211 ch 9 muscle pt a
Chapter 9
Muscles and Muscle Tissue: Part A
Muscle Tissue
• Three types
– Skeletal
– Cardiac
– Smooth
• prefixes for muscle:
• Skeletal muscles
– Organs attached to bones and skin
– Elongated cells called :
– Striated:
– Voluntary (i.e., conscious control)
– Contract rapidly; tire easily; powerful
– Require nervous system stimulation
• Cardiac muscle
– Only in heart; bulk of heart walls
– Striated
– Can contract without nervous system stimulation
– Involuntary
– More details in Chapter 18
• Smooth muscle
– In walls of hollow organs, e.g., stomach, urinary bladder, and airways
– Not :
– Involuntary
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211 ch 9 muscle pt a
Special Characteristics of Muscle Tissue
• Excitability (responsiveness or irritability): ability to receive and
respond to stimuli
• Contractility: ability to shorten forcibly when stimulated
• Extensibility: ability to be stretched
• Elasticity:
Muscle Functions
• Four important functions
– Movement of bones or fluids (e.g., blood)
– Maintaining posture and body position
– Stabilizing:
– Heat generation (especially:
)
• Additional functions
– Protects organs, forms valves, controls pupil size, causes
"goosebumps"
Skeletal Muscle
• Each muscle served by one artery, one nerve, and one or more
veins
– Enter/exit near central part and branch through connective tissue
sheaths
– Every skeletal muscle fiber supplied by nerve ending that controls its
activity
– Huge nutrient and oxygen need; generates large amount of waste
Skeletal Muscle
• Connective tissue sheaths of skeletal muscle
– Support cells; reinforce whole muscle
– External to internal
•
: dense irregular connective tissue
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211 ch 9 muscle pt a
surrounding entire muscle; may blend with fascia
•
: fibrous connective tissue surrounding
fascicles :
: fine areolar connective tissue
surrounding each muscle fiber
Skeletal Muscle: Attachments
• Attach in at least two places
– Insertion :
– Origin :
• Attachments direct or indirect
– Direct—epimysium fused to periosteum of bone or perichondrium of
cartilage
– Indirect—connective tissue wrappings extend beyond muscle as
ropelike tendon or sheetlike aponeurosis
Microscopic Anatomy of A Skeletal Muscle Fiber
• Long, cylindrical cell
– 10 to 100 µm in diameter; up to 30 cm long
•
•
•
•
Multiple peripheral nuclei
Sarcolemma = plasma membrane
Sarcoplasm = cytoplasm
Modified structures: myofibrils, sarcoplasmic reticulum, and
T tubules
Myofibrils
• Densely packed, rodlike (cylindrical) elements
• ~80% of cell volume
• Contain sarcomeres - contractile units
– Sarcomeres contain myofilaments
• Exhibit striations - perfectly aligned repeating series of dark A
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bands and light I bands
Striations
• H zone: lighter region in midsection of dark A band where
filaments do not overlap
• M line: line of protein myomesin bisects H zone
• Z disc (line): coin-shaped sheet of proteins on midline of light I
band that anchors thin filaments and connects myofibrils to one
another
• Thick filaments: run entire length of an A band
• Thin filaments: run length of I band and partway into A band
• Sarcomere: region between two successive Z discs
Sarcomere
• Smallest contractile unit (functional unit) of muscle fiber
• Align along myofibril like boxcars of train
• Contains A band with ½ I band at each end
• Composed of thick and thin myofilaments made of contractile
proteins
Myofibril Banding Pattern
• Orderly arrangement of actin and myosin myofilaments within
sarcomere
– Actin myofilaments = thin filaments
• Extend across I band and partway in A band
• Anchored to Z discs
– Myosin myofilaments = thick filaments
• Extend length of A band
• Connected at M line
Ultrastructure of Thick Filament
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211 ch 9 muscle pt a
• Composed of protein myosin
• Each composed of 2 heavy and four light polypeptide chains
– Myosin tails contain 2 interwoven, heavy polypeptide chains
– Myosin heads contain 2 smaller, light polypeptide chains that act as
cross bridges during contraction
• Binding sites for actin of thin filaments
• Binding sites for ATP
• ATPase enzymes
Ultrastructure of Thin Filament
• Twisted double strand of fibrous protein
F actin
• F actin consists of G (globular) actin subunits
• G actin bears active sites for myosin head attachment during
contraction
• Tropomyosin and troponin - regulatory proteins bound to actin
Structure of Myofibril
• Elastic filament
– Composed of protein titin
– Holds thick filaments in place; helps recoil after stretch; resists
excessive stretching
Sarcoplasmic Reticulum (SR)
• Network of smooth endoplasmic reticulum surrounding each
myofibril
– Most run longitudinally
• Pairs of terminal cisternae form perpendicular cross channels
• Functions in regulation of intracellular Ca2+ levels
– Stores and releases Ca2+
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211 ch 9 muscle pt a
T Tubules
•
•
•
•
•
Continuations of sarcolemma into muscle fiber
Lumen continuous with extracellular space
Increase muscle fiber's surface area
Penetrate cell's interior at each A band–I band junction
Associate with paired terminal cisterns to form triads that
encircle each sarcomere
Triad Relationships
• T tubules conduct impulses deep into muscle fiber; every
sarcomere
• Integral proteins protrude into intermembrane space from T
tubule and SR cistern membranes–act as voltage sensors
• SR foot proteins: gated channels that regulate Ca2+ release
from SR cisterns
Sliding Filament Model of Contraction
• Generation of force
• Does not necessarily cause shortening of fiber
• Shortening occurs when tension generated by cross bridges on
thin filaments exceeds forces opposing shortening
• In relaxed state, thin and thick filaments overlap only at ends of
A band
• Sliding filament model of contraction
– During contraction, thin filaments slide past thick filaments  actin and
myosin overlap more
– Occurs when myosin heads bind to actin  cross bridges
• Myosin heads bind to actin; sliding begins
• Cross bridges form and break several times, ratcheting thin
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211 ch 9 muscle pt a
filaments toward center of sarcomere
– Causes shortening of muscle fiber
– Pulls Z discs toward M line
• I bands shorten; Z discs closer; H zones disappear; A bands move closer
(length stays same)
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