Download 9- muscle tissue

Chp 9 - Muscles and Muscle Tissue
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BIO 200
Muscle Tissue Overview
The three types of muscle tissue
1. Skeletal
2. Cardiac
3. Smooth
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These types differ in structure, location, function, and means of activation
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Muscles Similarities
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Skeletal and smooth muscle cells are elongated and are called muscle fibers
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Muscle contraction depends on two kinds of myofilaments – actin and myosin
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Muscle terminology is similar
1. Sarcolemma – muscle plasma membrane
2. Sarcoplasm – cytoplasm of a muscle cell
3. Prefixes – myo, mys, and sarco all refer to muscle
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Skeletal Muscle Tissue - Cover the bony skeleton; have obvious stripes called striations; are
controlled voluntarily (conscious control) and contract rapidly but tire easily.
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Skeletal Muscle Tissue -is responsible for overall body motility of the body; it is extremely adaptable;
can exert forces ranging from just a fraction of an ounce to over 70 pounds
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Cardiac Muscle Tissue - Found only in the heart; are Striated like skeletal muscle; Involuntary
muscles movement and contract at a steady rate set by the heart’s pacemaker.
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Cardiac Muscle Tissue – are Neural controls allow the heart to respond to changes in bodily needs
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Smooth Muscle Tissue - are found in the walls of hollow visceral organs, such (stomach, urinary
bladder, respiratory tract); forces food and other substances through internal body channels
(intestinal wall) and are not striated muscle tissue. They are Involuntary muscle movement
Functional Characteristics of Muscle Tissue
1. Excitability - responsiveness or irritability; mean that it has the ability to receive and respond
to stimuli
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Contractility – the ability to shorten forcibly when stimulated.
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Extensibility – the ability to stretched or extend
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Elasticity – the ability to recoil and resume its original resting length
Muscle Function
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Muscles maintain posture, stabilize joints, and generate heat and Provide strength. Skeletal muscles
are responsible for locomotion
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Cardiac muscle is responsible for pumping the blood through the body
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Smooth muscles help maintain blood pressure, and squeezes or propels substances (food, feces)
through organs
Skeletal Muscle
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Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and
connective tissue. These tissues support each cell, reinforce the muscle and prevent bulging muscles
from bursting. They includes three connective tissue sheaths:
1. Epimysium – outside the muscle - an overcoat of dense regular connective tissue that
surrounds the entire muscle
2. Perimysium – around the muscle - fibrous connective tissue that surrounds groups of muscle
fibers called fascicles
3. Endomysium – within the muscle - fine sheath of connective tissue composed of reticular
fibers surrounding each muscle fiber
Skeletal Muscle –
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Skeletal Muscle: Nerve and Blood Supply. Each muscle is served by one nerve, an artery, and one or
more veins. Each skeletal muscle fiber is supplied with a nerve ending that controls contraction
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Contracting fibers require continuous delivery of oxygen and nutrients via arteries. Wastes must be
removed via veins
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Skeletal Muscle: Attachments. Most skeletal muscles are attached to bone in at least two places
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When muscles contract the movable bone, (muscle’s insertion) moves toward the immovable bone,
the muscle’s origin.
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Muscles attach:
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Directly – epimysium of the muscle is fused to the periosteum of a bone
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Indirectly – connective tissue wrappings extend beyond the muscle as a tendon
Microscopic Anatomy of a Skeletal Muscle Fiber
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Each fiber is a long, cylindrical cell, multiple nuclei. The Fibers are 10 to 100 m in diameter, and up
to hundreds of centimeters long
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Sarcoplasm has numerous glycosomes and a unique oxygen-binding protein called myoglobin
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Fibers contain the usual organelles, myofibrils, sarcoplasmic reticulum, and T tubules
Myofibrils are densely packed, rodlike contractile elements. They make up most of the muscle volume
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The arrangement of myofibrils are perfectly aligned repeating series of dark A bands and light I bands
Sarcomeres - The smallest contractile unit of a muscle
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The region of a myofibril between two successive Z discs
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Composed of myofilaments made up of contractile proteins: two types – thick and thin
Myofilaments: Banding Pattern
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Thick filaments – extend the entire length of an A band
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Thin filaments – extend across the I band and partway into the A band. do not overlap thick filaments
in the lighter H zone
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Z-disc – sheet of proteins (connectins) that anchors the thin filaments and connects myofibrils to one
another
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Thin filaments M lines appear dark due to protein desmin
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Ultrastructure of Myofilaments: Thick Filaments
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Thick filaments are composed of protein myosin
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Each myosin molecule has a rod-like tail and two globular heads
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Tails – two interwoven polypeptide chains
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Heads – two smaller, light polypeptide chains called cross bridges
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Ultrastructure of Myofilaments: Thin Filaments
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Thin filaments are composed of the protein actin
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Each actin molecule is a helical polymer of globular subunits called G actin
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The subunits contain active sites to which myosin heads attach during contraction
Tropomyosin and troponin are regulatory subunits bound to actin
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Arrangement of the Filaments in a Sarcomere
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Longitudinal section within one sarcomere
Sarcoplasmic Reticulum (SR)
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SR is a smooth endoplasmic reticulum that runs longitudinally and surrounds each myofibril
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Paired terminal cisternae
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Functions in regulating intracellular calcium levels
T Tubules
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T tubules are continuous with the sarcolemma
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They conduct impulses to the deep regions of the muscle
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These impulses signal Ca2+ release from terminal cisternae
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T tubules and SR provide tightly linked signals for muscle contraction
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T tubule proteins act as voltage sensors
Skeletal Muscle Contraction - In order to contract, a skeletal muscle must:
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Be stimulated by a nerve ending
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Propagate an electrical current, or action potential, along its sarcolemma
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Have a rise in intracellular Ca2+ levels, the final trigger for contraction
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Excitation-contraction coupling link the electrical signal to the contraction
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Nerve Stimulus of Skeletal Muscle and the Skeletal muscles are stimulated by motor neurons from
the nervous system
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Axons of these neurons travel in nerves to muscle cells and branch out profusely as they enter
muscles
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Neuromuscular Junction:
Axon endings that have small membranous sacs (synaptic vesicles) contain the neurotransmitter
acetylcholine (ACh)
The motor end plate of a muscle is a specific part of the sarcolemma that contains ACh receptors and helps
form the neuromuscular junction
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When a nerve impulse reaches the end of an axon at the neuromuscular junction:
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Voltage-regulated calcium channels open and allow Ca2+ to enter the axon
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Ca2+ inside the axon terminal causes vesicles to fuse with the membrane
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This fusion releases ACh into the synaptic cleft via exocytosis
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ACh diffuses across the synaptic cleft to ACh receptors on the sarcolemma
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Binding of ACh to its receptors initiates action potential in the muscle
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Destruction of Acetylcholine
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ACh bound to ACh receptors is quickly destroyed by the enzyme acetylcholinesterase
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This destruction prevents continued muscle fiber contraction in the absence of additional stimuli
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A transient depolarization event and polarity reversal of a sarcolemma (nerve cell membrane) that is
conducted along the membrane of a muscle cell or nerve fiber
Role of Acetylcholine (Ach)
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Depolarization – a local electrical event that loses polarity; loss or reduction of negative membrane
potential
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ACh binds its receptors at the motor end plate
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Na+ and K+ diffuse out and the interior of the sarcolemma becomes less negative
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Ignites the action potential that spreads throughout the neuromuscular junction across the
sarcolemma
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Repolarization – restores the sarcolemma to its initial polarized state
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Action Potential: Polarized Sarcolemma
Depolarization: an electrical event called end plate potential
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It ignites an action potential that spreads in all directions across the sarcolemma
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The outside (extracellular) face is positive, while the inside face is negative
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This difference in charge is the resting membrane potential
Action Potential: Polarized Sarcolemma
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The predominant extracellular ion is Na+ and the predominant intracellular ion is K+
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The sarcolemma is relatively impermeable to both ions
Action Potential: Depolarization
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An axonal terminal of a motor neuron releases ACh and causes a patch of the sarcolemma to become
permeable to Na+ (sodium channels open)
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Na+ enters the cell, and the resting potential is decreased (depolarization occurs). If the stimulus is
strong enough, an action potential is initiated
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Polarity reversal of the initial patch of sarcolemma changes the permeability of the adjacent patch
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Voltage-regulated Na+ channels now open in the adjacent patch causing it to depolarize
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The action potential travels rapidly along the sarcolemma
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Once initiated, the action potential is unstoppable, and ultimately results in the contraction of a
muscle
Action Potential: Repolarization
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Immediately after the depolarization wave passes, the sarcolemma permeability changes
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Na+ channels close and K+ channels open
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K+ diffuses from the cell, restoring the electrical polarity of the sarcolemma
Action Potential: Repolarization
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Repolarization occurs in the same direction as depolarization, and must occur before the muscle can
be stimulated again (refractory period)
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The ionic concentration of the resting state is restored by the
Na+-K+ pump
Action Potential: Repolarization
Excitation-Contraction (EC) Coupling
1. Action potential generated and propagated along sarcomere to T-tubules. Neurotransmitters are
released and Diffuse across the synaptic cleft and Attaches to ACh receptors on the sarcolemma.
(Sodium (Na +) initiates an action potential that propagates along the sarcomere down to the Ttubules
Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
Axon terminal
Synaptic
cleft
Synaptic
vesicle
Sarcolemma
T tubule
1
ACh
ACh
ACh
Net entry of Na+ Initiates
an action potential which
is propagated along the
sarcolemma and down
the T tubules.
1. Na +initiates an action
potential that propagates along
the sarcomere down to the Ttubules
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 9.10
2. Action potential triggers Ca2+ release. Action potential triggers voltage-sensitive receptors that
trigger Calcium (Ca2+) release from the SR.
3. Ca++ bind to troponin; blocks release of tropomyosin. Ca+ ions bind totroponin; troponin changes
shape and blocks release of tropomyosin. Actin active sites are exposed.
4. Contraction occurs via crossbridge formation and release of energy by ATP hyrdolysis
5. Removal of Ca+ by active transport occurs
6. Tropomyosin blockage is restorred and contraction ends and the muscle fiber relaxes
Contraction of Skeletal Muscle Fibers
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Contraction – refers to the activation of myosin’s cross bridges
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Shortening occurs when the tension generated by the cross bridge exceeds forces opposing
shortening
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Contraction ends when cross bridges become inactive, the tension generated declines, and
relaxation is induced
Contraction of Skeletal Muscle (Organ Level)
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Contraction of muscle fibers (cells) and muscles (organs) is similar
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The two types of muscle contractions are:
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Isometric contraction – increasing muscle tension (muscle does not shorten during
contraction)
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Isotonic contraction – decreasing muscle length (muscle shortens during contraction)
The Nerve-Muscle Functional: Motor Unit
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A motor unit is a motor neuron and all the muscle fibers it supplies
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The number of muscle fibers per motor unit can vary from four to several hundred
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Muscles that control fine movements (fingers, eyes) have small motor units
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Large weight-bearing muscles have large motor units
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Homeostasis imbalance
Rigor mortis
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Muscles stiffen 3-4 hrs after death; peak at 2 hrs.
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Gradual dissipation over the next 40-60 hrs.
Dying cells cannot exlude calcium
Calcium in muscle cells promotes myosin cross bridges – detachment of cross bridge is unable to cease –
actin & myosin cross-link become irreversible.
Rigor mortis disappears when muscle proteins break down.
BIO 200 - Muscle Tone
Chp 9b
Muscle Twitch
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Motor unit responds to a single action potential
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Muscle fibers contract quickly & relax
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3 distinct phases seen myogram
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Latent period – muscle tension being to increase but not response is seen
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Period of contraction – activation of cross-bridge
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Period of relaxation – contraction is followed by relaxation because calcium re-enters into
the SR
Graded muscle responses
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Healthy muscle contractions are smooth; very in strength
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Graded muscle contractions:
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Change in the frequency of stimulation – wave summations: tetanus
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Change in the strength of the stimulation – as stimulus strength increases, the contraction
force increases
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The larger the motor unit, the stronger the contractile force
Muscle Tone
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A constant but slightly contracted state of all muscles - which do not produce active
movements
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Keeps the muscles firm, healthy, and ready to respond to stimulus
Spinal reflexes (CNS) account for muscle tone by:
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Activating one motor unit and then another
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Responding to activation of stretch receptors in muscles and tendons
The Nerve-Muscle Functional: Motor Unit
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A motor unit is a motor neuron and all the muscle fibers it supplies
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The number of muscle fibers per motor unit can vary from four to several hundred
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Muscles that control fine movements (fingers, eyes) have small motor units
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Large weight-bearing muscles have large motor units
Principles of muscle mechanics
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1. Muscle fibers in skeltal muscle are huge in numbers and consistantly the same
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2. Force exerted by a contracting muscle on an object - muscle tension
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3. The opposing force exerted on the muscle by the weight of the object to be moved is – load
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4. A contracting muscle does not always shorten – isometric
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5. If the muscle tension overcomes the load, and the muscle shortens - isotonic
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Skeletal muscle contracts with different forces and for different periods of time. However dependent
on the frequency and intensity of the stimuli. Each muscle is served by a motor nerve (axon) and
several hundreds of motor neurons. As the axon enters the muscle, it branches into terminals, each
forming a neuromuscular junction with a single muscle fiber
Isotonic Contractions
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In isotonic contractions, the muscle changes in length (decreasing the angle of the joint) and moves
the load
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The two types of isotonic contractions are concentric and eccentric
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Concentric contractions – the muscle shortens and does work
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Eccentric contractions – the muscle contracts as it lengthens
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Isometric Contractions
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Tension increases to the muscle’s capacity, but the muscle neither shortens nor lengthens
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Occurs if the load is greater than the tension the muscle is able to develop
Muscle Metabolism: Energy for Contraction
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ATP is the only source used directly for contractile activity. As soon as available stores of ATP are
hydrolyzed (4-6 seconds), they are regenerated by:
1. The interaction of ADP with creatinine phosphate
2. Anaerobic glycolysis
3. Aerobic respiration
Muscle Metabolism: Anaerobic Glycolysis
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When muscle contractile and activity reaches 70% of maximum:
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Bulging muscles compress blood vessels
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Oxygen delivery is impaired
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Pyruvic acid is converted into lactic acid
The lactic acid:
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Diffuses into the bloodstream and iIs picked up and used as fuel by the liver, kidneys, and
heart. IIs converted back into pyruvic acid by the liver
Muscle fatigue – the muscle is in a state of physiological inability to contract. Muscle fatigue occurs
when:
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ATP production fails to keep pace with ATP use
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There is a relative deficit of ATP, causing contractures (cramps)
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Lactic acid accumulates in the muscle
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Ionic imbalances are present
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Intense exercise produces rapid muscle fatigue
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Na+-K+ pumps cannot restore ionic balances quickly enough
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SR is damaged and Ca2+ regulation is disrupted
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Low-intensity exercise produces slow-developing fatigue
Oxygen Debt
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Vigorous exercise causes dramatic changes in muscle chemistry. For a muscle to return to a resting
state:
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Oxygen reserves must be replenished
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Lactic acid must be converted to pyruvic acid
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Glycogen stores must be replaced
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ATP and CP reserves must be resynthesized
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Extra amount of O2 is needed for the above processes to be restored
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Heat Production During Muscle Activity
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Only 40% of the energy released in muscle activity is useful as work. The remaining 60% is given off
as heat. Dangerous heat levels are prevented by radiation of heat from the skin and sweating
Force of Muscle Contraction - the force of contraction is affected by:
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The number of muscle fibers contracting – the more motor fibers in a muscle, the stronger
the contraction
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The size of the muscle – the bulkier the muscle, the greater its strength
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Degree of muscle stretch – muscles contract strongest when muscle fibers are 80-120% of
their normal resting length
Muscle Fiber Type: Functional Characteristics
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Speed of contraction – determined by speed in which ATPases split ATP
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The two types of fibers are slow and fast
ATP-forming pathways
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Oxidative fibers – use aerobic pathways
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Glycolytic fibers – use anaerobic glycolysis
Muscle Fiber Type: Speed of Contraction
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Slow oxidative fibers contract slowly, have slow acting myosin ATPases and are fatigue resistant
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Fast oxidative fibers contract quickly, have fast myosin ATPases, and have moderate resistance to
fatigue
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Fast glycolytic fibers contract quickly, have fast myosin ATPases, and are easily fatigued
Effects of Aerobic Exercise - Aerobic exercise results in an increase of:
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Muscle capillaries
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Number of mitochondria
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Myoglobin synthesis
Typically anaerobic exercises, results in:
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Muscle hypertrophy
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Increased mitochondria, myofilaments, and glycogen stores
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The Overload Principle
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Forcing a muscle to work promotes increased muscular strength but Muscles adapt to increased
demands. Muscles must be overloaded to produce further gains
Smooth Muscle
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Composed of spindle-shaped fibers, 2-10 m in diameter and lengths = several hundred m
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Lack the coarse connective tissue sheaths of skeletal muscle, but have fine endomysium
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Organized into two layers (longitudinal and circular) of closely apposed fibers
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Found in walls of hollow organs (except the heart)
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Essentially vave the same contractile mechanisms as skeletal muscle
Peristalsis - when the longitudinal layer contracts, the organ dilates and contracts and When the circular
layer contracts, the organ elongates. Peristalsis is an alternating contractions and relaxations of smooth
muscles that mix and squeeze substances through the lumen of hollow organs.
Innervation of Smooth Muscle
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Smooth muscle lacks neuromuscular junctions
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Innervating nerves have bulbous swellings called varicosities
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Varicosities release neurotransmitters into wide synaptic clefts called diffuse junctions
Microscopic Anatomy of Smooth Muscle
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SR is less developed than in skeletal muscle and lacks a specific pattern; T tubules are absent and the
Plasma membranes have pouchlike infoldings called caveoli. There are no visible striations and no
sarcomeres
Contraction of Smooth Muscle
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Whole sheets of smooth muscle exhibit slow, synchronized contraction and they contract in unison.
Action potentials are transmitted from cell to cell. Some smooth muscle cells:
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Act as pacemakers and set the contractile pace for whole sheets of muscle
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Are self-excitatory and depolarize without external stimuli
Some Special Features of Smooth Muscle Contraction are Unique and include:

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Smooth muscle tone; Slow, prolonged contractile activity; they require low energy and
response to stretch
Response to Stretch: smooth muscle exhibits a phenomenon called stress-relaxation response in
which:
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Responds to stretch only briefly, and then adapts to its new length
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The new length retains its ability to contract
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This enables organs such as the stomach and bladder to temporarily store contents
Hyperplasia
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Certain smooth muscles can divide and increase their numbers by undergoing hyperplasia. This is
shown by estrogen’s effect on the uterus
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At puberty, estrogen stimulates the synthesis of more smooth muscle, causing the uterus to
grow to adult size
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During pregnancy, estrogen stimulates uterine growth to accommodate the increasing size of
the growing fetus
Types of Smooth Muscle: Single Unit
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The cells of single-unit smooth muscle visceral muscle:
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Contract rhythmically as a unit; they are electrically coupled to one another via gap
junctions; they often exhibit spontaneous action potentials; and they are arranged in
opposing sheets and exhibit stress-relaxation response
Types of Smooth Muscle: Multiunit
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Multiunit smooth muscles are found:
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In large airways to the lungs; In large arteries; In arrector pili muscles’ Attached to hair
follicles; and in the internal eye muscles
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Their characteristics include: Rare gap junctions; Infrequent spontaneous depolarizations;
Structurally independent muscle fibers ; A rich nerve supply, which, with a number of muscle
fibers, forms motor units; and Graded contractions in response to neural stimuli
Muscular Dystrophy
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Muscular dystrophy – group of inherited muscle-destroying diseases where muscles enlarge due to
fat and connective tissue deposits, but muscle fibers atrophy
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Duchenne muscular dystrophy (DMD)
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Inherited, sex-linked disease carried by females and expressed in males (1/3500). Diagnoses
occaurs between the ages of 2-10. Victims become clumsy and fall frequently as their
muscles fail
Muscular Dystrophy is:
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Progresses from the extremities upward, and victims die of respiratory failure in their 20s
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Caused by a lack of the cytoplasmic protein dystrophin
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There is no cure, but myoblast transfer therapy shows promise
Developmental Aspects
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Muscle tissue develops from embryonic mesoderm called myoblasts
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Male and Female differ:
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greater strength in men than in women
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Women’s skeletal muscle - 36% of body mass
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Men’s skeletal muscle - 42% of body mass
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hormone testosterone
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Body strength per unit muscle mass is the same in both sexes
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Developmental Aspects: Age Related
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With age, connective tissue increases and muscle fibers decrease
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Muscles become stringier and more sinewy
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By age 80, 50% of muscle mass is lost (sarcopenia)
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Regular exercise reverses sarcopenia
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Aging of the cardiovascular system affects every organ in the body

Atherosclerosis may block distal arteries, leading to intermittent claudication and causing severe leg
muscle pain