Chapter 10 Muscle Tissue

1
Chapter 10Muscle Tissue

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

2
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

3
3 Types of Muscle Tissue

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

4
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

5
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)

6
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

7
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

8
Connective Tissue Components
9
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

10
Fusion of Myoblasts into Muscle Fibers

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

11
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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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 Myofilaments

• Muscle fibers are filled with threads called
  myofibrils separated by SR (sarcoplasmic
  reticulum)
• 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

15
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

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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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Thick Thin Myofilaments

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

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Overlap of Thick Thin Myofilaments within a
Myofibril
Dark(A) light(I) bands visible with an electron
microscope
19
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

20
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

21
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.

22
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.

23
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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Other 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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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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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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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 high Ca2 level near thin filament

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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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Overview From Start to Finish

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

32
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

33
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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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

35
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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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)

37
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.

38
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.

39
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

40
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
• anaerobic cellular respiration

41
Creatine Phosphate

• 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?)

42
Anaerobic Cellular Respiration

• 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)

43
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

44
Muscle Fatigue

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

45
Oxygen Consumption after Exercise

• Muscle tissue has two sources of oxygen.
• diffuses in from the blood
• released by myoglobin inside muscle fibers
• Aerobic system requires O2 to produce ATP needed
  for prolonged activity
• increased breathing effort during exercise
• Recovery oxygen uptake
• elevated oxygen use after exercise (oxygen debt)
• lactic acid is converted back to pyruvic acid
• elevated body temperature means all reactions
  faster

46
The Motor Unit

• Motor unit one somatic motor neuron all the
  skeletal muscle cells (fibers) it stimulates
• 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

47
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
49
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

50
Wave Summation

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

51
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

52
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

53
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

54
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

55
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

56
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

57
Classification of Muscle Fibers

• Slow oxidative (slow-twitch)
• red in color (lots of mitochondria, myoglobin
  blood vessels)
• prolonged, sustained contractions for maintaining
  posture
• Fast 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

58
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.

59
Anabolic Steroids

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

60
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

61
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

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Appearance of Cardiac Muscle

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

63
Physiology of Cardiac Muscle

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

64
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

65
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

66
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

67
Physiology of Smooth Muscle

• Contraction starts slowly lasts longer
• no transverse tubules very little SR
• Ca2 must flows in from outside
• Calmodulin replaces troponin
• Ca2 binds to calmodulin turning on an enzyme
  (myosin light chain kinase) that phosphorylates
  the myosin head so that contraction can occur
• enzyme works slowly, slowing contraction

68
Smooth Muscle Tone

• Ca2 moves slowly out of the cell
• delaying relaxation and providing for state of
  continued partial contraction
• sustained long-term
• Useful for maintaining blood pressure or a steady
  pressure on the contents of GI tract

69
Regulation of Contraction

• Regulation of contraction due to
• nerve signals from autonomic nervous system
• changes in local conditions (pH, O2, CO2,
  temperature ionic concentrations)
• hormones (epinephrine -- relaxes muscle in
  airways some blood vessels)
• Stress-relaxation response
• when stretched, initially contracts then
  tension decreases to what is needed
• stretch hollow organs as they fill yet pressure
  remains fairly constant
• when empties, muscle rebounds walls firm up

70
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

71
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

72
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

73
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

74
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

75
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