MUSCLES

1
MUSCLES

• AND MUSCLE TISSUE

2
MUSCLE TYPES

• Skeletal

• Cardiac

• Smooth

3
OVERVIEW

• Skeletal and smooth muscle cells are elongated
• Muscle fibers
• Muscle contraction depends on two kinds of
  myofilaments
• Actin fibers
• Myosin fibers

4
SKELETAL MUSCLES

• Skeletal muscles
• Tissue packaged into organs
• Skeletal muscle fibers longest muscle cells
• Striated
• Obvious stripes due to overlapping of filaments
• Contraction involves movement of filaments
• Voluntary control
• Voluntary muscles
• Only type of muscle subject to conscious control

5
CARDIAC MUSCLES

• Cardiac muscles
• Tissue packaged into organs
• Present only in heart
• Constitutes bulk of heart wall
• Striated
• Obvious stripes due to overlapping of filaments
• Contraction involves movement of filaments
• Involuntary
• Involuntary muscles
• Not subject to conscious control

6
SMOOTH MUSCLES

• Smooth muscles
• Tissue packaged into organs
• Present in walls of hollow visceral organs
• Forces substances through internal body channels
• Nonstriated
• Filaments present, but no stripes apparent
• Involuntary
• Involuntary muscles
• Not subject to conscious control

7
MUSCLE CHARACTERISTICS

• Excitability (Irritability)
• Ability to receive and respond to a stimulus
• e.g., chemical, pH change, etc.
• Response is generation of an electrical impulse
• Contractility
• Ability to shorten forcibly when stimulated
• Extensibility
• Ability to be stretched/extended when relaxed
• Elasticity
• Ability of muscle fiber to recoil and resume
  resting length after being stretched

8
MUSCLE FUNCTIONS

• Produce movement
• Most movements result of muscular contraction
• Movement of body and within body
• Maintain posture
• Muscles continuously active
• Stabilize joints
• Major force stabilizing many joints
• Generate heat
• Contraction generates heat
• Heat required to maintain constant body
  temperature

9
SKELETAL MUSCLE

• Gross Anatomy
• Each skeletal muscle is a discrete organ
• Composed of several kinds of tissues
• Skeletal muscle fibers
• Blood vessels
• Nerve fibers
• Connective tissue

10
SKELETAL MUSCLE

• Gross Anatomy Nerve Blood Supply
• Muscle served by
• Single nerve
• Single artery
• One or more veins
• All branch profusely through connective tissue
  sheaths

11
SKELETAL MUSCLE

• Gross Anatomy Connective Tissue Sheaths
• Individual muscle fibers wrapped and held
  together by multiple connective tissue sheaths
• Endomysium
• Perimysium
• Epimysium
• Support each cell and reinforce muscle

12
SKELETAL MUSCLE

• Gross Anatomy Connective Tissue Sheaths
• Endomysium
• Surround each individual muscle fiber
• Fibers mainly reticular fibers

13
SKELETAL MUSCLE

• Gross Anatomy Connective Tissue Sheaths
• Perimysium
• Fibrous connective tissue
• Surrounds bundles wrapped muscle fibers
• Endomysium-wrapped bundles are fasicles

14
SKELETAL MUSCLE

• Gross Anatomy Connective Tissue Sheaths
• Epimysium
• Dense irregular connective tissue
• Surrounds entire muscle

15
SKELETAL MUSCLE

• Gross Anatomy Connective Tissue Sheaths
• All connective tissue sheaths are continuous with
  each other and with tendons
• Muscle fibers contract ?
• Contraction pulls on sheaths ?
• Sheaths transmit force to bones ?
• Bones move
• Contribute to natural elasticity of muscle tissue

16
SKELETAL MUSCLE

• Gross Anatomy Muscle Attachments
• Skeletal muscles span joints
• Attachment to bones in two places
• Insertion
• More movable bone
• Origin
• Less movable bone

17
SKELETAL MUSCLE

• Gross Anatomy Muscle Attachments
• Direct attachments
• Epimysium is attached to the bones periosteum
• Indirect attachments
• More common than direct attachments
• Smaller, more durable
• Connective tissue sheaths extend beyond the
  muscle
• Rope-like tendon
• Sheet-like aponeurosis
• Anchors muscle to the connective tissue covering
  of a bone, cartilage, or the fascia of another
  muscle

18
SKELETAL MUSCLE

• Microscopic Anatomy Skeletal Muscle Fiber
• Long, cylindrical cell
• 10 100 mm width
• 10X that of average body cell
• Up to 30 cm length
• Plasma membrane is termed sarcolemma
• Cytoplasm is termed sarcoplasm

19
SKELETAL MUSCLE

• Microscopic Anatomy Skeletal Muscle Fiber
• Sarcoplasm
• Large amounts of stored glycogen
• Glycosomes
• Large amount of myoglobin
• Oxygen-storing pigment similar to hemoglobin
• Hemoglobin genes descended from duplicated
  ancestral myoglobin gene
• Molecular evolution
• What is the function of these two components?

20
SKELETAL MUSCLE

• Microscopic Anatomy Skeletal Muscle Fiber
• Organelles
• The usual organelles are present
• Multiple nuclei beneath sarcolemma
• Skeletal muscles are syncytia, fusions of
  hundreds of embryonic cells
• Myofibrils
• Sarcoplasmic reticulum
• T-tubules

21
SKELETAL MUSCLE

• Microscopic Anatomy Skeletal Muscle Fiber
• Myofibrils
• Many myofibrils densely packed in each muscle
  cell
• More than 105 per cell
• 80 of cell volume
• Run parallel to length of cell
• Diameter of 1-2 mm
• Contain contractile elements

22
SKELETAL MUSCLE

• Microscopic Anatomy Myofibrils
• Striations
• Repeating pattern of dark and light bands
• Dark A bands
• Light I bands
• Near-perfect alignment of bands between
  myofibrils gives entire cell striped appearance

23
SKELETAL MUSCLE

• Microscopic Anatomy Myofibril Striations
• A-bands (dark bands)
• Lighter stripe in center
• H-zone
• Bisected by M line
• I-bands (light bands)
• Bisected by Z disc
• Sarcomere
• Region of a myofibril between two Z discs

24
SKELETAL MUSCLE

• Microscopic Anatomy Myofibril Striations
• Sarcomere
• Region of a myofibril between two Z discs
• A band flanked by half of an I band on each end
• 2 mm long
• Smallest contractile unit of a muscle fiber
• Myofibril consists of linked sarcomeres

25
SKELETAL MUSCLE

• Microscopic Anatomy Myofibril Striations
• Banding pattern results from an orderly
  arrangement of thick (myosin) and thin (actin)
  filaments
• Thick filaments extend entire length of A band
• Thick filaments absent across I band
• Z disc composed of the protein nebulin
• Anchors thin filaments
• M line in H zone darker due to protein strands
  linking adjacent thick filaments

26
SKELETAL MUSCLE

• Microscopic Anatomy Thick Filaments
• Composed mainly of 200 myosin molecules
• Rod-like tail
• Two interwoven polypeptide chains
• Form central part of thick filament
• Globular heads
• Present on ends of thick filaments
• Business end of myosin
• Cross bridges linking thick filaments to thin
  filaments during contraction

27
SKELETAL MUSCLE

• Microscopic Anatomy Thin Filaments
• Composed mainly of the protein actin
• Possess sites to which myosin heads bind during
  contraction
• Also contains regulatory proteins
• Tropomyosin
• Surround and stiffen actin core
• Block myosin binding sites in relaxed muscle
• Troponin
• Three-polypeptide complex
• TnI inhibitory subunit
• TnT helps position tropomyosin on actin
• TnC binds to Ca2

28
SKELETAL MUSCLE

• Microscopic Anatomy Other Filaments
• Elastic filaments
• Comprised of the giant protein titin
• Extends from Z disc to the thick filament
• Functions
• Holds thick filaments in place
• Resists excessive stretching
• Assists the muscle cell in springing back into
  shape after being stretched

29
SKELETAL MUSCLE

• Microscopic Anatomy Sarcoplasmic Reticulum
• Elaborate smooth endoplasmic reticulum
• Interconnecting tubules loosely surround each
  myofibril
• Regulates intracellular Ca2 levels
• Stores Ca2
• Ca2 released on demand during stimulation

30
SKELETAL MUSCLE

• Microscopic Anatomy T Tubules
• Invagination of plasma membrane (sarcolemma)
• Elongated tube penetrating cell interior
• Intimate contact with SR
• Electrical impulses traveling along sarcolemma
  also travel along T tubules
• Conduct impulses to deepest regions of muscle
• Transmit to every sarcomere
• Impulses signal Ca2 release from SR

31
SKELETAL MUSCLE

• Sliding Filament Model of Contraction
• During contraction, thin filaments slide past
  thick filaments
• Involves activation of myosins cross bridges
• Increases overlap between actin and myosin
• Relaxed muscle slight overlap
• Stimulated muscle increased overlap

32
SKELETAL MUSCLE

• Sliding Filament Model of Contraction
• How do filaments slide?
• Each myosin cross bridge attaches and detaches
  several times during a contraction
• Ratcheting action generates tension
• Thin filaments propelled toward center of
  sarcomere
• Occurs simultaneously in all sarcomeres
  throughout the cell
• Not all cells of the muscle
• Cell shortens

33
MUSCLE PHYSIOLOGY

• Excitation - Contraction Coupling
• Muscle contraction requires stimulation by nerve
• Electrical current is propagated
• Action potential
• Intracellular Ca2 rises
• Contraction is triggered

34
MUSCLE PHYSIOLOGY

• Neuromuscular Junction
• Skeletal muscles stimulated by motor neurons
• Components of somatic nervous system
• Nerves reside in brain or spinal cord
• Threadlike extensions travel to muscle cells
• Axon
• Divides profusely as it enters the muscle
• Each axonal ending forms branching neuromuscular
  junction with a single muscle fiber
• Only one neuromuscular junction per muscle fiber

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MUSCLE PHYSIOLOGY

• Neuromuscular Junction
• Axonal ending and muscle fiber very close
• Not touching
• 1 2 nanometers (nm) apart
• Separating space termed synaptic cleft
• Gel-like extracellular substance rich in
  glycoproteins

36
MUSCLE PHYSIOLOGY

• Neuromuscular Junction
• Axonal ending contains synaptic vesicles
• Membrane-enclosed sacs
• Contain neurotransmitter acetylcholine (Ach)
• Ach can be released into synaptic cleft by
  exocytosis

37
MUSCLE PHYSIOLOGY

• Neuromuscular Junction
• Nerve impulse reaches end of axon
• Voltage-gated Ca2 channels in axonal membrane
  open
• Ca2 enters axon from extracellular fluid
• Ca2 influx signals exocytosis
• Synaptic vesicles fuse with axonal membrane
• ACh enters synaptic cleft

38
MUSCLE PHYSIOLOGY

• Neuromuscular Junction
• Muscle cells sarcolemma highly infolded
• Junctional folds
• Increased surface area
• Membrane rich with ACh receptors

39
MUSCLE PHYSIOLOGY

• Neuromuscular Junction
• Released ACh diffuses across synaptic cleft
• ACh binds to sarcolemmas ACh receptors
• ACh ? acetic acid and choline
• Catalyzed by the enzyme acetylcholinesterase
• Sarcolemma-bound enzyme
• Binding triggers electrical events
• Muscle ultimately contracts

40
MUSCLE PHYSIOLOGY

• Homeostatic Imbalance Myasthenia Gravis
• Disease characterized by a shortage of ACh
  receptors
• Autoimmune disease
• Body destroys its own Ach receptors
• Interferes with neuromuscular junction events
• Drooping eyelids, difficulty swallowing
  talking, generalized weakness

41
MEMBRANE POTENTIAL

• A voltage exists across the plasma membrane
• Membrane potential
• Due to separation of oppositely charged ions
• Resting potential exhibited in cells resting
  state
• From -5 to -100 millivolts (mV)
• Inside of cell is negative relative to outside
• Cells are polarized

42
MUSCLE PHYSIOLOGY

• Neuromuscular Junction Depolarization
• Resting sarcolemma is polarized
• Resting potential
• Interior face slightly negatively charged
• Binding of ACh to receptors opens ligand-gated
  Na channels
• Na enters myofibril
• K exits cell, but to lesser degree
• Interior face of sarcolemma becomes less
  negative
• Depolarization
• End plate potential is formed

43
MUSCLE PHYSIOLOGY

• Action Potential
• Depolarization is initially a local electrical
  event
• End plate potential becomes action potential,
  which spreads rapidly along sarcolemma
• Membrane areas adjacent initial depolarization
  become depolarized
• Voltage-gated Na channels are activated
• Na enters cell, initiation action potential
• Depolarization wave spreads to adjacent areas,
  opening additional Na channels
• Action potential results in contraction

44
MUSCLE PHYSIOLOGY

• Action Potential
• Repolarization wave quickly follows
  depolarization wave
• Na channels close
• Voltage-gated K channels open
• K rapidly exits myofibril
• Electrical conditions of cell restored
• Na-K pump will restore ionic conditions
• Several contractions can occur before ionic
  imbalances adversely impact contractile
  activity

45
MUSCLE PHYSIOLOGY

• Action Potential
• Muscle cannot be stimulated again until
  repolarization is complete
• Refractory period

46
MUSCLE PHYSIOLOGY

• Excitation-Contraction Coupling
• Action potential propagates along sarcolemma
• Transmitted down T tubules
• Brief (1-2 milliseconds ms)
• Ends prior to visible signs of contraction
• AP triggers SR to release Ca2 into sarcoplasm
• Ca2 channels opened

47
MUSCLE PHYSIOLOGY

• Excitation-Contraction Coupling
• Ca2 binds to troponin TnC
• Shape altered, blocking action of tropomyosin
  removed
• Myosin heads attach to thin filaments
• Ratcheting pulls thin filaments toward center
  of sarcomere

48
MUSCLE PHYSIOLOGY

• Excitation-Contraction Coupling
• Ca2 actively pumped back into SR
• Ca2 signal ends within 30 ms
• Tropomyosin blockade reestablished
• Cross bridge activity ends
• Relaxation occurs

49
MUSCLE PHYSIOLOGY

• Excitation-Contraction Coupling
• Repeat
• Requires additional nerve impulse
• When nerve impulses are delivered rapidly
• Ca2 levels increase greatly
• Muscle cells do not completely relax between
  stimuli
• Contraction is stronger and more sustained

50
MUSCLE PHYSIOLOGY

• Sarcoplasmic Ca2 Concentrations
• Sarcoplasmic Ca2 levels are very low
• Phosphate levels in sarcoplasm are higher
• Ca2 and PO43- react to form crystals
• These crystals would kill the cell
• Ca2 and PO43- must be kept separated
• Intracellular Ca2 is tightly regulated by
  proteins
• e.g., calsequestrin, calmodulin

51
MUSCLE PHYSIOLOGY

• Muscle Fiber Contraction
• Cross bridge attachment of actin requires Ca2
• Low intracellular Ca2
• Muscle cell is relaxed
• Myosin binding sites on actin are physically
  blocked by tropomyosin molecules
• Rising Ca2 levels
• Ca2 ions bind to regulatory sites on troponin
  TnC
• TnC shape changes
• Tropomyosin removed from binding site
• Binding sites on actin are exposed

52
MUSCLE PHYSIOLOGY

• Muscle Fiber Contraction
• Once binding sites on actin are exposed
• Myosin head is already cocked
• Cocking fueled by hydrolysis of bound ATP
  molecule
• ADP Pi remain covalently bound to myosin

53
MUSCLE PHYSIOLOGY

• Muscle Fiber Contraction
• Once binding sites on actin are exposed
• Cross bridge formation
• Myosin heads attach to binding sites
• Approximately half of the myosin heads on a
  given thick filament are bound at any given time

54
MUSCLE PHYSIOLOGY

• Muscle Fiber Contraction
• Once binding sites on actin are exposed
• Power stroke
• Myosin head pivots, moving through 70o angle
• ADP Pi released from myosin

55
MUSCLE PHYSIOLOGY

• Muscle Fiber Contraction
• Once binding sites on actin are exposed
• Cross bridge detachment
• New ATP molecule covalently binds to myosin
  head
• Myosin and actin disassociate

56
MUSCLE PHYSIOLOGY

• Muscle Fiber Contraction
• Once binding sites on actin are exposed
• Myosin head is again cocked
• Repeat 30 times or more during a muscle
  contraction
• Continues as long as Ca2 and ATP
  concentrations are sufficient

57
MUSCLE PHYSIOLOGY

• Muscle Fiber Relaxation
• As Ca2 pumps of the SR reclaim Ca2
• Troponin again changes shape
• Myosin binding sites on actin again blocked by
  tropomyosin
• Contraction ends
• Muscle fiber relaxes

This same thing in reverse
58
MUSCLE PHYSIOLOGY

• Muscle Fiber Contraction Rigor Mortis
• Muscles begin to stiffen 3 4 hours after death
• Peak rigidity at 12 hours
• Dying cells are unable to exclude Ca2
• Intracellular Ca2 normally low
• Ca2 influx promotes formation of cross bridges
• What step requires ATP binding?
• How much ATP is made by dead cells?
• Why dont the muscles relax?

59
MUSCLE PHYSIOLOGY

• Contraction of a Skeletal Muscle
• A skeletal muscle consists of many muscle cells
• Principles governing the contraction of a muscle
  fiber and of a skeletal muscle are similar
• All contractions are not equal
• In response to different stimuli, muscles
  contract
• With varying force
• For varying periods of time

60
MUSCLE PHYSIOLOGY

• Contraction of a Skeletal Muscle
• Muscle tension
• The force exerted by a contracting muscle
• Load
• The opposing force exerted by the weight of the
  object being moved
• Isometric contraction
• Muscle tension develops, but load is not moved
• Isotonic contraction
• Muscle tension developed overcomes the load
• Muscle shortening occurs

61
MUSCLE PHYSIOLOGY

• Each muscle is served by one or more motor nerves
• Consist of axons of hundreds of motor neurons
• The axon of each motor neuron branches upon
  entering muscle
• Each branch ends at a neuromuscular junction

Motor Neuron
62
MUSCLE PHYSIOLOGY

• Motor Unit
• A single motor neuron and its associated muscle
  fibers
• From four to hundreds of muscle fibers per motor
  unit
• Multiple motor units per muscle
• Not clustered within muscle
• Firing of motor neuron causes contraction of all
  muscle fibers in motor unit
• Not all muscle fibers of muscle
• Entire muscle contracts
• More motor units stronger contraction

63
MUSCLE TWITCH

• Response of motor unit to single action potential
• Three phases
• Latent period
• Few msec following stimulation
• Muscle tension begins to increase
• Period of contraction
• 10 100 msec
• Cross bridges are active
• Tension develops, muscle may shorten
• Period of relaxation
• 10 100 msec
• Initiated by reentry of Ca2 into SR
• Muscle tension decreases to zero

64
MUSCLE TWITCH

• Twitch contractions of some muscles are rapid and
  brief
• e.g., muscles controlling eye movement
• Some muscles contract more slowly and remain
  contracted for longer periods
• e.g., calf muscles

65
GRADED RESPONSES

• Normal muscle contractions
• Not jerky
• Relatively smooth
• Vary in strength based on demand
• Graded muscle responses
• Graded muscle responses achieved by
• Altering the frequency of stimulation
• Altering the strength of the stimulus

66
GRADED RESPONSES

• Changes in Stimulation Frequency
• Two stimuli in rapid succession
• Second twitch will be stronger than the first
• Appears to ride on shoulders of first
• Wave summation
• Second contraction occurs before the muscle is
  completely relaxed following the first
  contraction
• Second contraction causes more shortening that
  first

67
GRADED RESPONSES

• Changes in Stimulation Frequency
• Multiple stimuli in rapid succession
• Relaxation time between twitches shortens
• Sarcoplasmic Ca2 increases
• Degree of summation increases
• Quivering contraction termed incomplete tetanus
• a.k.a., unfused tetanus

68
GRADED RESPONSES

• Changes in Stimulation Frequency
• Multiple stimuli in rapid succession
• Relaxation time between twitches shortens
• Sarcoplasmic Ca2 increases
• Degree of summation increases
• Quivering contraction termed incomplete tetanus
• Ultimately fused into smooth complete tetanus

69
GRADED RESPONSES

• Response to Stronger Stimuli
• Wave summation increases contractile force
• Main function is to produce smooth, continuous
  contractions
• Force of contraction increased by multiple motor
  unit summation
• Threshhold stimulus recruits first motor units
• Maximal stimulus recruits all of a muscles motor
  units

70
GRADED RESPONSES

• Smallest motor units
• Fewest and smallest muscle fibers
• Controlled by small, highly excitable motor
  neurons
• Tend to get activated first
• Larger motor units
• Contain large, coarse muscle fibers
• Controlled by larger, less excitable motor
  neurons
• Activated only when a stronger contraction is
  necessary
• DONT REALLY DO THIS, JUST PRETEND!!!!!
• Caress your neighbor, then slap him/her.
• What motor units are you activating?

71
GRADED RESPONSES

• Treppe
• Why do muscle contractions become slightly
  stronger with each successive stimulus during the
  initial phases of muscle activity?
• Increasing sarcoplasmic Ca2
• More binding sites exposed
• More heat liberated
• Enzymes work more efficiently
• Muscles become more pliable

72
MUSCLE TONE

• Relaxed skeletal muscles are always slightly
  contracted
• This state is termed muscle tone
• Stretch receptors in muscles and tendons are
  activated
• Spinal reflexes continually activate an
  alternating subset of motor neurons
• No active movement produced
• Mucles kept firm, healthy, and ready to respond
  to stimulation
• Helps stabilize joints and maintain osture

73
ISOTONIC CONTRACTIONS

• Muscle length changes and moves the load
• Cross bridges are moving thin filaments
• Once tension is sufficient to move load, tension
  remains relatively constant
• Two types
• Concentric contractions
• Muscle shortens and does work
• Eccetric contractions
• Muscle contracts as it lengthens
• e.g., calf muscle while walking up a hill
• More forceful than concentric contractions

74
ISOMETRIC CONTRACTIONS

• Tension builds but muscle length remains constant
• Muscle attempts to move a load greater than the
  force the muscle is able to develop
• (Try to lift your car or push this building
  over)
• Cross bridges are generating force but are not
  moving the thin filaments

75
MUSCLE METABOLISM

• Muscular contraction requires energy
• Actually, they require TONS of energy
• Energy for the movement of the cross bridges
• Energy for the operation fo the calcium pump
• Energy sources
• Stored ATP
• Creatine phosphate
• Aerobic respiration
• Fermentation

76
MUSCLE METABOLISM

• Stored ATP
• ATP provides the energy for
• Movement of the cross bridges
• Operation of the calcium pump
• ATP reserves provide this ATP for a little while
• Reserves depleted in 4 6 seconds at most
• How is this ATP get regenerated?

77
MUSCLE METABOLISM

• Phosphorylation of ADP by Creatine Phosphate
• Creatine phosphate is a high-energy molecule
  stored in muscle
• Amount stored exceeds ATP reserves
• Phosphate readily transferred to ADP
• ADP creatine-P ? ATP creatine
• (A-P-P creatine-P ? A-P-P-P creatine)
• Creatine phosphate provides ATP for a while
• Creatine phosphate deplete in up to 10 15
  seconds
• Now what do we do?

78
MUSCLE METABOLISM

• Aerobic Respiration
• Provides most of the ATP during rest and light to
  moderate exercise
• Sugar O2 ? ATP CO2 H2O
• Provides 36 ATP molecules per glucose
• Sugar
• Easily liberated from glycogen stored in muscle
• Present in blood
• Can be replaced by other fuels (e.g., fatty
  acids, amino acids)
• Oxygen
• Carried by hemoglobin in blood
• Stored by myoglobin in muscle
• Ultimately limits aerobic respiration

79
MUSCLE METABOLISM

• Fermentation
• Does not require O2
• Aerobic respiration does require O2
• Much less efficient than aerobic respiration
• 2 ATP per sugar as opposed to 30-something
• Faster than aerobic respiration
• Important early in strenuous exercise
• Glycolysis occurs as usual
• Glucose ? 2 pyruvate 2 ATP 2 NADH
• Pyruvate is reduced to form lactic acid
• Process regenerates NAD (which is required for
  glycolysis)
• Pyruvate 2 NADH ? lactic acid 2 NAD

80
MUSCLE METABOLISM

• Order of use of the various energy sources
• Stored ATP
• Creatine phosphate
• Fermentation
• Aerobic respiration
• Fermentation

81
MUSCLE FATIGUE

• O2 ultimately becomes limiting
• ATP use exceeds ATP production
• Intracellular lactic acid increases
• Intracellular pH drops
• Ionic imbalances occur (e.g., K, Ca2)
• Muscles contract less effectively
• Muscles ultimately become physiologically unable
  to contract even when receiving stimuli
• Muscle fatigue
• Do cardiac muscles experience fatigue?
• Why or why not?

82
OXYGEN DEBT

• For a muscle to return to its resting state
• Lactic acid must be removed
• O2 reserves must be replenished
• ATP reserves must be replenished
• Creatine phosphate must be replenished
• The amount of oxygen required for these processes
  is termed the oxygen debt
• Represents the difference between the amount of
  O2 needed for aerobic muscle activity and the
  amount of O2 actually used
• All non-aerobic sources of ATP contribute to debt

83
HEAT PRODUCTION

• Heat production is an incidental consequence of
  muscle contraction
• 40 of the energy released during muscle
  contraction is converted to useful work
• Remainder is given off as heat
• This heat is used to maintain a constant body
  temperature
• Excess heat is shed
• Do you remember how?
• Shivering produces additional heat when required

84
FORCE OF CONTRACTION

• Force of muscle contraction is affected by
• Number of muscle fibers stimulated
• More motor units greater force
• Relative size of fibers
• Larger fibers produce greater force
• Regular exercise causes muscle fiber hypertrophy
• Frequency of stimulation
• Force transferred from muscle to load
• Repeated stimulations produce sustained
  contraction
• Transfer more complete during sustained
  contractions
• Degree of muscle stretch
• Greatest force when muscle is slightly stretched

85
CONTRACTION

• Velocity and Duration of Contraction
• Affected by load
• Faster contractions with no added load
• Greater load causes
• Longer latent period
• Slower contraction
• Shorter contraction duration
• Affected by recruitment
• More motor units ? faster more prolonged
  contractions
• Affected by muscle fiber type
• Detailed on next image

86
CONTRACTION

• Velocity and Duration of Contraction
• Classification of muscle type by major ATP
  formation pathways
• Oxidative fibers
• Rely mainly on aerobic pathways for ATP
  generation
• Glycolytic fibers
• Rely mainly on fermentation for ATP generation

• Classification of muscle type by speed of
  contraction
• Slow fibers vs. fast fibers
• Differences reflect speed enzymatic hydrolysis
  of ATP
• Slow oxidative fibers
• Fast oxidative fibers
• Fast glycolytic fibers

87
EFFECTS OF EXERCISE

• Muscles change in response to the amount of work
  they do
• e.g., active muscles may increase in size and
  strength
• e.g., inactive muscles atrophy

88
EFFECTS OF EXERCISE

• Effects of aerobic (endurance) exercise
• (e.g., swimming, jogging, etc.)
• Increased of capillaries surrounding muscle
  fibers
• Increased number of mitochondria in muscle fibers
• Increased myoglobin synthesis
• Most dramatic changes in slow oxidative fibers
• Changes result in
• More efficient muscle metabolism
• Greater endurance and strength
• Greater resistance to fatigue
• NO significant muscle hypertrophy

89
EFFECTS OF EXERCISE

• Effects of resistance exercise
• (e.g., weight lifting, etc.)
• Significant muscle hypertrophy
• Individual muscle fibers increase in size
• Increase in number of mitochondria in muscle
  fibers
• Increase in number of myofilaments in muscle
  fibers
• Increased amount of connective tissue in muscle

90
SMOOTH MUSCLE

• Present in the walls of all hollow organs
• (Not including the heart)
• Contractions similar to those of skeletal muscle
• Smooth muscle has several differences

91
SMOOTH MUSCLE

• Spindle-shaped cells
• Smaller than skeletal muscle cells
• 2 5 micrometer diameter
• 100 400 micrometer length
• Single, centrally located nucleus
• Lack coarse connective tissue sheaths
• Possess small amount of fine connective tissue
  (endomysium) secreted by muscle cells
• Vascular, innervated

92
SMOOTH MUSCLE

• Generally organized into two sheets
• Fibers organized perpendicular to each other
• Why are no striations visible?
• Alternating contraction and relaxation of
  opposing layers mixes and moves substances in
  organs lumen
• Peristalsis

93
SMOOTH MUSCLE

• Lacks highly structured neuromuscular junctions
• Possess diffuse junctions
• Neurotransmitter released into wider area
• Sarcoplasmic reticulum less developed
• T-tubules absent
• Sarcolemma posesses Ca2-containing infoldings
• Caveoli

94
SMOOTH MUSCLE

• Organization of filaments differs
• No sarcomeres
• Thickthin ratio 113, not 12
• Thick filaments longer than skeletal counterparts
• Myosin heads on entire length of thick filaments
• Tropomyosin present, troponin absent
• Filaments arranged diagonally
• Contain noncontractile intermediate filaments
• Resist tension
• Attach to dense bodies

95
SMOOTH MUSCLE

• Slow, synchronized contractions
• Entire sheet responds to stimulus in unison
• Electrically coupled via gap junctions
• Some fibers in stomach and small intestine act as
  pacemakers
• Set contraction pace for other cells
• Some are self-excitatory
• Contract even without external stimulus
• Contract in response to neuronal or hormonal
  stimuli