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LECTURE NOTES
Contraction of Skeletal Muscle
Part 1: Review of Skeletal Muscle Anatomy
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Skeletal muscle is an example of muscle tissue, one of the four types of basic tissue
The essential characteristic of muscle tissue is that it shortens or contracts
There are three kinds of muscle tissue
1. Skeletal
2. Cardiac
3. Smooth
Special Characteristics of Muscle Tissue
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Excitability (irritability)- ability to respond to stimulus
Contractility- ability to shorten
Extensibility- ability to stretch
Elasticity- ability to resume cell’s original length once stretched
Characteristics of Skeletal Muscles
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Skeletal muscle is striated and voluntary
The word striated means “striped”
Skeletal muscle is the only type pf muscle that we can consciously control through our nervous
system
This is the reason it is also voluntary
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Muscle Functions
1. Produces movement
2. Aid in maintaining posture
3. Stabilize joints by exerting tension around the joint
4. Generate heat
Skeletal Muscle
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Each muscle cell has a nerve and blood supply
o This allows neural control
o Ensures adequate nutrient delivery and waste removal
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Skeletal muscle cells are also long and cylindrical
o For this reason a skeletal muscle cell can also be referred to as a skeletal muscle fiber
o A skeletal muscle fiber can be up to a foot long
o These long, highly specialized cells result from the fusion of many cells and as the cells
fuse their individual nuclei are retained. As a result, skeletal muscles are multinucleate.
 Connective Tissue Components
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Skeletal muscles as organs consist of muscle fibers bound by connective tissue
Connective tissue attaches skeletal muscle to the skeleton and other tissues and
transmits the force of a contraction to the moving part
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Connective tissue binds skeletal muscle fibers in a hierarchical pattern
o Endomysium (endo- below or within)
 Surrounds individual muscle fibers or cell
 Contains tiny capillaries & individual neurons providing
nutrients & nerve innervation of the fiber
o Fascicles
 Fibers aligned in bundles are called fascicles
 Package of muscle fibers
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Perimysium (peri-perimeter)
o Fascicles are in turn surrounded by a stronger sheath of connective
tissue called the perimysium
o Contains lots of blood vessels and nerves
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Epimysium (epi-on top)
o Fascicles are finally packaged in yet a stronger connective tissue called
epimysium
o The epimysium surrounds the entire muscle organ
Tendons & aponeurosis
o At the attachment points of the muscle all connective tissue elements
combine to form the connective tissue attachment of the muscle to
bone or tissue.
o If this attachment is round and cord-like, it is called a tendon
o If the attachment is broad and sheet-like it is called an aponeurosis
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Microscopic Anatomy of a Skeletal Muscle Fiber
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Skeletal muscle fibers are long cylindrical cells with multiple nuclei beneath the sarcolemma
The muscle fiber contains long cylindrical structures, the myofibrils
Myofibrils
o Almost entirely fill the cell and push the nuclei to the outer edges of the cell under the
sarcolemma
o 80% of cellular volume
o Contractile elements of the muscle cell
o Have light and dark bands
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Aligned with one another so that the light and dark bands are next to one another
This gives the cell its striated appearance
Consists of repeating units called sarcomeres
Overlapping myofilaments connected to Z discs at either end of the sarcomere
Sarcomere
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Functional unit of a muscle
Consists of a number of individual protein elements. Some of these proteins are thread-like
proteins called myofilaments
Two major types of myofilaments
o Actin
o Myosin
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Ultrastructure & Molecular Composition of the Myofilaments
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Thick filaments are composed of bundles of myosin molecules which have a head joined to a tail
by a flexible hinge region
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Thin filaments are composed of strands of f-actin, each actin-f filament is composed of g-actin
subunits
o Tropmyosin and troponin are regulatory proteins present in filaments
o They have the binding sites to which the heads of the myosin attach
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Thick myofilaments are made up of protein molecules called mysosin
o Myosin molecules are shaped like golf clubs with long shafts
o Myosin forms the thick myofilaments by forming the heads of the “golf clubs “. They
stick out at either end of the filament and the shafts form a “bare zone”
o Heads form attachments with the actin myofilaments
o These attachments are called cross bridges
o The heads are also the places that use the ATP molecule to power the muscle
contraction
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The Z Disc
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Composed of the protein alpha actinin
Connected to Z discs of adjacent myofibrils by intermediate filaments composed of desmin
Titin
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Anchors the thick filaments to Z discs and runs within the thick filaments of the M line
Helps muscle spring back into shape after contraction or stretching
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Striations of Skeletal Muscle
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Striations are due to repeating series of dark A bands (anisotropic, polarize visible light) and
light I bands (isotropic, don’t polarize visible light)
In the middle of the I bands there is a line called the Z line (or disc)
In the middle of the A bands (or dark bands) there is a light zone called the H zone
In the middle of the H zone there is another line, the M line
The precise arrangement of these features is due to a chain of functional units in the myofibrils,
the sarcomeres
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BANDING PATTERN AND THE SARCOMERE
Sarcomere
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An individual sarcomere extends from one Z line to the next
A bands
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This is where the thick myofilaments are positioned
These are the dark bands.
Thick and thin filaments overlap
I bands
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Corresponds to a region that overlap two adjacent sarcomeres where there are only thin
myofilaments
These are light bands
Only thin filaments are present
A band
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Is where the thick myofilaments are positioned
H Zone
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Region in the middle of the sarcomere where the thin filaments fail to overlap the thick
myofilaments
Only myosin present
M Line
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In the center of the sarcomere and A band is where proteins hold the thick myofilaments in
place
(A pneumonic to remember them is: Hotels-Motels In Alabang Are Busy. Many Customers Zapping
Excitement)
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Watch: https://www.youtube.com/watch?v=FfFBoIkdDgQ
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Sarcoplasmic Reticulum
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Smooth endoplasmic reticulum surrounding each myofibril
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T-Tubules (Transverse Tubules)
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Infoldings of the sarcolemma that conduct electrical impulses from the surface of the cell to the
terminal cisternae
Terminal cisternae are enlarged areas of the sarcoplasmic reticulum surrounding the
transverse tubules.