Muscular System: Histology and Physiology

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Muscular SystemHistology and Physiology

• Chapter 9

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Muscular System Functions

• Body movement
• Posture
• Respiration
• Production of body heat
• Communication
• Constriction of organs and vessels
• Heart beat

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Properties of Muscle

• Contractility
• Excitability
• Extensibility
• Elasticity

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Muscle Tissue Types

• Skeletal
• Smooth
• Cardiac

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Skeletal Muscle Structure

• Composed of muscle fibers (cells), CT, blood
  vessels, nerves
• Fibers are long, multinucleated
• Develop from myoblasts numbers remain constant
• Striated

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CT Layers

• External lamina. Reticular fibers. Surrounds
  sarcolemma
• Endomysium. Loose C.T. with reticular fibers
  surrounds muscle fiber
• Perimysium. Denser C.T. surrounding a group of
  muscle fibers called a fasciculus
• Epimysium. C.T. that surrounds a whole muscle
  (many fascicles) aka fascia

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• Fascia (epimysium) CT sheet
• Forms layer under the skin
• Holds muscles together separates them into
  groups
• Allows free movement
• Carries nerves (motor neurons, sensory neurons),
  blood vessels, and lymphatics
• Continuous with tendons and periosteum.

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Nerves

• Motor neurons stimulate muscle fibers to
  contract. Cell bodies in brain or spinal cord
  axons extend to skeletal muscle fibers through
  nerves
• Axons branch so that each muscle fiber is
  innervated

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Blood Supply

• Capillary beds surround muscle fibers

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Muscle Fiber Anatomy

• Cell packed with myofibrils in sarcoplasm
• Composed of protein myofilaments
• Actin (thin)
• Myosin (thick)
• Sarcomeres repeating units of actin myosin

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Sarcomeres Z Disk to Z Disk

• Z disk network of protein attachment for actin
  myofilaments
• Striations
• I bands
• A bands
• H zone
• M line

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Parts of a Muscle
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Other Muscle Fiber Structures

• Sarcolemma cell membrane
• Sarcoplasm cytoplasm
• Sarcoplasmic reticulum sER that acts as holding
  tank for Ca2 uses active transport to pump ions
  into lumen
• T-tubules tubes from sarcolemma that project
  into muscle fiber brings sarcomeres in contact
  with extracellular fluid

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Structure of Actin and Myosin
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Actin Myofilaments

• Proteins involved
• F (fibrous) actin forms attachment site for
  myosin filament
• Tropomyosin block F actin binding sites with
  troponin
• Troponin

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Myosin Myofilaments

• Myosin heads
• Attached to the rod portion by a hinge region
  that can bend and straighten during contraction
• Bind to active sites on the actin molecules to
  form cross-bridges
• Have ATPase activity

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Sliding Filament Model

• Actin myofilaments sliding over myosin to shorten
  sarcomeres

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Sarcomere Shortening
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Physiology of Skeletal Muscle

• Nervous system controls muscle contractions
  through action potentials
• Resting membrane potentials (RMP)
• Voltage difference across membranes (polarized)
• Protein molecules
• More K on inside than outside
• Na/K pump maintains RMP

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Neuromuscular Junction

• NMJ
• Presynaptic terminal
• Synaptic cleft
• NT
• Acetylcholinesterase
• Motor end-plate (postsynaptic membrane)

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Cont.

• Synaptic vesicles
• Neurotransmitter substance released from a
  presynaptic membrane, diffuses across the
  synaptic cleft, and stimulates (or inhibits) the
  production of an action potential in the
  postsynaptic membrane
• Acetylcholine (ACh)

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Events of Contraction

1. Action potential in motor neuron causes Ca2
   channels to open
2. Ca2 diffuses into axon terminal and causes
   release of Ach from synaptic vesicles
3. Ach diffuses across synaptic cleft and binds to
   receptors on sarcolemma at motor end plate
4. Ligand-gated Na channels open and Na diffuses
   into muscle fiber causing depolarization
5. Depolarization triggers an a.p. in the muscle
   fiber

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Con.t

• 6. T-tubules depolarize and cause Ca2
  voltage-gated channels to open in sarcoplasmic
  reticulum
• 7. Ca2 ruches out of SR into sarcoplasm and
  binds to troponin on actin filament
• 8. Binding sites on F actin are exposed and
  crossbridges form between actin and myosin
• 9. ATP is used to move myosin head and then
  release crossbridges

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Excitation-Contraction Coupling

• Mechanism where an action potential causes muscle
  fiber contraction

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Action Potentials and Muscle Contraction
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Cross-Bridge Movement
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Events of Relaxation

1. Acetylcholinesterase removes excess Ach from
   synaptic cleft
2. Sarcoplasmic reticulum pumps Ca2 back into its
   lumen (active transport)
3. Troponin releases Ca2 and actin binding sites
   are blocked

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Muscle Twitch

• Muscle contraction in response to a stimulus that
  causes action potential in one or more muscle
  fibers
• Phases
• Lag or latent
• Contraction
• Relaxation

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Stimulus Strength

• All-or-none law
• Contraction of equal force in response to each
  action potential
• Sub-threshold stimulus no action potential no
  contraction
• Threshold stimulus action potential contraction
• Motor units a single motor neuron and all muscle
  fibers innervated by it

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Contraction of Entire Muscle

• Strength of contraction is graded
• Multiple motor unit summation strength of
  contraction depends upon recruitment of motor
  units

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Multiple-Wave Summation

• As the frequency of action potentials increase,
  the frequency of contraction increases
• Incomplete tetanus
• Complete tetanus
• Multiple-wave summation

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Treppe

• Graded response
• Occurs in muscle rested for prolonged period
• Each subsequent contraction is stronger than
  previous until all equal after few stimuli

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Types of Contractions

• Isometric
• Isotonic
• Muscle tone

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Fatigue

• Types
• Psychological
• Muscular
• Synaptic

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Rigor Mortis
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Energy Sources

• ATP 3 sources
• Creatine phosphate
• During resting conditions stores energy to
  synthesize ATP
• Anaerobic respiration
• Aerobic respiration
• Oxygen debt oxygen taken in by the body above
  that required for resting metabolism after
  exercise

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Muscle Fiber Types

• Slow-twitch or high-oxidative (red)
• Fast-twitch or low-oxidative
• Effects of exercise change in size of muscle
  fibers
• Hypertrophy
• Atrophy

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Heat production

• Exercise
• Excess heat lost due to vasodilation and sweating
• Shivering

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Smooth Muscle

• Not striated, smaller fibers
• Single nucleus
• More actin than myosin
• Caveolae
• Ca2 required to initiate contractions
  Cross-bridging occurs but takes longer
• Relaxation caused by enzyme myosin phosphatase

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Types of Smooth Muscle

• Visceral (unitary) cells in sheets function as
  a unit
• GI, urinary, reproductive tracts
• Numerous gap junctions waves of contraction
• Often autorhythmic
• Multiunit cells act as independent units
• Sheets (blood vessels) bundles (arrector pili
  and iris) single cells (capsule of spleen)
• Only contract when stimulated by neuron or hormone

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Electrical Properties (of Smooth Muscle)

• Slow waves of depolarization and repolarization
  transferred from cell to cell
• Does not follow all-or-none law
• May have pacemaker cells
• Contraction regulated by nervous system and by
  hormones

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Functional Properties

• Some visceral muscle exhibits autorhythmic
  contractions
• Contracts in response to sudden stretch
• Constant tension smooth muscle tone
• Intensity of contraction constant
• Aerobic respiration

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Smooth Muscle Regulation

• Innervated by ANS
• Neurotransmitters acetylcholine, norepinephrine
• Hormones epinephrine
• Cell membranes have receptors

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Cardiac Muscle

• Only in heart
• Striated
• One nucleus
• Has intercalated disks and gap junctions
• Autorhythmic pacemaker cells (SA node)
• Action potentials of longer duration and longer
  refractory period
• Ca2 regulates contraction

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Effects of Aging

• Increased time
• Loss of muscle fibers more slow than fast
• Decreased density of capillaries in muscle