Download Skeletal Muscle Tissue

10 PART 1
Skeletal Muscle Tissue
Muscle
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Muscle—a Latin word for “little mouse”
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Muscle is the primary tissue in the:
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Heart (cardiac muscle tissue)
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Walls of hollow organs (smooth muscle tissue)
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Skeletal muscle
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Makes up nearly half the body’s mass
Properties of Muscle Tissue
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Contractility
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Myofilaments are responsible for shortening of muscles cells
• Actin and myosin are two type of myofilaments
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Excitability
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Nerve signals excite muscle cells, causing electrical impulses to travel along the
sarcolemma
Properties of Muscle Tissue
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Extensibility
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Contraction of a skeletal muscle stretches the opposing muscle
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Smooth muscle is stretched by substances within that hollow organ
• Food in stomach; urine in urinary bladder
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Elasticity
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Recoils after being stretched
Terminology Specific to Muscle Tissue
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Myo and mys—prefixes meaning “muscle”
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Sarco—prefix meaning “flesh”
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Sarcolemma—plasma membrane of muscle cells
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Sarcoplasm—cytoplasm of muscle cells
Functions of Muscle Tissue
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Produce movement
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Skeletal muscle—attached to skeleton
• Moves body by moving the bones
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Smooth muscle—squeezes fluids and other substances through hollow organs
Functions of Muscle Tissue (continued)
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Open and close body passageways
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Sphincters function as valves
• Open to allow passage of a substance
• Contract to close the passageway
Functions of Muscle Tissue (continued)
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Maintain posture and stabilize joints
Enables the body to remain sitting or standing
Muscle tone helps stabilize many synovial joints
Heat generation
Muscle contractions produce heat
Helps maintain normal body temperature
Types of Muscle Tissue
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Skeletal muscle tissue
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Packaged into skeletal muscles
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Makes up 40% of body weight
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Cells are striated
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Skeletal muscle is innervated by voluntary division of the nervous system
Types of Muscle Tissue
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Cardiac muscle tissue
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Occurs only in the walls of the heart
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Cells are striated
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Contraction is involuntary
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Smooth muscle tissue
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Occupies the walls of hollow organs
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Cells lack striations
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Innervated by involuntary division of the nervous system
Skeletal Muscle
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Each muscle is an organ
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Consists mostly of muscle tissue
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Skeletal muscle also contains:
• Connective tissue
• Blood vessels
• Nerves
Gross Anatomy of a Skeletal Muscle
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Connective tissue and fascicles
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Sheaths of connective tissue bind a skeletal muscle and its fibers together
• Epimysium—dense regular connective tissue surrounding entire muscle
• Perimysium—surrounds each fascicle
(group of muscle fibers)
• Endomysium—a fine sheath of connective tissue wrapping each muscle cell
Gross Anatomy of a Skeletal Muscle
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Connective tissue sheaths are continuous with tendons
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When muscle fibers contract, pull is exerted on all layers of connective tissue are
tendon
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Sheaths provide elasticity and carry blood vessels and nerves
Gross Anatomy of a Skeletal Muscle
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Nerves and blood vessels
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Each skeletal muscle supplied by branches of
• One nerve
• One artery
• One or more veins
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Nerves and vessels branch repeatedly
• Smallest branches serve individual muscle fibers
Gross Anatomy of a Skeletal Muscle
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Muscle attachments
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Most skeletal muscles run from one bone to another
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One bone will move; other bone remains fixed
• Origin—less movable attachment
• Insertion—more movable attachment
Gross Anatomy of a Skeletal Muscle
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Muscle attachments (continued)
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Muscles attach to origins and insertions by connective tissue (CT)
• Fleshy attachments—CT fibers are short
• Indirect attachments—CT forms a tendon or aponeurosis
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Bone markings present where tendons meet bones
• Tubercles, trochanters, and crests
Microscopic and Functional Anatomy of Skeletal Muscle Tissue
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The skeletal muscle fiber
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Fibers are long and cylindrical
• Are huge cells—diameter is 10–100 µm
• Length—several centimeters to dozens of centimeters
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Each cell formed by fusion of embryonic cells
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Cells are multinucleate
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Nuclei are peripherally located
Myofibrils and Sarcomeres
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Striations result from internal structure of myofibrils
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Myofibrils
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Are long rods within cytoplasm
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Make up 80% of the cytoplasm
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Are a specialized contractile organelle found in muscle tissue
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Are a long row of repeating segments called sarcomeres (functional unit of skeletal
muscle tissue)
Myofibrils and Sarcomeres
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Basic unit of contraction of skeletal muscle
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Z disc (Z line)—boundaries of each sarcomere
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Thin (actin) filaments—extend from Z disc toward the center of the sarcomere
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Thick (myosin) filaments—located in the center of the sarcomere
• Overlap inner ends of the thin filaments
• Contain ATPase enzymes
Myofibrils and Sarcomeres
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A bands—full length of the thick filament
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Includes inner end of thin filaments
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H zone—center part of A band where no thin filaments occur
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A bands and I bands refract polarized light differently
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A bands—anisotropic
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I bands—isotropic
Myofibrils and Sarcomeres
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M line—in center of H zone
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Contains tiny rods that hold thick filaments together
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I band—region with only thin filaments
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Lies within two adjacent sarcomeres
Titin and Other Myofibrils
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Titin is a springlike molecule that resists overstretching
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Titin molecules extend from the Z disc to thick filaments to the M line
• Two functions
• Holds thick filaments in place
• Unfolds when muscle is stretched
Sarcoplasmic Reticulum and T Tubules
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Sarcoplasmic reticulum
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A specialized smooth ER
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Interconnecting tubules surround each myofibril
• Some tubules form cross-channels called terminal cisterns
• Cisterns occur in pairs on either side of a
t tubule
Sarcoplasmic Reticulum and T Tubules
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Sarcoplasmic reticulum
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Contains calcium ions—released when muscle is stimulated to contract
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Calcium ions diffuse through cytoplasm
• Trigger the sliding filament mechanism
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T tubules—deep invaginations of sarcolemma
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Triad—T tubule flanked by two terminal cisterns
Mechanism of Contraction
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Two major types of contraction
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Concentric contraction—muscle shortens to do work
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Eccentric contraction—muscle generates force as it lengthens
• Muscle acts as a “brake” to resist gravity
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“Down” portion of a pushup is an example
Mechanism of Contraction
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Sliding filament mechanism
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Explains concentric contraction
• Myosin head attach to thin filaments at both ends of a sarcomere
• Then pull thin filaments toward the center of the sarcomere
• Thin and thick filaments do not shorten
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Initiated by release of calcium ions from the sarcoplasmic reticulum
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Powered by ATP
Sliding Filament Mechanism
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Contraction changes the striation pattern
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Fully relaxed—thin filaments partially overlap thin filaments
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Contraction—Z discs move closer together
• Sarcomere shortens
• I bands shorten; H zone disappears
• A band remains the same length
Microscopic and Functional Anatomy
of Skeletal Muscle Tissue
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Muscle extension
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Muscle is stretched by a movement opposite that which contracts it
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Muscle fiber length and force of contraction
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Greatest force produced when a fiber starts out slightly stretched
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Myosin heads can pull along the entire length of the thin filaments
The Role of Titin
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Titin—a springlike molecule in sarcomeres
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Resists overstretching
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Holds thick filaments in place
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Unfolds when muscle is stretched
Innervation of Skeletal Muscle
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Motor neurons innervate skeletal muscle tissue
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Neuromuscular junction is the point where nerve ending and muscle fiber meet
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Terminal boutons (axon terminals)
• Located at ends of axons
• Store neurotransmitters
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Synaptic cleft
• Space between axon terminal and sarcolemma
10 PART 2
Skeletal Muscle Tissue
Types of Skeletal Muscle Fibers
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Skeletal muscle fibers are categorized according to two characteristics
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How they manufacture energy (ATP)
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How quickly they contract
Oxidative fibers—produce ATP aerobically
Glycolytic fibers—produce ATP anaerobically by glycolysis
Types of Skeletal Muscle Fibers
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Skeletal muscle fibers
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Are divided into three classes
• Slow oxidative fibers
• Red slow oxidative fibers
• Fast glycolytic fibers
• White fast glycolytic fibers
• Fast oxidative fibers
• Intermediate fibers
Types of Skeletal Muscle Fibers
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Slow oxidative fibers
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Red color due to abundant myoglobin
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Obtain energy from aerobic metabolic reactions
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Contain a large number of mitochondria
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Richly supplied with capillaries
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Contract slowly and resistant to fatigue
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Fibers are small in diameter
Types of Skeletal Muscle Fibers
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Fast glycolytic fibers
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Contain little myoglobin and few mitochondria
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About twice the diameter of slow oxidative fibers
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Contain more myofilaments and generate more power
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Depend on anaerobic pathways
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Contract rapidly and tire quickly
Types of Skeletal Muscle Fibers
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Fast oxidative fibers
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Have an intermediate diameter
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Contract quickly like fast glycolytic fibers
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Are oxygen dependent
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Have high myoglobin content and rich supply of capillaries
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Somewhat fatigue resistant
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More powerful than slow oxidative fibers
Disorders of Muscle Tissue
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Muscle tissues experience few disorders
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Heart muscle is the exception
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Skeletal muscle
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Remarkably resistant to infection
Smooth muscle
• Problems stem from external irritants
Disorders of Muscle Tissue
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Muscular dystrophy
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A group of inherited muscle destroying disease
• Affected muscles enlarge with fat and connective tissue
• Muscles degenerate
• Types of muscular dystrophy
• Duchenne muscular dystrophy
• Myotonic dystrophy
Disorders of Muscle Tissue
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Myofascial pain syndrome
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Pain is caused by tightened bands of muscle fibers
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Fibromyalgia
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A mysterious chronic-pain syndrome
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Affects mostly women
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Symptoms—fatigue, sleep abnormalities, severe musculoskeletal pain, and headache
Muscle Tissue Throughout Life
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Muscle tissue develops from myoblasts
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Myoblasts fuse to form skeletal muscle fibers
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Skeletal muscles contract by week 7 of development
Muscle Tissue Throughout Life
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Cardiac muscle
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Pumps blood 3 weeks after fertilization
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Satellite cells
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Surround skeletal muscle fibers
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Resemble undifferentiated myoblasts
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Fuse into existing muscle fibers to help them grow
Muscle Tissue Throughout Life
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Differences in skeletal muscle between males and females
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Females—approximately 36% of body mass
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Males—approximately 42% of body mass
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Difference is due to androgens in males
Muscle Tissue Throughout Life
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With increased age
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Amount of connective tissue increases in muscles
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Number of muscle fibers decreases
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Loss of muscle mass with aging
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Decrease in muscular strength is 50% by age 80
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Sarcopenia—muscle wasting