Chapter 6 The Muscular System Biology 112 Tri-County Technical College Pendleton, SC.
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Transcript of Chapter 6 The Muscular System Biology 112 Tri-County Technical College Pendleton, SC.
![Page 1: Chapter 6 The Muscular System Biology 112 Tri-County Technical College Pendleton, SC.](https://reader036.fdocuments.net/reader036/viewer/2022062802/56649e9d5503460f94b9d923/html5/thumbnails/1.jpg)
Chapter 6 The Muscular System
Biology 112
Tri-County Technical College
Pendleton, SC
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Functions of Muscles
• Essential function of muscle is CONTRACTION (shortening)
• Separates it from other body tissue
• Responsible for essentially all body movement
• Can be viewed as “machines” of body
• Makes up nearly half of the body’s mass
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Types of Muscle• Skeletal, Cardiac, and Smooth
• Share certain characteristics– Muscle cells are elongated and are called
MUSCLE FIBERS– Ability to shorten depends on TWO types of
MYOFILAMENTS• Muscle cells =“microfilaments of cytoskeleton”
• “Myo,” “Mys,” and “Sarco” refer to muscle
• Sarcoplasm-cytoplasm of muscle cells
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Types of Muscles, visual
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Skeletal Muscle
• Skeletal muscle fibers packed into organs called skeletal muscles that attach to skeleton
• Known as striated muscle = fibers appear striped
• Only muscle subject to CONSCIOUS control
• Each fiber enclosed in CT sheath called ENDOMYSIUM
• Several sheathed muscle fibers wrapped by membrane called PERIMYSIUM to form bundle of fibers called a FASCICLE
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Skeletal Muscle, cont.
• Many fascicles bound by EPIMYSIUM which covers entire muscle
• EPIMYSIS blend into strong, cordlike TENDONS (sheetlike aponeuroses) which attach muscles indirectly to bone, cartilages, or connective tissue covering of each other
• Spend some time on 6.1; page 164
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Skeletal Muscle, visual
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Sarcomere & Myofibrils
• Plasma membrane of muscle cell called SARCOLEMMA
• Long ribbonlike organelles called MYOFIBRILS nearly fill cytoplasm
• Alternating light (I) and dark (A) bands along length of myofibril = striped appearance
• Light I band has midline interruption; a darker area called the Z DISC (line)
• Dark A band has lighter central area called the H zone
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Sarcomere, cont.
• Myofibrils are chains of tiny contractile units called SARCOMERES which are aligned end to end like boxcars in train
• IT is arrangement of smaller MYOFILAMENTS within sarcomere that actually produces banding pattern
• Spend some quality time on Figure 6.3; page 167
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Myofilament Arrangement
• Each sarcomere contains two types of myofilaments• Large thick filaments (MYOSIN) extend entire length
of dark A band– Midparts of thick filaments are smooth but ends are studded
with small projections (myosin heads or cross bridges)
• Thin filaments (ACTIN) anchored to Z line which is actually disclike membrane
• Light I band is area that includes parts of two adjacent sarcomeres and contains ONLY thin filaments
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Myofilament Arrangement, cont.
• Thin filaments overlap ends of thick filaments but DO NOT extend into middle of relaxed sarcomere
• Thus central region (H zone) looks bit lighter
• Contraction occurs, actin containing filaments slide toward each other into center of sarcomere and light zones disappear– Actin and myosin filaments completely overlap
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Sarcomere Contraction• Fibers activated by NS, cross bridges on myosin attach
to myosin binding sites of actin filaments• Attaches and detaches several times during contraction
and PULLS thin filaments toward center of sarcomere• Event occurs simultaneously in sarcomeres throughout
cell, muscle cell shortens• Z lines move closer together• H zone disappears• A bands move closer together but do NOT change in
length• Millions of sarcomeres in millions of fibers =
contraction of entire skeletal muscle
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Contraction Visual
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Connective Tissue Wrappings
• Skeletal muscle wrapped by connective tissue
• ENDOMYSIUM wraps each individual muscle fiber (cell)
• PERIMYSIUM wraps bundles of fibers into a FASCICLE
• EPIMYSIUM covers the entire muscle– Epimysium is continuous with tendons or
aponeuroses
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Smooth Muscle• Has NO striations and is involuntary• Walls of hollow visceral organs like stomach, urinary
bladder, digestive tract, bronchi, uterus, and blood vessels
• Visceral-nonstriated-involuntary• Spindled-shaped fibers with single nucleus and
arranged in sheets/layers• Contractions SLOW and SUSTAINED-does Not tire
easily• Movement of food through digestive tract, emptying
bowels and bladder, & maintenance of blood pressure
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Smooth Muscle, Visual
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In a heartbeat…so to speak
• Cardiac muscle best described as CARDIAC, STRIATED, AND INVOUNTARY
• Branching cells joined by special junctions called intercalated disks
• Cardiac muscle arranged in spiral shape• Allows contractions to be closely
coordianted
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Nerve Aspects
• Each muscle fiber must be stimulated separately by nerve impulses to contract
• MOTOR UNIT-one motor neuron (nerve cell) and all the skeletal muscle cells it stimulates
• Threadlike extensions of neuron (axon/nerve fiber) branch into number of axonal terminals at muscle– Each axonal terminal forms junctions with
sarcolema of different muscle cell
• Junctions called neuromuscular junctions
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Motor Unit, Visual
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Neuromuscular Junction, Visual
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Nerve Aspects, cont.
• Nerve endings and muscle cells’ membranes NEVER TOUCH– Gap between them called synaptic cleft and is
filled with interstitial fluid
• Nerve impulse reaches axonal terminals, a neurotransmitter is released– Specific neurotransmitter that simulates muscle
cells is ACETYLCHOLINE (Ach)
• Acetylcholine diffuses across synaptic cleft
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Nerve Aspects, cont.
• Acetylcholine attaches to receptors on sarcolemma– If enough released, sarcolemma becomes temporary
permeable to sodium ions (Na+) which rush into muscle cell
• Generates electrical current called action potential
• AP travels over entire surface of sarcolemma conducting impulse from one end of cell to the other
• Result is CONTRACTION of the cell
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Action Potential, Visual
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Sliding Filament Theory
• Nerve impulseneuromuscular junction acetylcholine releasedAP in sarcolemma
• AP in sarcolemma causes sarcoplasmic reticulum to releases stored calcium ions into sarcoplasm
• Calcium ions cause cross-bridges to from • Thin myofilaments (actin) pulled over thick
(myosin) myofilaments– Energy provided by ATP
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SF Theory, cont.
• Sacromere contracts
• AP ends, calcium ions reabsorbed
• Cross-bridges turn loose & sarcomere relaxes
• Neurotransmitter acetylcholine degraded by enzymes in synaptic cleft
• Prevents continuous stimulation of muscle fiber
• Acetylcholinesterase (care for some Raid, anyone?)
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Contraction Mechanism, Visual
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To twitch or not to twitch
• Muscle fiber contracts in all-or-none fashion• Whole muscles do NOT contract that way• Skeletal muscles are organs composed of
1000s of muscle cells which react to stimuli with GRADED RESPONSES (different degrees of shortening)
• Graded muscle contractions produced in 2 ways– Changing speed of muscle stimulation– Changing number of muscle cells being stimulated
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Twitching time, cont.
• Muscle twitch is single, brief, jerky contraction that occurs as result of certain nervous system problems– NOT the way muscle normally operates– Single stimulus-contraction-relaxation
sequence in muscle fiber– DOES NOT accomplish anything useful in
skeletal muscle
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Twitch and more, cont.
• Incomplete tetanus results when nerve impulses delivered to muscle at very high rate
• Delivered so rapidly cells do not get chance to relax completely between stimuli
• Stimulation continues and muscle never allowed to relax completely will cause tension to peak
• Muscle producing peak tension during rapid cycles of contraction/relaxation said to be in incomplete tetanus
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Enough on twitching already
• Complete tetanus results when muscle is stimulated so rapidly that NO evidence of relaxation is seen
• Contractions are completely smooth and sustained• Complete tetanus major role = smooth and prolonged
muscle contractions• Force of muscle contraction depends on how many
of its cells are stimulated• Few cells = contraction as whole is slight• All cells = muscle contraction as strong as it can be
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Fatigue and Debt
• Muscle subject to continual contraction for long time = muscle fatigue occurs
• Muscle is fatigued when unable to contract even though still be stimulated
• Without rest, active/working muscle begins to tire and contracts more weakly until finally ceases reacting and stops contracting
• MF believed to result from oxygen debt that occurs during prolonged muscle activity
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Fatigue & Debt, cont.
• Work muscle can do and how long it can work w/o becoming fatigued depend on how good its blood supply is
• If muscle runs out of O2, it must depend on glycolysis for ATP and converts pyruvic acid to lactic acid
• Lack of adequate ATP and >acidity cause muscle to contract less effectively and finally to stop contracting all together
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Regeneration of ATP, Visual
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Isotonic Contractions
• Isotonic (same tone/tension) contractions most familiar
• Myofilaments are successful in sliding movements, muscle shortens, and movement occurs
• Bending knee, rotating arms, and smiling are examples of isotonic contractions
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Isometric Contractions
• Isometric (same measurement/length) contractions are contractions in which muscles do NOT shorten
• Mysoin myofilaments skidding their “wheels” and tension in muscle keeps increasing
• Trying to slide but muscle pitted against some more/less immovable object(s)
• Trying to lift 400 lb dresser along or pushing against immovable wall
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Muscle tone
• When muscle voluntarily relaxed, some of its fibers are contracting (one group then another)
• As result, muscle remains firm, healthy, and ready for action
• This state of continuous partial contractions is called MUSCLE TONE
• Is the result of different motor units scattered through muscle being stimulated by nervous system in systematic way
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Muscle tone, cont.
• If nerve supply to muscle is destroyed, muscle NO longer stimulated in this manner
• It loses TONE, and becomes paralyzed
• Soon after, becomes flaccid (soft/flabby) and begins to atrophy (waste away)