Muscle Tissue

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Muscle Tissue
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Muscle Tissue Classification
Skeletal Muscle
Cardiac Muscle
Intercalated Disc
Smooth Muscle
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Skeletal Muscle

• directly or indirectly attached to bones of
  skeleton

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Functions

• movement
• simple-breathing to highly coordinated
  ones-swimming
• posture body position
• maintenance or stability
• constant muscle contraction holds the head up
• store move substances in the body
• maintains body temperature
• muscle contraction requires energy when energy
  is used some energy is converted to heat?keeps
  body temperature within the normal range
• when cold shivering occurs

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Gross Anatomy

• entire muscle is surrounded by epimysium
• fuses into connective tissue sheets called fascia
• groups of muscle fibers are arranged in bundles
  called fascicles wrapped in connective tissue
  layer-perimysium
• contains blood vessels nerves
• endomysium surrounds each individual muscle fiber
• connective tissue layers are continuous through
  length of muscle
• at end of muscle, collagen fibers of epi-,
  peri- and endomysium come together to form
  tendons aponeurosis

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Microscopic Anatomy

• muscle cell myofibril or fiber is thin very
  long
• Multinucleate-maybe hundreds present
• arranged around periphery just beneath cell
  membrane
• sarcolemma-plasma membrane surrounds sarcoplasm
  or cytoplasm
• contains long protein bundles called myofibrils,
  a great deal of glycogen and a red pigment,
  myoglobin
• Smooth endoplasmic reticulum-SR or sarcoplasmic
  reticulum
• forms network around each myofibril and
  periodically expands into terminal cisternae
• sarcolemma has tubular infoldings called T
  (transverse) tubules which are associated with
  two terminal cisternae
• t tubule plus adjacent terminal cisternae is a
  Triad
• stores releases calcium needed for contractions
  
• T tubules conduct action potential through the
  entire muscle fiber

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Myofibril Composition

• made of myofilaments
• arranged in repeating patterns
• appear as striations under a microscope
• two types actin myosin
• one repeat is a sarcomere
• smallest, functional unit of skeletal muscle
• narrow plates called Z discs separate the
  sarcomeres
• a sarcomere extends from one Z disc to the next

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Sarcomere Structure

• A band
• darker, middle part
• myosin actin
• I Bands
• lighter areas
• actin only
• Z disc
• passes through middle of each I band
• defines one sarcomere
• H zone
• either side of M line
• M line
• center of H zone

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Proteins in Muscle Fibers

• Contractile Proteins
• actin
• myosin
• Regulatory Proteins
• tropomyosin
• troponin
• Structural Proteins
• titin
• alpha actinin
• myomesin
• nebulin
• dystrophin

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Myosin

• contractile protein
• comprised of 2 subunits
• twisted around one another
• forming? long coiled tail
• pair of heads
• project toward m
• line

MYOSIN-THICK FILAMENT
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Actin

• contractile protein
• comprises thin filaments
• composed of two intertwined strands of fibrous
  (F) actin-contractile protein
• each F-actin is made up of subunits called
  G-Actin
• each G-actin has an active site which can bind a
  myosin head

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Regulatory Proteins

• Control contraction-turn it on off
• Tropomyosin
• winds around actin
• covers myosin binding sites preventing
  actin-myosin interactions
• Troponin
• calcium binding protein each
• bound to each tropomyosin
• When calcium binds to troponin?changes
  shape?pulls tropomyosin off actin?myosin binding
  site exposed?crossbridges form

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Structural Proteins

• Titin
• huge elastic molecule
• recoils after stretching
• anchors myosin to Z-disc
• Nebulin
• helps anchor thin filaments to Z discs
• helps stabilize thick filament
• Alpha actinin
• comprises z discs
• Myomesin
• forms M line
• Dystrophin
• under sarcolemma
• attaches actin to membrane proteins

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Sliding Filament Theory

• theory of how muscle contraction takes place
• under microscope, during muscle contraction
• H zone I bands get smaller
• H zone almost disappears
• zones of overlap get larger
• Z lines move closer together
• width length of A band remains constant
• only make sense if thin filaments slide to center
  of each sarcomere
• actin slides over myosin which causes sarcomere
  to shorten
• ultimately entire muscle cell shortens

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Sliding Filament Theory
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Contraction

• calcium binds to troponin ? tropomyosin is pulled
  toward actin groove
• myosin binding site uncovered
• myosin heads interact with actin
• forming cross bridges
• like hinges
• myosin head pivots at its base
• pulls on actin
• causing it to move to center of sacromere
• muscle shortens

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Muscle Cell Contraction

• Skeletal muscles only contract when activated by
  motor neurons from CNS

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NEURON STRUCTURE

• Dendrites
• Receive information
• Typically many
• Axons
• Send information
• Covered with Myelin Sheath
• End in Terminal Buttons

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Neuromuscular Junction

• communication between muscles nerves occurs at
  neuromuscular junction
• each branch of a motor nerve fiber ends in a
  synaptic knob
• nestled in a depression on sarcolemma?motor end
  plate (MEP)
• exhibits many junctional folds
• contains receptors

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Neuromuscular Juncion
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Neuromuscular Junction

• cells do not touch
• separated by a tiny gap-synaptic cleft
• synaptic knobs contain vesicles of
  acetylcholine-ACH
• neurotransmitter
• the cleft sarcolemma contain ACHE or
  acetylcholinesterase
• Breaks down ACH

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Excitation Contraction Coupling

• Transfer of an impulse from somatic motor neuron
  to muscle cell is excitation contraction coupling
• 4 steps
• ACH release
• Activation of ACH receptors
• Production of Muscle Action Potential
• Termination of ACH activity

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STEP 1 ACH release

• action potential reaches synaptic terminal
• opens calcium gates
• calcium enters neuron causing synaptic vesicles
  to fuse with cell membrane which releases ACH via
  exocytosis into synaptic cleft
• ACH diffuses across cleft

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STEP 2 Activation of ACH Receptors

• ACH binds to receptors on motor end plate
• opens sodium gates
• sodium rushes into sarcoplasm

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STEP 3 Production of Muscle Action Potential

• positive charges of sodium accumulate
• membrane potential of cell moves toward zero
• as concentration of sodium increases threshold is
  reached
• muscle cell depolarizes
• Action potential begins and spreads in all
  directions
• invaginates at T tubules
• muscle cell contracts

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STEP 4 Termination of ACH Activity

• influx of calcium continues until
  acetylcholinesterase degrades ACH removing it
  from receptors
• component parts are recycled
• calcium is pumped back into the SR
• muscle cell relaxes

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Muscle Cell Contraction

• arrival of action potential
• releases ACH into cleft
• binds to receptors
• sodium rushes into cell
• causes an Action Potential in muscle cell

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Muscle Cell Contraction

• action potential is propagated across entire
  membrane
• when reaches t tubule?travels down t tubules
• t tubules terminal cisternae of sarcoplasmic
  reticulum form a triad
• triad releases calcium from sarcoplasmic reticulum

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Muscle Cell Contraction

• calcium binds to troponin
• changes its shape
• tropomyosin swings away from active site
• exposes myosin binding sites on actin
• cross-bridges form
• initiates contraction
• effect of calcium is instantaneous
• contraction cycle begins

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Contraction Cycle Steps

• 1. ATP Hydrolysis
• 2. Attachment of Myosin to Actin forming
  Cross-Bridges
• 3. Power Stroke
• 4. Detachment of Myosin from Actin

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Step 1- ATP Hydrolysis

• each myosin head must have an ATP bound to it to
  initiate contraction
• head contains myosin ATPase hydrolyzes ATP?ADP
  Pi energy
• ADP Pi still attached to myosin head

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Steps 2 3-Attachment of Myosin to Actin Power
Stroke

• energized myosin binds to exposed active site on
  actin forming a cross-bridge
• myosin releases ADP phosphate
• flexes into a bent, low energy position bringing
  the thin filament with it
• the power stroke

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Step 4-Detachment of Myosin From Actin

• at end of power stroke myosin remains attached to
  actin until nyosin binds another ATP
• upon binding more ATP, myosin releases actin and
  it is ready to begin the process again by
  hydrolyzing the ATP
• each cycle shortens the sarcomere 10 nm
• each myosin head continues to attach, pivot
  detach as long as calcium ATP are available

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Relaxation

• duration of muscle contractions depend on
  duration of stimulus at neuromuscular junction
• ACH does not last long-chewed up by ACHE
• contraction continues only if more action
  potentials arrive at synaptic terminal in rapid
  succession
• muscle fiber sarcoplasm return to normal or
  relax in two ways
• active transport of calcium across cell membrane
  into extracellular fluid
• active transport of calcium into the sarcoplasmic
  reticulum
• more important way
• almost as soon as calcium is released-SR begins
  to absorb calcium from surrounding sarcoplasm
• here calcium binds to calsequestrin is stored
  until stimulated again
• as calcium in sarcoplasm decreases, calcium
  detaches from troponin causing it to return to
  its original position recovering active sites
  with tropomyosin
• once contraction has ended sarcomere does not
  automatically return to its original length
• Sacromeres actively shorten but there is no
  active mechanism to reverse the process
• combination of elastic forces, opposing muscle
  contractions and gravity return muscle to its
  uncontracted state

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Tension Production

• muscle cells contract shorten causing them to
  pull on collagen fibers ? generates tension
• collagen fibers resist building tension
• as muscle continues to pull on collagen
  fibers?fibers transmit force and pull on
  something else
• what happens depends on what fibers are attached
  to and how muscle cells are arranged
• muscles are attached to at least 2 different
  structures
• usually bone occasionally soft tissue
• as muscle contracts, one attachment
  moves?insertion
• other attachment remains stationary? origin
• developing tension pulls object toward source of
  tension

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Tension Production

• tension produced by an individual muscle fiber
  varies
• depends on
• resting length of fiber at time of stimulation
• determines amount of overlap between thin thick
  filaments
• frequency of stimulation
• effects internal calcium concentration
• number of muscle fibers stimulated in one muscle

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Length-Tension Relationship

• amount of tension depends on how stretched or
  contracted it was prior to being stimulated
• length-tension relationship
• amount of tension produced by a muscle is related
  to number of cross bridges formed
• number of cross bridges that can form depends on
  degree of overlap between thick thin filaments
• only myosin heads in zone of overlap can bind to
  active sites on actin produce tension
• Sarcomeres work most efficiently in an optimal
  range of lengths
• Outside optimal range?muscle cannot produce as
  much tension
• optimal range is range where maximum number of
  cross bridges can form?making most tension
• when sarcomeres are short thick filaments are
  jammed up against Z line
• cross bridges form but myosin heads cannot
  pivot?no tension production
• sarcomeres with length longer than optimal range
  has reduced zone of overlap?less cross bridges
  can form?less tension

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Frequency of Stimulation

• Increasing the number of nerve impulses to the
  muscles will keep ACH being released
• which will keep calcium being released
• which will keep cross bridges forming
• which will keep the muscle contracting
• which will cause the development of more tension

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Muscle Twitch

• one above threshold stimulus to a muscle produces
  one contraction/relaxation cycle-twitch
• vary in duration with type, location, temperature
  environmental conditions
• eye twitch-7.5msec
• soleus (calf muscle) twitch- 100msec
• too brief to be part of normal activity
• to show what a twitch looks like a myogram is
  used
• twitch can be divided into three parts
• 1) latent period
• 2) contraction phase
• 3) relaxation phase

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Muscle Twitch

• latent phase begins as stimulation of muscle
  begins-lasts 2msec
• as tension rises to a peak contraction phase
  begins (10-100msec)
• during relaxation phase tension decreases to
  resting levels (10-100msdc)

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Treppe

• twitches produce no work
• sending more more stimulation to muscle in
  short period of time results in changes to
  initial twitch
• when skeletal muscle is stimulated for a second
  time immediately after a relaxation phase treppe
  contraction develops

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TREPPE

• myogram tracing shows a slightly higher tension
  than the first tension
• tension increases over first 30-50 stimulations
  and thereafter amount of tension remains constant
• increase in tension is due to increases in
  calcium in sarcoplasm
• stimuli are arriving so rapidly that calcium is
  not reabsorbed into the SR
• thus there is more Ca in cytosol when the second
  stimulus arrives
• resulting in slightly more tension production a
  slightly higher tracing

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Wave Summation

• as frequency of stimuli increase before previous
  twitch has ended each new twitch rides piggy back
  on previous one
• wave summation
• result of one wave of contraction being added to
  another
• produces sustained contraction called incomplete
  tetanus

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TETANUS

• at a still higher frequency?muscle has no time to
  relax between stimuli
• twitches fuse into a smooth, prolonged
  contraction called complete tetanus

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Tension Production

• tension developed depends on number of muscle
  fibers involved
• each muscle fiber is innervated by one motor
  neuron
• when nerve signal approaches end of axon-it
  spreads to all of axons terminal branches
  stimulates all muscle fibers supplied by them
• makes all muscle fibers connected to neuron
  contract at same time
• one nerve fiber all muscle fibers innervated by
  it is one motor unit

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Motor Units

• some motor neurons control few muscle fibers
• others control hundreds
• number of neurons innervating a muscle indicates
  how fine movement can be in that muscle
• eye muscles need to have precise control
• neuron to muscle in eye controls 4-6 fibers
• leg muscles do not need precise control
• neuron to leg muscle can control 1000-2000 muscle
  fibers

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MOTOR UNITS

• neuron fires?contracts all muscle cells in one
  motor unit
• greater tension can be be generated by recruiting
  more motor units
• smooth steady increase in muscle tension is
  produced by increasing number of active motor
  units
• recruitment
• peak tension occurs when all motor units in a
  muscle contract to tetanus
• such powerful contractions do not last long
• sustained contractions are maintained by
  asynchronous recruitment
• motor units are activated on a rotating basis
• some rest recover while others contract

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Tension Production Movement

• amount of tension produced in a skeletal muscle
  depends on several factors
• before movement is possible, tension must
  overcome resistance
• passive force opposing movement
• amount of resistance depends on objects weight,
  shape, friction and other factors
• when tension is greater than resistance? object
  moves

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Contraction Types

• contractions types are based on pattern of
  tension development
• Isometric
• Isotonic
• Concentric
• Eccentric

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Isometric Contractions

• tension develops with no length change in the
  muscle
• tension never exceeds resistance
• occurs when you begin to use a muscle
• occurs when you push against a locked door
• cross bridges form?tension rises to a peak?muscle
  cannot overcome resistance
• Example-carrying a bag of groceries-arm muscles
  are contracting to hold the bag, but the arm
  itself is not moving

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Isotonic Contractions

• when tension in a muscle increases produces a
  change in muscle length
• two types eccentric concentric
• concentric contractions
• muscles shorten as it maintains tension
• eccentric contractions
• muscle lengthens as it maintains tension
• Example-
• bicep shortens then stretches as a dumbell is
  curled
• tension on muscle remains the same in a muscle as
  length increases or decreases
• concentric is shortening (b) and eccentric is
  lengthening (c)
• To review-Isometric contraction is when a muscle
  is used but it does not shorten (a)
• Both isotonic and isometric contractions are used
  in normal activities

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Muscle Metabolism

• contracting muscles use enormous amounts of ATP
• one muscle fiber may have 15 billion thick
  filaments
• during contraction each filament breaks down 2500
  ATP molecules/sec

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Muscle Metabolism

• ATP is also needed for
• cross bridge release
• to pump calcium back into SR
• to restore sodium potassium levels to
  precontraction conditions
• cannot have all ATP needed for contraction before
  contraction begins
• ATP stores are depleted in 6 sec.-time
• enough for 8 twitches
• for a cell is to continue to contract more ATP
  must be generated
• muscle fiber generates ATP at same rate it is
  used

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ATP

• energy used to power all activity in cells the
  body
• three high energy phosphate bonds
• breaking off one phostphate yields about 7kcal of
  energy
• ATP ? ADP Pi 7Kcal

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Ways to Acquire ATP

• Creatine Phosphate
• Glycolysis-anerobic cellular respiration
• Aerobic Cellular Respiration

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Ways to Acquire ATP
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Creatine Phosphate

• at rest muscle produce more ATP than they use
• excess ATP is used to make creatine phosphate
  (phosphocreatine)
• in working muscles-creatine kinase transfers a
  high energy phosphate from phosphocreatine to
  ADP ? creatine ATP
• provides energy needed for short burst of intense
  activity
• 1 minute of brisk walking or 6 second of
  sprinting

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Glycolysis

• once creatine phosphate stores have been used
  respiratory cardiovascular systems cannot
  deliver oxygen to muscles fast enough to use
  aerobic respiration to produce ATP
• ATP is provided by anaerobic cellular
  respiration-glycolysis
• occurs in cytoplasm
• oxidizes glucose to 2 molecules of pyruvic acid
  and 2 ATP molecules (net)
• produces enough ATP for 30-40 seconds of maximum
  activity

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Glycolysis

• without oxygen? pyruvic acid?lactic acid ATP
• organic acid
• can lower blood pH
• eventually pH changes alter functional
  characteristics of enzymes
• muscle fibers cannot continue to contract
• end result?muscle soreness fatigue

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Aerobic Respiration

• after 40 seconds or so? cardiopulmonary system
  catches up ? delivers oxygen to muscles fast
  enough for aerobic respiration
• requires oxygen
• occurs in mitochondria
• First-TCA (tri-carboxylic acid) or Krebs Cycle
• Second-electron transport chain or oxidative
  phosphorylation
• starting product -pyruvic acid
• muscles using aerobic respiration can contract
  for long periods of time
• 36 molecules of ATP are produced
• in exercise lasting 10 minutes or more 90 of ATP
  is produced aerobically

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Metabolism Overview
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Types of Muscle Fibers

• slow oxidative fibers
• fast glycolytic fibers
• fast oxidative-glycolytic fibers

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Slow Oxidative Muscle Fibers

• called red fibers
• contain a great deal of mitochondria, blood
  capillaries myoglobin-red pigment like
  hemoglobin which binds oxygen
• provide dramatically higher oxygen supply
• gives fibers dark red color
• muscle dominated by slow twitch fibers is
  referred to as dark meat in chicken
• contract slowly
• require 3X as long to contract after stimulation
  as fast twitch fibers
• fatigue resistant
• specialized to contract for long periods of time
• keep contracting long after fast fibers fatigue
• diameters are half that of fast fibers
• less dependent on anaerobic metabolism
• obtain ATP via aerobic respiration
• used almost constantly to maintain posture, to
  stand and to walk

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Fast Glycolytic Fibers

• White fibers
• Muscles appear pale
• termed white muscle
• contract 0.01 sec. after stimulation
• 2-3X faster than slow twitch fibers
• faster speed leads to faster tension development
• muscles dominated by fast fibers display powerful
  contractions
• have large diameters, densely packed myofibrils,
  large glycogen reserves and few mitochondria
• use massive amounts of ATP
• rely on anaerobic respiration
• fatigue more rapidly due to lactic acid build up

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Fast-Oxidative Glycolytic Fibers

• Intermediate fibers
• combine fast twitch response with aerobic fatigue
  resistant metabolism
• contain large amount of myoglobin capillaries
• dark red in color
• get ATP by aerobic mechanisms
• fast due to presence of a faster type of ATPase
• moderately resistant to fatigue

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Thought questions Why do chickens have white
breast meat and dark leg meat? What does this
say about the activities of the associated
muscles? Why do ducks have dark breast meat?
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Muscle Composition

• muscles are composed of all three fiber types
• proportion of each differs from muscle to muscle
• no slow twitch fibers in eye or hand muscles
• need swift contractions
• people with different types levels of physical
  activity differ in the proportion of each fiber
  type
• muscle performance distribution of muscle
  fibers is genetically determined
• proportions of different muscle fibers can change
  with physical conditioning

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Muscle Performance

• rated in terms of
• Power
• maximum amount of tension produced
• Endurance
• amount of time muscle can perform particular
  activity
• two factors determine these performance
  capabilities
• type of muscle fiber
• physical conditioning
• training of that muscle

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Aerobic Endurance

• length of time muscle can contract while
  supported by mitochondrial activities
• determined by substrate availability-break down
  of carbohydrates, lipids amino acids
• involves sustained low level muscle
  activity-jogging
• training-alters characteristics of muscle fibers
• fasts fibers will develop characteristics of
  intermediate fibers
• improves performance of cardiovascular system
  which delivers oxygen nutrients to muscles
• does not promote hypertrophy

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Anaerobic Endurance

• length of time muscle contraction can be
  supported by glycolysis by existing ATP
  creatine phosphate reserves
• limited-amounts of ATP creatine phosphate,
  amounts of glycogen ability of muscles to
  tolerate lactic acid
• improve-frequent, brief, intensive workouts
• weight lifting body building
• produce muscle hypertrophy-
• repeated, exhaustive stimulation causes muscle
  fibers to develop more mitochondria, more
  glycolytic enzymes more glycogen
• muscle will develop more myofibrils-have more
  thin thick filaments
• when muscles are not used?become flaccid,
  smaller-atrophy

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Muscular Strength Conditioning-Training
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Smooth Muscle

• found in almost every organ
• walls of hollow internal structures, blood
  vessels, stomach, intestine, gallbladder
  urinary bladder
• important in homeostasis
• contraction changes shape of organs
• generate force to move materials through the
  lumens of organs

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Smooth Muscle Structure

• long, slender, spindle-shaped
• no striations, no myofibrils and no sarcomeres
• contains myosin actin filaments
• no t-tubules
• SR forms loose network through sarcoplasm
• Actin is attached to dense bodies (like Z discs)
• intermediate fiber bundles are attached to dense
  bodies
• arranged so entire surface of actin is covered
  by myosin heads
• continuous line of myosin heads allows actin to
  slide down myosin without interruption? producing
  tension
• dense bodies intermediate filaments anchor thin
  filaments
• when sliding they slide against each other to
  produce contraction

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Smooth Muscle Contraction

• dense bodies are not found in a straight line
• during contraction causes cell to twist like a
  cork screw

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Types of Smooth Muscle

• Multiunit types
• Single-unit types or Visceral Smooth Muscle

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Multiunit Smooth Muscles

• innervated like skeletal muscle
• neural activity generates an action potential
  which is propagated over the sarcolemma
• found-some large arteries, pulmonary air
  passages, piloerector muscles and the iris
• cells contract or relax depending on type of
  neurotransmitter released

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Visceral Smooth Muscle

• arranged in sheets or layers
• adjacent cells connected by gap junctions
• one muscle cell contracts? electrical impulse?
  travels to adjacent muscle cells? contraction
  spreads in waves soon involving all cells
• initial stimulus may be motor neuron
• also contracts in response to chemicals,
  hormones, oxygen, CO2, stretching irritation

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Excitation-Contraction Coupling

• trigger for contraction?calcium in sarcoplasm
• calcium enters from extracellular fluid
• more is released by the sarcoplasmic reticulum
• calcium interacts with calmodulin, a calcium
  binding protein which activates light chain
  myokinase to break down ATP
• starts contraction
• relaxation occurs when calcium is removed from
  cytosol
• accomplished by a Ca-Na antiport exchange by
  Ca-ATPase