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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:
(a)Describe the structure of neuromuscular junction and explain impulse transmission at thejunction
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Objective:
(a)Describe the structure of neuromuscular junction and explain impulse transmission at thejunction
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Objective:
(a)Describe the structure of neuromuscular junction and explain impulse transmission at thejunction
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Objective:
(a)Describe the structure of neuromuscular junction and explain impulse transmission at thejunction
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Skeletal Muscle
STRUCTURE
Muscle fiber
Myofibrils
Actin(thin filament)
Myosin
(thick filament)
Sarcoplasm
Sarcoplasmicreticulum
cytoplasm
Internal membranes
Objective:
(a)Describe the structure of neuromuscular junction and explain impulse transmission at thejunction
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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:
(a)Describe the structure of neuromuscular junction and explain impulse transmission at thejunction
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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
Structure of Skeletal Muscle
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Terminal cisternae
Enlargement of the sarcoplasmic reticulum adjacent to
the T tubule
Store calcium ions
Terminal
cisternae
Objective:
(a)Describe the structure of neuromuscular junction and explain impulse transmission at thejunction
Structure of Skeletal Muscle
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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:
(a)Describe the structure of neuromuscular junction and explain impulse transmission at thejunction
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Objective:
(b) Describe the structure of sarcomere
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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
Objective:(b) Describe the structure of sarcomere
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The H band Dark region
A portion of thick filamentonly
No overlapping
Objective:(b) Describe the structure of sarcomere
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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
Objective:(b) Describe the structure of sarcomere
S f S
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Structure of Sarcomere
[contracted and relaxed stated]
Objective:(b) Describe the structure of sarcomere
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A Skeletal Muscle Fiber
Objective:(b) Describe the structure of sarcomere
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Objective:(b) Describe the structure of sarcomere
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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
Objective:(c) Describe the mechanism of muscular contraction based on sliding filament theory
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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
Objective:(c) Describe the mechanism of muscular contraction based on sliding filament theory
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Objective:(c) Describe the mechanism of muscular contraction based on sliding filament theory
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
Objective:(c) Describe the mechanism of muscular contraction based on sliding filament theory
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Muscle Contraction
Objective:(c) Describe the mechanism of muscular contraction based on sliding filament theory
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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