:


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http://c/Documents%20and%20Settings/kmpk/Desktop/coordination/D:/10.Coordination/Animation/0010-swf_action_potenti.swf

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Impulse Transmission at the

Neuromuscular Junction

An action potential arrives at the junction

Ca2+ ions enter the synaptic knob

Acetylcholine is released from the synaptic vesicles

Acetylcholine diffuses across the synaptic cleft

Depolarized the postsynaptic membrane of muscle cells

Objective:

(a)Describe the structure of neuromuscular junction and explain impulse transmission at theunction

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The action potential produced is

propagated along the sarcolema

A wave of depolarization spreads to the T tubules and

the terminal cisternae

Causing the Ca2+ channels to open

Ca2+ ions diffuse from the sarcoplasmic

reticulum into the sarcoplasm

Triggers muscle contractionObjective:


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Objective:

(b) Describe the structure of sarcomere
http://c/Documents%20and%20Settings/kmpk/Desktop/coordination/D:/10.Coordination/Animation/strufast.swf

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The Sarcomere

The functional unit of the

muscle

Consist of thick and thin

filaments between the 2 Z

lines

Form alternating dark andlight bands

Objective:(b) Describe the structure of sarcomere

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The A band

Darkest band

Overlapping thick and

thin filaments Central dark line M line

Contains proteins to hold

the thick filaments


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The H band Dark region

A portion of thick filamentonly

No overlapping


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The I band Light region

Thin filaments only

Has a Z line

Contains protein

Attachment site for thethin filaments

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The distance between

2 Z lines represents 1

sarcomere


S f S

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Structure of Sarcomere

[contracted and relaxed stated]


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A Skeletal Muscle Fiber


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Mechanism of Muscle

Contraction

Consist of myosin protein molecules

Lie parallel to each other

Structure:

Head

2 globular structures

Extend away from the body of the myosin filament At rest (not contracting),

ATP bound to the myosin

Thick filamentMYOSIN

Objective:(c) Describe the mechanism of muscular contraction based on sliding filament theory

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Myosin is an adenosine triphosphatase (ATPase)

An enzyme that splits ATP forming ADP and

inorganic phosphate(Pi)

Attach to specific sites on actinOperate like a hook

Tail

Joins together with other myosin tails

Forming the body of the thick filament

Cross Bridges


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Structure:

Consist of 2 helical strands of actin molecules intertwined

with 2 accessory proteins

Tropomyosin

Long thin strand

Fits in the groove of the actin strands

Covers the binding sites of myosin heads

Troponin

Positioned at regular intervals along tropomyosin

Has Ca2+ ion binding sites

Thin filamentACTIN


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M h f M l

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Mechanism of Muscle

Contraction

The sliding filament theory

Muscle contracts when the thin and thick filaments slide

past each other

Formation of cross-bridges

Reduction in the distance between Z-lines of the

sarcomere


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Muscle Contraction


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At rest, The tropomyosin blocks the myosin binding sites

ATP is bound to the myosin

Low energy configuration

When stimulated by the nerve impulse,

The wave of depolarization spreads over the muscle

Passes from the sarcolemma into the T tubules

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When the impulse reaches the vesicles of thesarcoplasmic reticulum,

Ca2+ ions are released into the sarcoplasm

Ca2+ ions concentration rises

Ca2+ ions bind to the troponin,

Change its conformation

Tropomyosin-troponin complexmove to a newposition on each actin molecule

Exposing the myosin-binding sites

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ATP is hydrolysed to ADP +

Pi

Energy released is transferred

to the myosin head

Changes shape to a high-

energy configuration

Myosin heads attach to the

actin filament,

Forming cross bridge

C B id F ti i

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Cross-Bridge Formation in

Muscle Contraction

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ADP + Piare released

Myosin head returns to its low-energy

configuration

The myosin head bends The actin filament is pulled towards

the center of the sarcomere

ADP + Pi

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Myosin head binds with ATP

Low-energy configuration

Detaches from actin

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MuscleCon

tractio

n

Th Slidi Fil t M d l f

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The Sliding Filament Model of

Muscle Contraction

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Summary of Muscle
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Summary of Muscle

ContractionMotor neuron releases acetylcholine

Acetylcholine combines with receptors on muscle fiber

Depolarization of sarcolemma

Action potential spreads through T tubules

Ca2+ released from sarcoplasmic reticulum

Ca

2+

binds to troponin, causing conformational change

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Troponin pushes tropomyosin away,

exposing the myosin head binding site

ATP attached to the myosin head is hydrolized

Myosin head binds to exposed active site on actin filament

forming cross bridge

ADP and Pi released from myosin head

Myosin head bend and actin filament pulled

towards center of sarcomere

Myosin head binds ATP and detaches from actin

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NEXT LECTURE