Muscular System: Histology and Physiology

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Muscular SystemHistology and Physiology

• Chapter 9

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Functions of the Muscular System

• Body movement (skeletal muscles attached to
  bones)
• Maintenance of posture
• Respiration (skeletal muscles of thorax are
  responsible for the movement necessary for
  respiration)
• Production of body heat (when skeletal muscles
  contact, heat is given off as a
  by-product)
• Communication (speaking, writing)
• Constriction of organs and vessels (contraction
  of smooth muscle)
• Heart beat (contraction of cardiac muscle)

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General Functional Characteristics of Muscle

• Contractility ability of a muscle to shorten
  with force
• Excitability capacity of muscle to respond to a
  stimulus (by nerve or hormone)
• Extensibility muscle can be stretched to its
  normal resting length and beyond to a limited
  degree
• Elasticity ability of muscle to recoil to
  original resting length after stretched

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Types of Muscle Tissue

• Skeletal
• Responsible for locomotion, facial expressions,
  posture, respiratory movements, other types of
  body movement
• Voluntary
• Smooth
• Walls of hollow organs, blood vessels, eye,
  glands, skin
• Some functions propel urine, mix food in
  digestive tract, dilating/constricting pupils,
  regulating blood flow
• In some locations, autorhythmic
• Controlled involuntarily by endocrine and
  autonomic nervous systems
• Cardiac
• Heart major source of movement of blood
• Autorhythmic
• Controlled involuntarily by endocrine and
  autonomic nervous systems

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Skeletal Muscle Structure

• Composed of muscle cells (fibers), connective
  tissue, blood vessels, nerves
• Fibers are long, cylindrical, multinucleated
• Tend to be smaller diameter in small muscles and
  larger in large muscles. 1 mm- 4 cm in length
• Develop from myoblasts (they are converted to
  muscle fibers as contractile proteins accumulate
  within their cytoplasm) numbers remain constant
  ( of muscle fibers remain constant after
  birth----so, enlargement of muscles is an
  increase in size rather than )
• Striated appearance due to light and dark banding

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Connective Tissue Coverings of Muscle

• Layers
• External lamina. Delicate, reticular fibers.
  Surrounds sarcolemma (P.M.)
• Endomysium. Loose C.T. with reticular fibers.
• Perimysium. Denser C.T. surrounding a group of
  muscle fibers. Each group called a fasciculus
• Epimysium. C.T. that surrounds a whole muscle
  (many fascicles)
• Fascia connective tissue sheet
• Forms layer under the skin
• Holds muscles together and separates them into
  functional groups.
• Allows free movements of muscles.
• Carries nerves (motor neurons, sensory neurons),
  blood vessels, and lymphatics.
• Continuous with connective tissue of tendons and
  periosteum.

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Nerves and Blood Vessel Supply

• Motor neurons stimulate muscle fibers to
  contract. Nerve cells with cell bodies in brain
  or spinal cord axons extend to skeletal muscle
  fibers through nerves
• Axons branch so that each muscle fiber is
  innervated
• Capillary beds surround muscle fibers

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Muscle Fibers

• Nuclei just inside sarcolemma
• Cell packed with myofibrils within cytoplasm
  (sarcoplasm cytoplasm without myofibrils)
• Threadlike (extends from one end of muscle fiber
  to the other)
• Composed of protein threads called myofilaments
  thin (actin 8nm) and thick (myosin 12nm)
• Sarcomeres actin myosin myofilaments form
  highly ordered units called sarcomeres. They are
  joined end to end to form the myofibrils.

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Parts of a Muscle
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Actin and Myosin Myofilaments
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Actin (Thin) Myofilaments

• Two strands of fibrous (F) actin form a double
  helix extending the length of the myofilament
  attached at either end at sarcomere.
• Composed of G actin monomers each of which has an
  active site
• Actin site can bind myosin during muscle
  contraction.
• Tropomyosin an elongated protein winds along the
  groove of the F actin double helix.

• Troponin is composed of three subunits one that
  binds to actin, a second that binds to
  tropomyosin, and a third that binds to calcium
  ions. Spaced between the ends of the tropomyosin
  molecules in the groove between the F actin
  strands.
• The tropomyosin/troponin complex regulates the
  interaction between active sites on G actin and
  myosin.

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Myosin (Thick) Myofilament

• Many elongated myosin molecules shaped like golf
  clubs.
• Molecule consists of two heavy myosin molecules
  wound together to form a rod portion lying
  parallel to the myosin myofilament and two heads
  that extend laterally.

• Myosin heads
• Can bind to active sites on the actin molecules
  to form cross-bridges.
• Attached to the rod portion by a hinge region
  that can bend and straighten during contraction.
• Have ATPase activity activity that breaks down
  adenosine triphosphate (ATP), releasing energy.
  Part of the energy is used to bend the hinge
  region of the myosin molecule during contraction

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Sarcomeres Z Disk to Z Disk

• Z disk filamentous network of protein. Serves as
  attachment for actin myofilaments
• Striated appearance
• I bands from Z disks to ends of thick filaments
• A bands length of thick filaments
• H zone region in A band where actin and myosin
  do not overlap
• M line middle of H zone delicate filaments
  holding myosin in place

• In muscle fibers, A and I bands of parallel
  myofibrils are aligned.
• Titin filaments elastic chains of amino acids
  make muscles extensible and elastic

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Sliding Filament Model

• Actin myofilaments sliding over myosin to shorten
  sarcomeres
• Actin and myosin do not change length
• Shortening sarcomeres responsible for skeletal
  muscle contraction
• During relaxation, sarcomeres lengthen because of
  some external force, like forces produced by
  other muscles (contraction of antagonistic
  muscles) or by gravity.
• - agonist muscle that accomplishes a
  certain movement, such as flexion.
• - antagonist muscle acting in
  opposition to agonist.

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Sarcomere Shortening
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Physiology of Skeletal Muscle Fibers

• Nervous system controls muscle contractions
  through action potentials
• Resting membrane potentials
• Membrane voltage difference across membranes
  (polarized)
• Inside cell more negative due to accumulation of
  large protein molecules. More K on inside than
  outside. K leaks out (through leak channels) but
  not completely because negative molecules hold
  some back.
• Outside cell more positive and more Na on
  outside than inside.
• Na /K pump maintains this situation.
• Must exist for action potential to occur

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Ion Channels

• Types
• Ligand-gated. Ligands are molecules that bind to
  receptors. Receptor protein or glycoprotein with
  a receptor site
• Example neurotransmitters
• Gate is closed until neurotransmitter attaches to
  receptor molecule. When Ach (acetylcholine)
  attaches to receptor on muscle cell, Na gate
  opens. Na moves into cell due to concentration
  gradient
• Voltage-gated
• Open and close in response to small voltage
  changes across plasma membrane
• Each is specific for one type of ion

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Action Potentials

• Phases
• Depolarization Inside of plasma membrane becomes
  less negative. If change reaches threshold,
  depolarization occurs
• Repolarization return of resting membrane
  potential. Note that during repolarization, the
  membrane potential drops lower than its original
  resting potential, then rebounds. This is because
  Na plus K together are higher, but then Na/K pump
  restores the resting potential
• All-or-none principle like camera flash system
• Propagate Spread from one location to another.
  Action potential does not move along the
  membrane new action potential at each successive
  location.
• Frequency number of action potential produced
  per unit of time

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Gated Ion Channels and the Action Potential
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Action Potential Propagation
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Neuromuscular Junction

• Synapse axon terminal resting in an invagination
  of the sarcolemma
• Neuromuscular junction (NMJ)
• Presynaptic terminal axon terminal with synaptic
  vesicles
• Synaptic cleft space
• Postsynaptic membrane or motor end-plate

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Function of Neuromuscular Junction

• Synaptic vesicles
• Neurotransmitter substance released from a
  presynaptic membrane that diffuses across the
  synaptic cleft and stimulates (or inhibits) the
  production of an action potential in the
  postsynaptic membrane.
• Acetylcholine
• Acetylcholinesterase A degrading enzyme in
  synaptic cleft. Prevents accumulation of ACh

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Excitation-Contraction Coupling

• Mechanism by which an action potential causes
  muscle fiber contraction
• Involves
• Sarcolemma
• Transverse (T) tubules invaginations of
  sarcolemma
• Terminal cisternae
• Sarcoplasmic reticulum smooth ER
• Triad T tubule, two adjacent terminal cisternae
• Ca2
• Troponin

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Action Potentials and Muscle Contraction
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Cross-Bridge Movement
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Relaxation

• Ca2 moves back into sarcoplasmic reticulum by
  active transport. Requires energy
• Ca2 moves away from troponin-tropomyosin complex
  
• Complex re-establishes its position and blocks
  binding sites.

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Muscle Twitch

• Muscle contraction in response to a stimulus that
  causes action potential in one or more muscle
  fibers
• Muscle contraction measures as force, also called
  tension. Requires up to a second to occur.
• Phases
• Lag or latent (neuromuscular junction step 1
  of cross-bridge movement)
• Contraction (step 2 - 6 of cross-bridge
  movement)
• Relaxation (powerpoint slide 28)

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Stimulus Strength and Muscle Contraction

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

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Contraction of the Whole Muscle

• Whole muscles exhibit characteristics that are
  more complex than those of individual muscle
  fibers or motor units. Instead of responding in
  an all-or-none fashion, whole muscles respond to
  stimuli in a graded fashion, which means that the
  strength of the contractions can range from weak
  to strong.
• Remember There are many muscle fibers in one
  fasciculi and many fasciculi in one
  whole muscle.
• Strength of contraction in whole muscle is
  graded ranges from weak to strong depending on
  stimulus strength
• Multiple motor unit summation the force in which
  a whole muscle contracts depends on the number of
  motor units stimulated to contract. (force of
  contraction increases as more more motor units
  are stimulated). A muscle has many motor units
• Submaximal stimuli
• Maximal stimulus
• Supramaximal stimuli

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Contraction of the Whole Muscle
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Stimulus Frequency and Muscle Contraction

• Relaxation of a muscle fiber is not required
  before a second action potential can stimulate a
  second contraction.
• As the frequency of action potentials increase,
  the frequency of contraction increases
• Incomplete tetanus muscle fibers partially relax
  between contraction
• Complete tetanus no relaxation between
  contractions
• Multiple-wave summation muscle tension increases
  as contraction frequencies increase

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Types of Muscle Contractions

• Isometric no change in length of muscle but
  tension increases during contraction
• Postural muscles of body ex muscles hold spine
  erect while person is sitting or
  standing
• Isotonic change in length but tension constant
  ex waving using computer keyboard
• Concentric tension is so great it overcomes
  opposing resistance and muscle shortens
  ex raising of a
  weight during a bicep curl.
• Eccentric tension maintained but muscle
  lengthens ex person slowly lowers a heavy
  weight
• Muscle tone constant tension by muscles for long
  periods of time

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Fatigue

• Decreased capacity to work and reduced efficiency
  of performance
• Types
• Psychological depends on emotional state of
  individual ex burst of activity in tired athlete
  in response to encouragement from spectators
  shows how psychological fatigue can be overcome
• Muscular results from ATP depletion ex fatigue
  in lower limbs of marathon runners or in upper
  lower limbs of swimmers
• Synaptic occurs in NMJ due to lack of
  acetylcholine ex rare-----only under extreme
  exertion

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Physiological Contracture and Rigor Mortis

• Physiological contracture state of extreme
  fatigue (extreme exercise) where due to lack of
  ATP neither contraction nor relaxation can occur
• Rigor mortis development of rigid muscles
  several hours after death. Ca2 leaks into
  sarcoplasm and attaches to myosin heads and
  crossbridges form but no ATP available to bind to
  myosin---------so the cross-bridges are unable to
  release. Rigor ends as tissues start to
  deteriorate.

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Energy Sources

• ATP provides immediate energy for muscle
  contractions. Produced from three sources
• Creatine phosphate
• During resting conditions stores energy to
  synthesize ATP
• ADP Creatine phosphate------------------?
  Creatine 1ATP
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  (Creatine Kinase)
• Anaerobic respiration
• Occurs in absence of oxygen and results in
  breakdown of glucose to yield ATP and lactic acid
• Aerobic respiration
• Requires oxygen and breaks down glucose to
  produce ATP, carbon dioxide and water
• More efficient than anaerobic

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Slow and Fast Fibers

• Slow-twitch oxidative
• Contract more slowly, smaller in diameter, better
  blood supply, more mitochondria (also called
  oxidative because carry out aerobic respiration),
  more fatigue-resistant than fast-twitch, large
  amount of myoglobin (dark pigment which binds
  oxygen acts as a muscle reservoir for oxygen
  when blood does not supply adequate amount).
• Postural muscles, more in lower than upper limbs.
  Dark meat of chicken.
• Functions Maintenance of posture performance
  in endurance activities.
• Fast-twitch
• Respond rapidly to nervous stimulation, contain
  myosin that can break down ATP more rapidly than
  that in Type I, less blood supply, fewer and
  smaller mitochondria than slow-twitch (adapted to
  perform anaerobic respiration)
• Lower limbs in sprinter, upper limbs of most
  people. White meat in chicken.
• Comes in oxidative and glycolytic forms
• Functions Rapid, intense movements of short
  duration
• Distribution of fast-twitch and slow-twitch
• Most muscles have both but varies for each muscle
• Exercise weight lifting enlarges fast-twitch
  aerobic training enlarges slow-twitch
• Effects of exercise change in size of muscle
  fibers
• Hypertrophy increase in muscle size
• Increase in myofibrils
• Increase in nuclei due to fusion of satellite
  cells
• Increase in strength
• Atrophy decrease in muscle size
• Reverse except in severe situations where cells
  die

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Smooth Muscle

• Not striated, fibers smaller than those in
  skeletal muscle
• Spindle-shaped single, central nucleus
• More actin than myosin
• Caveolae indentations in sarcolemma may act
  like T tubules
• Dense bodies instead of Z disks as in skeletal
  muscle have noncontractile intermediate
  filaments
• Ca2 required to initiate contractions binds to
  calmodulin (protein). Calmodulin molecules with
  Ca bound to them activate an enzyme called
  myosin kinase, which transfers a phosphate group
  from ATP to heads of myosin molecules.
  Cross-bridging occurs
• Relaxation caused by enzyme myosin phosphatase

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Electrical Properties of Smooth Muscle

• Slow waves of depolarization and repolarization
  transferred from cell to cell
• Depolarization caused by spontaneous diffusion of
  Na and Ca2 into cell
• Does not follow all-or-none law
• Contraction regulated by nervous system and by
  hormones (ex epinephrine)

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Regulation of Smooth Muscle

• Innervated by autonomic nervous system (composed
  of nerve fibers that send impulses from CNS to
  smooth muscle, cardiac muscle, glands)
• Neurotransmitters are acetylcholine and
  norepinephrine (increases cardiac output, blood
  glucose levels)
• Hormones important as epinephrine and oxytocin
• Receptors present on plasma membrane which
  neurotransmitters or hormones bind determines
  response

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Cardiac Muscle

• Found only in heart
• Striated
• Each cell usually has one nucleus
• Has intercalated disks and gap junctions
• Autorhythmic cells
• Action potentials of longer duration
• The depolarization of cardiac muscle results from
  influx of Na and Ca2 across the plasma membrane

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Effects of Aging on Skeletal Muscle

• Reduced muscle mass
• Increased time for muscle to contract in response
  to nervous stimuli
• Reduced stamina
• Increased recovery time
• Loss of muscle fibers