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muscle tissue:
definition
3 types
prefixes
nearly half of body's mass; Transforms chemical energy (ATP) to directed mechanical energy --> exerts force
- 3 types:
- skeletal; cardiac; smooth
- prefixes for muscle tissue and muscle cells:
- myo; mys; sarco
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skeletal muscles
- organs attached to bones and skin
- elongated cells called muscle fibers (single cell within muscle tissue)
- striated
- vountary
- contract rapidly; tire easily; powerful
- require nervous system stimulation for contraction
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cardiac muscle
- only in heart; bulk of heart walls
- striated
- con contract without nervous system stimulation
- involuntary
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smooth muscle
- in walls of hollow organs (stomach, urinary bladder, airways)
- not striated
- con contract without nervous system stimulation
- involuntary
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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: ability to recoil to resting length
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4 important muscle functions + additional functions
- 1) movement of bones or fluids (blood)
- 2) maintaining posture and body position
- 3) stabilizing joints
- 4) heat generation (especially skeletal muscles)
- additional functions= protects organs, forms valves, controls pupil size, causes goosebumps
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skeletal muscle anatomy
- each muscle is 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)
- connective tissue sheaths contain support cells and reinforce the skeletal muscle
- From external to internal:
- Epimysium = dense irregular connective tissue surrounding entire muscle; may blend with fascia (sheet of fibrous connective tissue surrounding muscle)
- Perimysium = fibrous connective tissue surrounding fascicles (groups of muscle fibers)
- Endomysium = fine areolar connective tissue surrounding each muscle fiber (muscle cell)
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skeletal muscle: attachments
- attach in at least 2 places:
- -insertion = movable bone
- -origin = immovable (less movable) bone
- 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 (biceps brachii muscle) or sheet-like aponeurosis (palmaris longus muscle)
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myofibrils
- densely packed, rodlike elements of skeletal and cardiac muscle cells
- ~80% of cell volume
- contain sarcomeres - contractile units (sarcomeres contain myofilaments)
- exhibit striations - perfectly aligned repeating series of: dark A bands and light I bands
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striations
(need to see diagram)
- 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 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
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sarcomere
(need to see diagram)
- smallest contractile unit (functional unit) of muscle fiber
- align along myofibril like boxcars of a train
- contains A band with 1/2 I band at each end
- composed of thick and thin myofilaments made of contractile proteins
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myofibril banding pattern
(see diagram)
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
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ultrastructure of thick filament
- composed of protein myosin
- each composed of 2 heavy and 4 light polypeptide chains
- -- myosin tails contain 2 interwoven, heavy long 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)
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ultrastructure of thin filament
- twisted double strand of fibrous protein F actin
- F actin (polypeptide chain) consists of G (globular) actin subunits
- G actin (monomer) bears active sites for myosin head attachment during contraction
- Tropomyosin and Troponin are regulatory proteins bound to actin
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structure of myofibril (fyi)
- elastic filament: composed of protein titin; holds thick filaments in place; helps recoil after stretch; resists excessive stretching
- dystophin: links thin filaments to proteins of sarcolemma
- nebulin, myomesin, C proteins bind filaments of sarcomeres together; maintain alignment
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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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T tubules
- continuations of sarcolemma
- 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
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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
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sliding filament model of contraction
(see diagram)
- in relaxed state, thin and thick filaments overlap only at ends of A band
- myosin heads bind to actin; sliding begins
- cross bridges form and break several times, ratcheting thin 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)
- 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
- 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
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physiology of skeletal muscle fibers: for skeletal muscle to contract
- Activation (at neuromuscular junction)
- must be nervous system stimulation -- voluntary
- must generate action potential in sarcolemma
- Excitation-contraction coupling
- action potential propogated along sarcolemma
- intracellular Ca2+ levels must rise briefly
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neuromuscular junction (NMJ)
- situated midway along length of muscle fiber
- axon terminal and muscle fiber separated by gel-filled space called synaptic cleft
- synaptic vesicles of axon terminal contain neurotransmitter acetylcholine (ACh)
- junctional folds of sarcolemma contain ACh receptors
- NMJ includes axon terminals, synaptic cleft, junctional folds
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the nerve stimulus
- skeletal muscles are stimulated by somatic motor neurons
- axons of motor neurons travel from central nervous system via nerves to skeletal muscle
- each axon (nerve fiber) forms several branches as it enters muscle
- each axon ending forms neuromuscular junction with single muscle fiber
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events at neuromuscular junction - 6 steps
- 1) AP arrives at axon terminal
- 2) voltage-gated Ca2+ channels in axon terminal open. Ca2+ moves down electochemical gradient
- 3)Ca2+ entry causes ACh (a neurotransmitter) to be released by exocytosis
- 4) ACh diffuses across synaptic cleft to muscle -- binds to its receptors on the sarcolemma
- 5) ACh binding to ion channels opens them (allows simultaneous passage of Na+ into muscle fiber and K+ out of muscle fiber. More Na+ ions enter than K+ ions exit, which produces local change in membrane potential called the end plate potential)
- 6) Extracellular ACh is broken down (ACh effects are terminated by its breakdown in the synaptic cleft by acetylcholinesterase and diffusion away from junction)
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