Muscle Tissue

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Chapter 10

• Muscle Tissue
• Lecture Outline

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INTRODUCTION

• Motion results from alternating contraction
  (shortening) and relaxation of muscles the
  skeletal system provides leverage and a
  supportive framework for this movement.
• The scientific study of muscles is known as
  myology.

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Chapter 10Muscle Tissue

• Alternating contraction and relaxation of cells
• Chemical energy changed into mechanical energy

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OVERVIEW OF MUSCLE TISSUE

• Types of Muscle Tissue
• Skeletal muscle tissue is primarily attached to
  bones. It is striated and voluntary.
• Cardiac muscle tissue forms the wall of the
  heart. It is striated and involuntary.
• Smooth (visceral) muscle tissue is located in
  viscera. It is nonstraited (smooth) and
  involuntary.
• Table 4.4 compares the different types of muscle.

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3 Types of Muscle Tissue

• Skeletal muscle
• attaches to bone, skin or fascia
• striated with light dark bands visible with
  scope
• voluntary control of contraction relaxation

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3 Types of Muscle Tissue

• Cardiac muscle
• striated in appearance
• involuntary control
• autorhythmic because of built in pacemaker

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3 Types of Muscle Tissue

• Smooth muscle
• attached to hair follicles in skin
• in walls of hollow organs -- blood vessels GI
• nonstriated in appearance
• involuntary

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Functions of Muscle Tissue

• Producing body movements
• Stabilizing body positions
• Regulating organ volumes
• bands of smooth muscle called sphincters
• Movement of substances within the body
• blood, lymph, urine, air, food and fluids, sperm
• Producing heat
• involuntary contractions of skeletal muscle
  (shivering)

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Properties of Muscle Tissue

• Excitability
• respond to chemicals released from nerve cells
• Conductivity
• ability to propagate electrical signals over
  membrane
• Contractility
• ability to shorten and generate force
• Extensibility
• ability to be stretched without damaging the
  tissue
• Elasticity
• ability to return to original shape after being
  stretched

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SKELETAL MUSCLE TISSUE

• Each skeletal muscle is a separate organ composed
  of cells called fibers.

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Skeletal Muscle -- Connective Tissue

• Superficial fascia is loose connective tissue
  fat underlying the skin
• Deep fascia dense irregular connective tissue
  around muscle
• Connective tissue components of the muscle
  include
• epimysium surrounds the whole muscle
• perimysium surrounds bundles (fascicles) of
  10-100 muscle cells
• endomysium separates individual muscle cells
• All these connective tissue layers extend beyond
  the muscle belly to form the tendon

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Connective Tissue Components
13
Connective Tissue Components

• Tendons and aponeuroses are extensions of
  connective tissue beyond muscle cells that attach
  muscle to bone or other muscle.
• A tendon is a cord of dense connective tissue
  that attaches a muscle to the periosteum of a
  bone (Figure 11.22).
• An aponeurosis is a tendon that extends as a
  broad, flat layer (Figure 11.4c).

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Nerve and Blood Supply

• Each skeletal muscle is supplied by a nerve,
  artery and two veins.
• Each motor neuron supplies multiple muscle cells
  (neuromuscular junction)
• Each muscle cell is supplied by one motor neuron
  terminal branch and is in contact with one or two
  capillaries.
• nerve fibers capillaries are found in the
  endomysium between individual cells

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Muscle Fiber or Myofibers

• Muscle cells are long, cylindrical
  multinucleated
• Sarcolemma muscle cell membrane
• Sarcoplasm filled with tiny threads called
  myofibrils myoglobin (red-colored,
  oxygen-binding protein)

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Sarcolemma, T Tubules, and Sarcoplasm

• Skeletal muscle consists of fibers (cells)
  covered by a sarcolemma (Figure 10.3b).
• The fibers contain T tubules and sarcoplasm
• T tubules are tiny invaginations of the
  sarcolemma that quickly spread the muscle action
  potential to all parts of the muscle fiber.
• Sarcoplasm is the muscle cell cytoplasm and
  contains a large amount of glycogen for energy
  production and myoglobin for oxygen storage.

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Transverse Tubules

• T (transverse) tubules are invaginations of the
  sarcolemma into the center of the cell
• filled with extracellular fluid
• carry muscle action potentials down into cell
• Mitochondria lie in rows throughout the cell
• near the muscle proteins that use ATP during
  contraction

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Myofibrils and Sarcoplasmic Reticulum

• Each fiber contains myofibrils that consist of
  thin and thick filaments (myofilaments) (Figure
  10.3b).

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Myofibrils Myofilaments

• Muscle fibers are filled with threads called
  myofibrils separated by SR (sarcoplasmic
  reticulum)
• The sarcoplasmic reticulum encircles each
  myofibril. It is similar to smooth endoplasmic
  reticulum in nonmuscle cells and in the relaxed
  muscle stores calcium ions.
• Myofilaments (thick thin filaments) are the
  contractile proteins of muscle

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Sarcoplasmic Reticulum (SR)

• System of tubular sacs similar to smooth ER in
  nonmuscle cells
• Stores Ca2 in a relaxed muscle
• Release of Ca2 triggers muscle contraction

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Filaments and the Sarcomere

• Thick and thin filaments overlap each other in a
  pattern that creates striations (light I bands
  and dark A bands)
• The I band region contains only thin filaments.
• They are arranged in compartments called
  sarcomeres, separated by Z discs.
• In the overlap region, six thin filaments
  surround each thick filament

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Sarcomere

• Figure 10.5 shows the relationships of the zones,
  bands, and lines as seen in a transmission
  electron micrograph.
• Exercise can result in torn sarcolemma, damaged
  myofibrils, and disrupted Z discs (Clinical
  Application).

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Thick Thin Myofilaments

• Supporting proteins (M line, titin and Z disc
  help anchor the thick and thin filaments in place)

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Thick Thin Myofilaments Overlap
Dark(A) light(I) bands (electron microscope)
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Muscle Proteins
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The Proteins of Muscle

• Myofibrils are built of 3 kinds of protein
• contractile proteins
• myosin and actin
• regulatory proteins which turn contraction on
  off
• troponin and tropomyosin
• structural proteins which provide proper
  alignment, elasticity and extensibility
• titin, myomesin, nebulin and dystrophin

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The Proteins of Muscle -- Myosin

• Thick filaments are composed of myosin
• each molecule resembles two golf clubs twisted
  together
• myosin heads (cross bridges) extend toward the
  thin filaments
• Held in place by the M line proteins.

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The Proteins of Muscle -- Actin

• Thin filaments are made of actin, troponin,
  tropomyosin
• The myosin-binding site on each actin molecule is
  covered by tropomyosin in relaxed muscle
• The thin filaments are held in place by Z lines.
  From one Z line to the next is a sarcomere.

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Structural Proteins

• Structural proteins keep the thick and thin
  filaments in the proper alignment, give the
  myofibril elasticity and extensibility, and link
  the myofibrils to the sarcolemma and
  extracellular matrix.
• Titin helps a sarcomere return to its resting
  length after a muscle has contracted or been
  stretched.
• Myomesin forms the M line.
• Nebulin helps maintain alignment of the thin
  filaments in the sarcomere.
• Dystrophin reinforces the sarcolemma and helps
  transmit the tension generated by the sarcomeres
  to the tendons.
• Table 10.1 reviews the type of proteins in
  skeletal muscle.

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The Proteins of Muscle -- Titin

• Titan anchors thick filament to the M line and
  the Z disc.
• The portion of the molecule between the Z disc
  and the end of the thick filament can stretch to
  4 times its resting length and spring back
  unharmed.
• Role in recovery of the muscle from being
  stretched.

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Structural Proteins

• The M line (myomesin) connects to titin and
  adjacent thick filaments.
• Nebulin, an inelastic protein helps align the
  thin filaments.
• Dystrophin links thin filaments to sarcolemma and
  transmits the tension generated to the tendon.

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Sliding Filament Mechanism Of Contraction

• Myosin cross bridgespull on thin filaments
• Thin filaments slide inward
• Z Discs come toward each other
• Sarcomeres shorten.The muscle fiber shortens. The
  muscle shortens
• Notice Thick thin filaments do not change in
  length

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Overview From Start to Finish

• Basic Structures
• Nerve ending
• Neurotransmitter
• Muscle membrane
• Stored Ca2
• ATP
• Muscle proteins

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How Does Contraction Begin?

• Nerve impulse reaches an axon terminal synaptic
  vesicles release acetylcholine (ACh)
• ACh diffuses to receptors on the sarcolemma Na
  channels open and Na rushes into the cell
• A muscle action potential spreads over sarcolemma
  and down into the transverse tubules
• SR releases Ca2 into the sarcoplasm
• Ca2 binds to troponin causes
  troponin-tropomyosin complex to move reveal
  myosin binding sites on actin--the contraction
  cycle begins

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Contraction Cycle

• Repeating sequence of events that cause the thick
  thin filaments to move past each other.
• 4 steps to contraction cycle
• ATP hydrolysis
• attachment of myosin to actin to form
  crossbridges
• power stroke
• detachment of myosin from actin
• Cycle keeps repeating as long as there is ATP
  available there is a high Ca2 level near the
  filaments.

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Steps in the Contraction Cycle

• Notice how the myosin head attaches and pulls on
  the thin filament with the energy released from
  ATP

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ATP and Myosin

• Myosin heads are activated by ATP
• Activated heads attach to actin pull (power
  stroke)
• ADP is released. (ATP released P ADP energy)
• Thin filaments slide past the thick filaments
• ATP binds to myosin head detaches it from actin
• All of these steps repeat over and over
• if ATP is available
• Ca level near the troponin-tropomyosin complex
  is high

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Excitation - Contraction Coupling

• All the steps that occur from the muscle action
  potential reaching the T tubule to contraction of
  the muscle fiber.

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Relaxation

• Acetylcholinesterase (AChE) breaks down ACh
  within the synaptic cleft
• Muscle action potential ceases
• Ca2 release channels close
• Active transport pumps Ca2 back into storage in
  the sarcoplasmic reticulum
• Calcium-binding protein (calsequestrin) helps
  hold Ca2 in SR (Ca2 concentration 10,000 times
  higher than in cytosol)
• Tropomyosin-troponin complex recovers binding
  site on the actin

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Overview From Start to Finish

• Nerve ending
• Neurotransmittor
• Muscle membrane
• Stored Ca2
• ATP
• Muscle proteins

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Length-Tension Relationship

• The forcefulness of muscle contraction depends on
  the length of the sarcomeres within a muscle
  before contraction begins.
• Figure 10.10 plots the length-tension
  relationships for skeletal muscle.

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Length Tension Curve

• Graph of Force of contraction(Tension) versus
  Length of sarcomere
• Optimal overlap at the topof the graph
• When the cell is too stretchedand little force
  is produced
• When the cell is too short, againlittle force is
  produced

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Length of Muscle Fibers

• Optimal overlap of thick thin filaments
• produces greatest number of crossbridges and the
  greatest amount of tension
• As stretch muscle (past optimal length)
• fewer cross bridges exist less force is
  produced
• If muscle is overly shortened (less than optimal)
• fewer cross bridges exist less force is
  produced
• thick filaments crumpled by Z discs
• Normally
• resting muscle length remains between 70 to 130
  of the optimum

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Neuromuscular Junction (NMJ) or Synapse

• NMJ myoneural junction
• end of axon nears the surface of a muscle fiber
  at its motor end plate region (remain separated
  by synaptic cleft or gap)

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Structures of NMJ Region

• Synaptic end bulbs are swellings of axon
  terminals
• End bulbs contain synaptic vesicles filled with
  acetylcholine (ACh)
• Motor end plate membrane contains 30 million ACh
  receptors.

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Events Occurring After a Nerve Signal

• Arrival of nerve impulse at nerve terminal causes
  release of ACh from synaptic vesicles
• ACh binds to receptors on muscle motor end plate
  opening the gated ion channels so that Na can
  rush into the muscle cell
• Inside of muscle cell becomes more positive,
  triggering a muscle action potential that travels
  over the cell and down the T tubules
• The release of Ca2 from the SR is triggered and
  the muscle cell will shorten generate force
• Acetylcholinesterase breaks down the ACh attached
  to the receptors on the motor end plate so the
  muscle action potential will cease and the muscle
  cell will relax.

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Pharmacology of the NMJ

• Botulinum toxin blocks release of
  neurotransmitter at the NMJ so muscle contraction
  can not occur
• bacteria found in improperly canned food
• death occurs from paralysis of the diaphragm
• Curare (plant poison from poison arrows)
• causes muscle paralysis by blocking the ACh
  receptors
• used to relax muscle during surgery
• Neostigmine (anticholinesterase agent)
• blocks removal of ACh from receptors so
  strengthens weak muscle contractions of
  myasthenia gravis
• also an antidote for curare after surgery is
  finished

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Muscle MetabolismProduction of ATP in Muscle
Fibers

• Muscle uses ATP at a great rate when active
• Sarcoplasmic ATP only lasts for few seconds
• 3 sources of ATP production within muscle
• creatine phosphate
• anaerobic cellular respiration
• aerobic cellular respiration

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MUSCLE METABOLISM

• Creatine phosphate and ATP can power maximal
  muscle contraction for about 15 seconds and is
  used for maximal short bursts of energy (e.g.,
  100-meter dash) (Figure 10.13a).
• Creatine phosphate is unique to muscle fibers.

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MUSCLE METABOLISM

• The partial catabolism of glucose to generate ATP
  occurs in anaerobic cellular respiration (Figure
  10.13b). This system can provide enough energy
  for about 30-40 seconds of maximal muscle
  activity (e.g., 300-meter race).
• Muscular activity lasting more than 30 seconds
  depends increasingly on aerobic cellular
  respiration (reactions requiring oxygen). This
  system of ATP production involves the complete
  oxidation of glucose via cellular respiration
  (biological oxidation) (Figure 10.13c).

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Creatine Phosphate Details

• Excess ATP within resting muscle used to form
  creatine phosphate
• Creatine phosphate 3-6 times more plentiful
  than ATP within muscle
• Its quick breakdownprovides energy for creation
  of ATP
• Sustains maximal contraction for 15 sec (used for
  100 meter dash).
• Athletes tried creatine supplementation
• gain muscle mass but shut down bodies own
  synthesis (safety?)

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Anaerobic Cellular Respiration Details

• ATP produced from glucose breakdown into pyruvic
  acid during glycolysis
• if no O2 present
• pyruvic converted to lactic acid which diffuses
  into the blood
• Glycolysis can continue anaerobically to provide
  ATP for 30 to 40 seconds of maximal activity (200
  meter race)

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Aerobic Cellular Respiration

• ATP for any activity lasting over 30 seconds
• if sufficient oxygen is available, pyruvic acid
  enters the mitochondria to generate ATP, water
  and heat
• fatty acids and amino acids can also be used by
  the mitochondria
• Provides 90 of ATP energy if activity lasts more
  than 10 minutes

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Muscle Fatigue

• Inability to contract after prolonged activity
• Factors that contribute to fatigue
• central fatigue is feeling of tiredness and a
  desire to stop (protective mechanism)
• insufficient release of acetylcholine from motor
  neurons
• depletion of creatine phosphate
• decline of Ca2 within the sarcoplasm
• insufficient oxygen or glycogen
• buildup of lactic acid and ADP

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The Motor Unit

• Motor unit one somatic motor neuron all the
  skeletal muscle cells (fibers) it stimulates (10
  cells to 2,000 cells)
• muscle fibers normally scattered throughout belly
  of muscle
• One nerve cell supplies on average 150 muscle
  cells that all contract in unison.
• Total strength of a contraction depends on how
  many motor units are activated how large the
  motor units are

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CONTROL OF MUSCLE TENSION

• A twitch contraction is a brief contraction of
  all the muscle fibers in a motor unit in response
  to a single action potential.
• A record of a muscle contraction is called a
  myogram and includes three periods latent,
  contraction, and relaxation (Figure 10.15).
• The refractory period is the time when a muscle
  has temporarily lost excitability with skeletal
  muscles having a short refractory period and
  cardiac muscle having a long refractory period.

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Twitch Contraction

• Brief contraction of all fibers in a motor unit
  in response to
• single action potential in its motor neuron
• electrical stimulation of the neuron or muscle
  fibers
• Myogram graph of a twitch contraction
• the action potential lasts 1-2 msec
• the twitch contraction lasts from 20 to 200 msec

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Myogram of a Twitch Contraction
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Parts of a Twitch Contraction

• Latent Period--2msec
• Ca2 is being released from SR
• slack is being removed from elastic components
• Contraction Period
• 10 to 100 msec
• filaments slide past each other
• Relaxation Period
• 10 to 100 msec
• active transport of Ca2 into SR
• Refractory Period
• muscle can not respond and has lost its
  excitability
• 5 msec for skeletal 300 msec for cardiac muscle

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Frequency of Stimulation

• Wave summation is the increased strength of a
  contraction resulting from the application of a
  second stimulus before the muscle has completely
  relaxed after a previous stimulus (Figure 10.16a,
  b).
• A sustained muscle contraction that permits
  partial relaxation between stimuli is called
  incomplete (unfused) tetanus (Figure 10.16c) a
  sustained contraction that lacks even partial
  relaxation between stimuli is called complete
  (fused) tetanus (Figure 10.16d).
• The process of increasing the number of active
  motor units is called recruitment (multiple motor
  unit summation).
• It prevents fatigue and helps provide smooth
  muscular contraction rather than a series of
  jerky movements.

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Wave Summation

• If second stimulation applied after the
  refractory period but before complete muscle
  relaxation---second contraction is stronger than
  first

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Complete and Incomplete Tetanus

• Unfused tetanus
• if stimulate at 20-30 times/second, there will be
  only partial relaxation between stimuli
• Fused tetanus
• if stimulate at 80-100 times/second, a sustained
  contraction with no relaxation between stimuli
  will result

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Explanation of Summation Tetanus

• Wave summation both types of tetanus result
  from Ca2 remaining in the sarcoplasm
• Force of 2nd contraction is easily added to the
  first, because the elastic elements remain
  partially contracted and do not delay the
  beginning of the next contraction

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Motor Unit Recruitment

• Motor units in a whole muscle fire asynchronously
• some fibers are active others are relaxed
• delays muscle fatigue so contraction can be
  sustained
• Produces smooth muscular contraction
• not series of jerky movements
• Precise movements require smaller contractions
• motor units must be smaller (less fibers/nerve)
• Large motor units are active when large tension
  is needed

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Muscle Tone

• Involuntary contraction of a small number of
  motor units (alternately active and inactive in a
  constantly shifting pattern)
• keeps muscles firm even though relaxed
• does not produce movement
• Essential for maintaining posture (head upright)
• Important in maintaining blood pressure
• tone of smooth muscles in walls of blood vessels

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Isotonic and Isometric Contraction

• Isotonic contractions a load is moved
• concentric contraction a muscle shortens to
  produce force and movement
• eccentric contractions a muscle lengthens while
  maintaining force and movement
• Isometric contraction no movement occurs
• tension is generated without muscle shortening
• maintaining posture supports objects in a fixed
  position

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TYPES OF SKELETAL MSUCLE FIBERS

• On the basis of structure and function, skeletal
  muscle fibers are classified as
• slow oxidative,
• oxidative-glycolytic, or
• fast glycolytic fibers.

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Variations in Skeletal Muscle Fibers

• Myoglobin, mitochondria and capillaries
• red muscle fibers
• more myoglobin, an oxygen-storing reddish pigment
  
• more capillaries and mitochondria
• white muscle fibers
• less myoglobin and less capillaries give fibers
  their pale color
• Contraction and relaxation speeds vary
• how fast myosin ATPase hydrolyzes ATP
• Resistance to fatigue
• different metabolic reactions used to generate ATP

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Classification of Muscle Fibers

• Slow oxidative (slow-twitch)
• red in color (lots of mitochondria, myoglobin
  blood vessels)
• prolonged, sustained contractions for maintaining
  posture
• Oxidative-glycolytic (fast-twitch A)
• red in color (lots of mitochondria, myoglobin
  blood vessels)
• split ATP at very fast rate used for walking and
  sprinting
• Fast glycolytic (fast-twitch B)
• white in color (few mitochondria BV, low
  myoglobin)
• anaerobic movements for short duration used for
  weight-lifting

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Fiber Types within a Whole Muscle

• Most muscles contain a mixture of all three fiber
  types
• Proportions vary with the usual action of the
  muscle
• neck, back and leg muscles have a higher
  proportion of postural, slow oxidative fibers
• shoulder and arm muscles have a higher proportion
  of fast glycolytic fibers
• All fibers of any one motor unit are same.
• Different fibers are recruited as needed.

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Distribution and Recruitment of Different Types
of Fibers

• Although the number of different skeletal muscle
  fibers does not change, the characteristics of
  those present can be altered by various types of
  exercise.
• The use of anabolic steroids by athletes to
  increase muscle size, strength, and endurance has
  been shown to have very serious side effects,
  some of which are life-threatening. (Clinical
  Application)

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Anabolic Steroids

• Similar to testosterone
• Increases muscle size, strength, and endurance
• side effects
• liver cancer
• kidney damage
• heart disease
• mood swings
• facial hair voice deepening in females
• atrophy of testicles baldness in males

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Anatomy of Cardiac Muscle

• Striated , short, quadrangular-shaped, branching
  fibers
• Single centrally located nucleus
• Cells connected by intercalated discs with gap
  junctions
• Same arrangement of thick thin filaments as
  skeletal

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CARDIAC MUSCLE TISSUE - Overview

• Cardiac muscle tissue is found only in the heart
  wall (see Chapter 20).
• Its fibers are arranged similarly to skeletal
  muscle fibers.
• Cardiac muscle fibers connect to adjacent fibers
  by intercalated discs which contain desmosomes
  and gap junctions (Figure 4.1e).
• Cardiac muscle contractions last longer than the
  skeletal muscle twitch due to the prolonged
  delivery of calcium ions from the sarcoplasmic
  reticulum and the extracellular fluid.
• Cardiac muscle fibers contract when stimulated by
  their own autorhythmic fibers.
• This continuous, rhythmic activity is a major
  physiological difference between cardiac and
  skeletal muscle tissue.

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Cardiac versus Skeletal Muscle

• More sarcoplasm and mitochondria
• Larger transverse tubules located at Z discs,
  rather than at A-l band junctions
• Less well-developed SR
• Limited intracellular Ca2 reserves
• more Ca2 enters cell from extracellular fluid
  during contraction
• Prolonged delivery of Ca2 to sarcoplasm,
  produces a contraction that last 10 -15 times
  longer than in skeletal muscle

76
Appearance of Cardiac Muscle

• Striated muscle containing thick thin filaments
• T tubules located at Z discs less SR

77
Physiology of Cardiac Muscle

• Autorhythmic cells
• contract without stimulation
• Contracts 75 times per min needs lots of O2
• Larger mitochondria generate ATP aerobically
• Extended contraction is possible due to slow Ca2
  delivery
• Ca2 channels to the extracellular fluid stay
  open

78
SMOOTH MUSCLE

• Smooth muscle tissue is nonstriated and
  involuntary and is classified into two types
  visceral (single unit) smooth muscle (Figure
  10.18a) and multiunit smooth muscle (Figure
  10.18b).
• Visceral (single unit) smooth muscle is found in
  the walls of hollow viscera and small blood
  vessels the fibers are arranged in a network and
  function as a single unit.
• Multiunit smooth muscle is found in large blood
  vessels, large airways, arrector pili muscles,
  and the iris of the eye. The fibers operate
  singly rather than as a unit.

79
Two Types of Smooth Muscle

• Visceral (single-unit)
• in the walls of hollow viscera small BV
• autorhythmic
• gap junctions cause fibers to contract in unison
• Multiunit
• individual fibers with own motor neuron ending
• found in large arteries, large airways, arrector
  pili muscles,iris ciliary body

80
Microscopic Anatomy of the Smooth Muscle

• Sarcoplasm of smooth muscle fibers contains both
  thick and thin filaments which are not organized
  into sarcomeres.
• Smooth muscle fibers contain intermediate
  filaments which are attached to dense bodies.
  (Figure 10.19)

81
Microscopic Anatomy of Smooth Muscle

• Small, involuntary muscle cell -- tapering at
  ends
• Single, oval, centrally located nucleus
• Lack T tubules have little SR for Ca2 storage

82
Microscopic Anatomy of Smooth Muscle

• Thick thin myofilaments not orderly arranged
  so lacks sarcomeres
• Sliding of thick thin filaments generates
  tension
• Transferred to intermediate filaments dense
  bodies attached to sarcolemma
• Muscle fiber contracts and twists into a helix as
  it shortens -- relaxes by untwisting

83
Physiology of Smooth Muscle

• Contraction starts slowly lasts longer
• no transverse tubules very little SR
• Ca2 must flows in from outside
• In smooth muscle, the regulator protein that
  binds calcium ions in the cytosol is calmodulin
  (in place of the role of troponin in striated
  muscle)
• calmodulin activates the enzyme myosin light
  chain kinase, which facilitates myosin-actin
  binding and allows contraction to occur at a
  relatively slow rate.

84
Smooth Muscle Tone

• The prolonged presence of calcium ions in the
  cytosol of smooth muscle fibers provides for
  smooth muscle tone, a state of continued partial
  contraction.
• Smooth muscle fibers can stretch considerably
  without developing tension this phenomenon is
  termed the stress-relaxation response.
• Useful for maintaining blood pressure or a steady
  pressure on the contents of GI tract

85
DEVELOPMENT OF MUSCLE

• With few exceptions, muscles develop from
  mesoderm (Figure 6.13a)
• Skeletal muscles of the head and extremities
  develop from general mesoderm the remainder of
  the skeletal muscles develop from mesoderm of
  somites (Figure 10.20a).

86
Fusion of Myoblasts into Muscle Fibers

• Mature muscle cells developed from 100 myoblasts
  that fuse together in the fetus. (multinucleated)
• Mature muscle cells are not known to divide.
• Muscle growth is a result of cellular enlargement
  (hypertrophy) not cell division (hyperplasia)
• Satellite cells retain the ability to regenerate
  new cells.

87
Developmental Anatomy of the Muscular System

• Develops from mesoderm
• Somite formation
• blocks of mesoderm give rise to vertebrae and
  skeletal muscles of the back
• Muscles of head limbs develop from general
  mesoderm

88
Regeneration of Muscle

• Skeletal muscle fibers cannot divide after 1st
  year
• growth is enlargement of existing cells
• repair
• satellite cells bone marrow produce some new
  cells
• if not enough numbers---fibrosis occurs most
  often
• Cardiac muscle fibers cannot divide or regenerate
• all healing is done by fibrosis (scar formation)
• Smooth muscle fibers (regeneration is possible)
• cells can grow in size (hypertrophy)
• some cells (uterus) can divide (hyperplasia)
• new fibers can form from stem cells in BV walls

89
Aging and Muscle Tissue

• Skeletal muscle starts to be replaced by fat
  beginning at 30
• use it or lose it
• Slowing of reflexes decrease in maximal
  strength
• Change in fiber type to slow oxidative fibers may
  be due to lack of use or may be result of aging

90
Myasthenia Gravis

• Progressive autoimmune disorder that blocks the
  ACh receptors at the neuromuscular junction
• The more receptors are damaged the weaker the
  muscle.
• More common in women 20 to 40 with possible line
  to thymus gland tumors
• Begins with double vision swallowing
  difficulties progresses to paralysis of
  respiratory muscles
• Treatment includes steroids that reduce
  antibodies that bind to ACh receptors and
  inhibitors of acetylcholinesterase

91
Muscular Dystrophies

• Inherited, muscle-destroying diseases
• Sarcolemma tears during muscle contraction
• Mutated gene is on X chromosome so problem is
  with males almost exclusively
• Appears by age 5 in males and by 12 may be unable
  to walk
• Degeneration of individual muscle fibers produces
  atrophy of the skeletal muscle
• Gene therapy is hoped for with the most common
  form Duchenne muscular dystrophy

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Abnormal Contractions

• Spasm involuntary contraction of single muscle
• Cramp a painful spasm
• Tic involuntary twitching of muscles normally
  under voluntary control--eyelid or facial muscles
• Tremor rhythmic, involuntary contraction of
  opposing muscle groups
• Fasciculation involuntary, brief twitch of a
  motor unit visible under the skin

93
Atrophy and Hypertrophy

• Atrophy
• wasting away of muscles
• caused by disuse (disuse atrophy) or severing of
  the nerve supply (denervation atrophy)
• the transition to connective tissue can not be
  reversed
• Hypertrophy
• increase in the diameter of muscle fibers
• resulting from very forceful, repetitive muscular
  activity and an increase in myofibrils, SR
  mitochondria

94
Exercise-Induced Muscle Damage

• Intense exercise can cause muscle damage
• electron micrographs reveal torn sarcolemmas,
  damaged myofibrils an disrupted Z discs
• increased blood levels of myoglobin creatine
  phosphate found only inside muscle cells
• Delayed onset muscle soreness
• 12 to 48 Hours after strenuous exercise
• stiffness, tenderness and swelling due to
  microscopic cell damage

95
Rigor Mortis

• Rigor mortis is a state of muscular rigidity
  that begins 3-4 hours after death and lasts about
  24 hours
• After death, Ca2 ions leak out of the SR and
  allow myosin heads to bind to actin
• Since ATP synthesis has ceased, crossbridges
  cannot detach from actin until proteolytic
  enzymes begin to digest the decomposing cells.

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• end