The Muscular System

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Chapter 10

• The Muscular System

• Muscles are able to transform chemical energy
  (ATP) into mechanical energy and enable the
  exertion of force.

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• Muscle from Latin word for mouse (musculus)
• myo- or mys- muscle
• sarco flesh
• words with myo-, mys- or sarco refer to muscle,
  e.g., myoglobin.

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Types of Muscle Tissue

• Skeletal Muscle Tissue Striated, voluntary
• Cardiac Muscle Tissue Striated, involuntary
• Smooth Muscle Tissue Nonstriated, involuntary

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Types of Muscle Tissue

• Skeletal Muscle Tissue Striated, voluntary
• Cardiac Muscle Tissue Striated, involuntary
• Smooth Muscle Tissue Nonstriated, involuntary

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Just because its interesting.

• smooth and striated muscle are also
  characteristic of invertebrate animals.
• striated muscle appears in invertebrate groups as
  diverse as primitive cnidarians (jellyfish) and
  advanced arthropods (lobsters).

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Functions of Muscle Tissue

• Produce body movement
• Stabilizing body position
• Storing and moving substances within the body
• Generating heat (thermogenesis)

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Properties of Muscle Tissue

• Electrical excitability
• Ability to respond to electrical stimuli
• Contractility
• Ability to contract fully when stimulated
• Extensibility
• Ability to stretch without being damaged
• Elasticity
• Ability to return to original shape after
  contraction or extension

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Skeletal Muscle Tissue

• Each skeletal muscle is a discrete organ
• Each muscle (organ) has muscle tissue, connective
  tissue, blood vessels, nerve fibers
• Innervated by motor neurons of somatic NS ?
  voluntary (keep this in mind for chapter 12)

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Connective Tissue Components

• Endomysium sheath of connective tissue
  surrounding individual fibers
• Fascicle bundle of fibers
• Perimysium collagen surrounding individual
  fascicle
• Epimysium fibrous connective tissue binding all
  fascicles together
• surrounds entire muscle
• Deep fascia dense irregular connective tissue
• external to epimysium
• binds muscle into functional group

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Connective Tissue Components

• Fascia
• Superficial Fascia
• Separates muscle from skin
• Contains areolar and adipose tissue
• subcutaneous layer
• Deep Fascia
• Holds muscles with similar functions together
• Contains dense irregular connective tissue

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Layers of Connective Tissue

• Epimysium
• Surrounds whole muscle
• Perimysium
• Surrounds muscle bundles
• Endomysium
• Surrounds individual muscle fibers

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• All three layers of connective tissue connect at
  each end of a muscle and form a TENDON
• Aponeurosis flat tendon-like structure where
  attachments are broad

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Nerve and Blood Supply

• Each muscle fiber (cell) is supplied with a nerve
  ending of a somatic motor neuron.
• Connected at the neuromuscular junction

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Nerve and Blood Supply

• Muscle Tissue is well supplied with arteries
• High O2 requirement
• Numerous veins
• High metabolic waste production

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Nerve and Blood Supply

• In general, 1 artery and 1 or more veins serve
  each muscle
• Blood vessels and nerve fibers typically enter
  central part of muscle and branch

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Microscopic Anatomy of a Skeletal Muscle Fiber

• Muscle cell muscle fiber
• Each fiber is a single multinucleate cell and
  runs the length of the muscle.
• Huge compared to most other human cells can
  reach length of 30 cm (1 foot).
• Each fiber is actually produced by the fusion of
  many cells during embryonic development.
• Muscle fiber --gt myofibrils --gt myofilaments
  (thick and thin)

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• Sarcolemma plasma membrane of muscle cell/fiber
  runs deep into muscle fiber via transverse
  tubules.
• Sarcoplasm cytoplasm of muscle fiber
• contains unusually large amounts of stored
  glycogen and myoglobin (oxygen-binding protein)
  not found in other cell types.

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• Transverse tubules (T tubules)
• Extend from sarcolemma into the sarcoplasm
• Channel electric impulses which trigger muscle
  contraction

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• Myofibrils

• contractile elements made of muscle proteins
  (filaments)

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• Myofibrils
• Multiple cylindrical structures within each
  muscle fiber
• Contain bundles of thick and thin myofilaments

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• Sarcoplasmic reticulum
• specialized ER stores Ca
• Active transport of Ca INTO SR
• Keeps level of Ca in sarcoplasm low ? would
  bind with P ions and form hydroxyapatite
  crystals, hard salts found in bone matrix. Such
  calcified cells would die.
•  

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• Sarcoplasmic reticulum
• Network of membranous channels surrounding each
  myofibril
• Extends throughout sarcoplasm
• Terminal cisternae
• Expanded ends of SR on either side of T tubule
• Triad
• Two terminal cisternae and a T tubule

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Filaments

• Short protein structures arranged into sarcomeres
• Thin actin filaments
• Thick myosin filaments

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• Sarcomere "muscle segment" functional unit of
  muscle
• segment between specific areas of myofibrils
  (between Z-lines)
• smallest contractile unit of fiber
• Sarcomeric arrangement striations
• striations repeating series of dark and light
  bands
• sarcomere area between 2 z-lines

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Sarcomeres

• A - band region of entire thick filament (dark)
• I-band region of thin filament only (light)
• H-zone region of thick filament only
• DECREASES W/ CONTRACT
• Z-line - see picture

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Sarcomeres

• Functional (contractile) units of skeletal muscle
• Z discs separate one sarcomere from the next
• A bands appear dArk
• I bands appear lIght

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A bands appear dArk

• H zone
• Contains thick but no thin filaments
• Zone of overlap
• Contains both thick and thin filaments
• M line
• In middle of sarcomere

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I bands appear lIght

• Only thin filaments
• Extend form A band of one sarcomere to A band of
  the next sarcomere

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Muscle Proteins

• Thick filaments contain mostly myosin
• Have heads (crossbridges) and tails
• Function as motor protein

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Muscle Proteins

• Thick filament primarily protein myosin
• Myosin filament many myosin molecules
• Each myosin molecule has distinctive structure
  rod-like tail, or axis, which terminates in two
  globular heads (sometimes called "cross bridges"
  due to binding).
• Each thick filament within sarcomere contains
  about 200 myosin molecules.
•  

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Muscle Proteins

• Thick filament heads have ATPase activity
• Two positions
• 1. High energy state ATP cleaved ADP and Pi
  held by head
• energy stored in conformational change.
• able to bind to G-actin
• 2. Resting state
• following power stroke ADP and Pi released
  unable to bind.

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Muscle Proteins

• Thin filament - commonly referred to as "actin"
  but has three components
• a. Actin
• G-actin "globular actin" -- individual protein
  molecules
• binding site for myosin head.
• F-actin "fibrous actin" -- rows of G-actin two
  coiled rows per filament.
• Looks like twisted double strand of pearls.

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Muscle Proteins

• b. Tropomyosin 2 molecules per filament
• winds around F-actin, stabilize covers myosin
  binding sites on G-actin.

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Muscle Proteins

• c. Troponin complex of three proteins
• attaches tropomyosin to actin
• holds tropomyosin over binding sites on G-actin.
• has binding site for Ca
• troponin I bound to actin
• troponin T bound to tropomyosin
• troponin C binds Ca
• binding of Ca causes troponin to "shift" and
  remove tropomyosin from G-actin binding sites

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Muscle Proteins

• Thin filaments contain mostly Actin
• Have a myosin binding site
• Tropomyosin covers binding site
• Troponin holds Tropomyosin in place

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III. CONTRACTION AND RELAXATION OF SKELETAL
MUSCLE FIBERS
A. The Sliding Filament Mechanism 1. The
contraction cycle 2. Excitation-Contraction
coupling 3. Length-tension relationship B. The
Neuromuscular Junction 1. Release of ACh 2.
Activation of the ACh receptors 3. Production of
muscle action potential 4. Termination ACh
activity
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Sliding filament mechanism of muscle contraction,
as it occurs in two adjacent sarcomeres. During
muscle contractions, thin filaments move toward
the M line of each sarcomere.
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Contraction and Relaxation of Skeletal Muscle
Fibers

• The sliding filament mechanism

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The contraction cycle

• When stimulated, crossbridges attach to myosin
  binding site on actin
• Each cross bridge attaches and detaches several
  times during a contraction

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The contraction cycle

• Crossbridges act like tiny ratchets to propel
  thin filaments toward center of sarcomere

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The contraction cycle

• Thus, Sarcomere is shortened
• Distance between Z lines is shortened
• I bands and H zones are shortened
• A bands move closer together, but do not change
  in length

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The contraction cycle. Sarcomeres exert force and
shorten through repeated cycle during which the
myosin heads attach to actin (crossbridges),
rotate, and detach. During the power stroke of
contraction, crossbridges rotate and move the
thin filaments past the thick filaments toward
the center of the sarcomere.
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Excitation-Contraction Coupling

• The events involved from
• excitation (receiving and propagating a stimulus)
  
• to contraction (actual sliding of the filaments).

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Length-Tension Relationship

• Tension depends on the degree of overlap of thin
  and thick myofilaments

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Length-Tension Relationship

• Lots of overlap little tension

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Length-Tension Relationship

• Some overlap allows for maximum tension

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Length-Tension Relationship

• Little overlap little tension
• No overlap no tension

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The Neuromuscular junction

• Contractions are controlled by nervous system via
  Neuromuscular Junctions
• Neuromuscular Junctions
• area of contact between neuron and sarcolemma

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Terminology associated with the Neuromuscular
Junction

• Synapse
• region where nerve cell communicates with other
  cell through the release of neurotransmitters
  called Acetylcholine (Ach)

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Terminology associated with the Neuromuscular
Junction

• Synaptic gap
• space between the synaptic terminal and the
  sarcolemma
• Synaptic end bulbs
• expanded end of axonal branches

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• Motor End Plate
• area on the sarcolemma where acetylcholine
  receptors are found

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Contraction of Skeletal Muscle Fibers

• Nerve impulses from the axon release ACh via
  exocytosis into the synaptic cleft
• ACh diffuses across synaptic cleft and binds to
  receptors on sarcolemma

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Contraction of Skeletal Muscle Fibers

• ACh binding opens sodium ion channels and causes
  an influx of sodium ions into the sarcoplasm
• This brings on an ACTION POTENTIAL that spreads
  all over via T tubules

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Contraction of Skeletal Muscle Fibers

• Sarcoplasmic reticulum releases calcium ions
• Calcium ions bind with troponin and expose the
  myosin binding site
• Muscle fibers then contract

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Relaxation of Skeletal Muscle Fibers

• Release of ACh stops
• Thus release of calcium ions stops
• Calcium is actively pumped back into sarcoplasmic
  reticulum
• Calcium ions detach from troponin
• Tropomyosin covers up myosin binding site
• Thus cross bridges cannot attach
• Contraction stops

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• Rigor Mortis
• No ATP production because youre dead!
• No ATP no Ca pump
• No Ca pump Ca remains in sarcoplasm
• Binding sites remain exposed and myosin can
  always bind.
• A few last power strokes that use up extra ATP
• Myosin heads get "stuck" to G-actin in low energy
  state/position and cannot release without ATP.
• Thick and thin filaments bond in partial
  contraction.

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Where does the come from?
ENERGY
ATP
Adenosine Triphosphate
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ATP

• ATP provides energy for contractions
• ATP is needed to detach cross bridges
• ATP is needed to pump calcium ions back into the
  sarcoplasmic reticulum
• ATP needs to be regenerated for contractions to
  continue

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regeneration
ATP

• 1. Creatine Phosphate
• Resting skeletal muscle needs little ATP
• Excess ATP is temporarily stored as creatine
  phosphate
• Used to form ATP to supply quick bursts of energy
• creatine phosphate transfers phosphate to ADP to
  create ATP
• another 5-10 seconds of contraction

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regeneration
ATP

• 2. Anaerobic Respiration
• Occurs in sarcoplasm
• Provides ATP during peak levels of muscular
  activity
• Requires no oxygen
• Produces small amounts of ATP
• Produces lactic acid as byproduct

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regeneration
ATP

• 3. Aerobic Respiration
• Occurs in mitochondria

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regeneration
ATP

• 2. Aerobic Respiration
• Requires oxygen
• Produces vast amounts of ATP
• Provides 90 of ATP for activities that last
  more than ten minutes

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Muscle Fatigue

• Muscle Fatigue inability of muscle to contract
  forcefully after prolonged activity.
• Possible causes
• Inadequate release of Ca from SR
• Depletion of creatine phosphate
• Insufficient oxygen
• Depletion of glycogen
• Buildup of lactic acid
• Insufficient ACh

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Oxygen Consumption after Exercise

• Oxygen Debt is Paid
• Extra amount of oxygen are needed for the
  restorative effort
• Evident in heavy breathing after exertion
• Lactic Acid Removal and Recycling
• Lactic acid is converted to pyruvic acid (used by
  mitochondria to produce ATP)

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Oxygen Consumption after Exercise

• Oxygen debt additional O2 that must be taken in
  to replenish E stores and O2 levels in muscle
  fiber
• To convert lactic acid back to glycogen
• To resynthesize creatine phosphate and ATP
• To replace oxygen removed from myoglobin

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Oxygen Consumption after Exercise

• Elevated body temperature increases rate of
  chemical reactions
• Heart and respiratory muscles work harder
• Tissue repair is ongoing

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Control of Muscle Tension

• Total force of contraction depends on
• Total number of muscle fibers stimulated
• Frequency of stimulation
• Amount of stretch prior to contraction
• Nutrient and oxygen availability

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Motor Unit

• Motor unit a single neuron and all the
  muscle fibers it innervates
• A single neuron can stimulate hundreds of muscle
  fibers

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Motor Unit

• Motor unit a single neuron and all the
  muscle fibers it innervates
• Each muscle fiber is innervated by only one motor
  neuron, but one motor neuron may innervate many
  fibers.

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Motor Unit

• In muscles requiring fine control (eye movement),
  a motor neuron may innervate only one fiber.
• In larger muscles with less precise movements, a
  single motor neuron may innervate 100's of fibers
• muscle fibers of any single motor neuron are
  scattered throughout the muscle. Stimulating a
  single motor unit results in weak contraction of
  entire muscle.

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Motor Units

• Each muscle has numerous motor units

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Motor Unit

• Smaller motor units in muscles for fine movement
• Not all motor units are active at all times
• Recruitment increasing tension through an
  increase in active motor units

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Twitch Contraction

• Single stimulus-contraction-relaxation occurrence
• Has three distinct phases
• Latent period
• Contraction period
• Relaxation phase

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Frequency of Stimulation

• Wave Summation
• Second contraction occurs before first
  contraction completely relaxes
• Waves of contraction with increasing intensity
  result

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Frequency of stimulation

• Unfused Tetanus
• Stimuli arrive close together
• Partial relaxation still occurring

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Frequency of stimulation

• Fused Tetanus
• Stimuli arrive VERY close together
• NO relaxation possible
• Occurs in most normal muscle contractions

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Motor Unit Recruitment

• Defined as the process in which the number of
  active motor units increases
• Not all motor units are active at all times
• Some are active, some rest
• Delays onset of muscle fatigue

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• Muscle Tone resting tension in a
  skeletal muscle
• Some motor units are always active but not
  enough to produce movement.
• Ex. Postural muscles

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Motor Unit

• Poor muscle tone flaccid, limp muscle
• Possibly due to injury
• Good muscle tone firm, solid muscle

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Types of Contraction

• Isotonic muscle changes in length as
  tension rises
• a. Concentric isotonic contraction muscle
  shortens as tension exceeds resistance
• b. Eccentric isotonic contraction
• muscle lengthens as resistance exceeds tension

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Types of Contraction

• Isometric muscle does NOT change in
  length as tension rises
• Tension never exceeds resistance
• Ex. Pushing against an immovable object

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Muscle Performance

• Depends on types of muscle fibers AND physical
  conditioning or training

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Types of Muscle Fibers

• - classified based on
• 1. amount of myoglobin
• 2. speed of contraction
• Three types
• Slow oxidative fibers (SO)
• Fast oxidative-glycolytic fibers (FOG)
• Fast glycolytic fibers (FG)

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Types of Muscle Fibers

• Slow Oxidative Fibers
• Take three times as long to contract as fast
  fibers
• Half the diameter of fast fibers
• Dont fatigue easily
• Have lots of mitochondria
• Have lots of capillaries and myoglobin
• Appear dark red

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Types of Muscle Fibers

• Fast Oxidative-Glycolytic Fibers
• Have intermediate properties
• Have lots of capillaries and myoglobin
• Appear dark red
• Generate ATP via aerobic cellular respiration
• Great for aerobic endurance

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Types of Skeletal Muscle Fibers

• Fast Glycolytic Fibers
• Contract quickly and strongly
• Large diameter lots of myofibrils
• Have few mitochondria and capillaries and less
  myoglobin
• Fatigue easily

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Distribution and Recruitment of Different Types
of Fibers

• most muscles have all 3 types in different
  proportions
• muscles for posture - more SO
• eye muscles - more FG
• these proportions are genetically determined, but
  can be changed with training 

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Exercise and Skeletal Muscle Tissue

• Distribution of muscle fiber types is genetically
  determined
• Endurance exercises can transform some fast
  glycolytic fibers into fast oxidative-
  glycolytic fibers

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Exercise and Skeletal Muscle Tissue

• Exercises that focus on quick and strong
  contractions can increase size and strength of
  fast glycolytic fibers
• More thick and thin
• Results in hypertrophy

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• SMOOTH MUSCLE TISSUE
• Types of smooth muscle
• Visceral or Single-unit SM
• cells connected by gap-junctions ? AP spreads to
  all connected cells
• all contract as a unit SYNCYTIUM
• some contract spontaneously, e.g., SM of
  digestive tract

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• SMOOTH MUSCLE TISSUE
• Types of smooth muscle
• 2. Multi-unit SM
• gap junctions are rare
• spontaneous and synchronous depolarizations
  infrequent
• individually innervated, cells act independently
  
• examples - iris ciliary muscle, sm of large
  airways to lungs, sm in large arteries, arrector
  pili muscles of skin (hair follicles)

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A. Microscopic Anatomy of Smooth
Muscle Differences from striated

• innervated by autonomic NS
• 116 thick thin
• no sarcomere arrangement (no striations) ? more
  random arrangement.
• microfilaments run in two layers
• longitudinal layer contraction shortens cell
• circular layer contraction lengthens cell

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A. Microscopic Anatomy of Smooth
Muscle Differences from striated

• peristalsis alternating contraction and
  elongation of these layers.
• no highly structured neuromuscular junction
• diffuse junction wide synaptic cleft in general
  area of cells
• nt released into ECF surrounding cell ?able to
  bind over entire surface and allows effects of
  hormones in blood

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A. Microscopic Anatomy of Smooth
Muscle Differences from striated

• poorly developed SR Ca from ECF
• thin filament has no troponin ?tropomyosin not
  over binding sites on actin.
• myosin head in low-energy state during rest (or
  they would bind).
• calmodulin complex is Ca receptor attached to
  thick filaments
• more efficient, less energy, slower to contract,
  slower power stroke, can sustain contraction
  longer, involuntary control - autonomic NS
  

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B. Physiology of Smooth Muscle

• Remember
• myosin heads in low energy state/position during
  rest
• binding sites exposed
• no neuromuscular junction
• Diffuse Junction neurotransmittert Acetylcholine
  and Norepinephrine (also Epinephrine thru circ
  system) released in to ECF surrounding cell
  (effect of hormones in blood)

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Physiology of Smooth Muscle

• AP thru neurons of autonomic NS
• nt released into diffuse junction (or epinephrine
  thru circ. system)
• nt binds to receptors over entire cell surface
• sarcolemma depolarization
• Ca channels in sarcolemma open
• Ca enters from ECF
• Ca forms complex with calmodulin (part of thick
  filament)
• complex binds with myosin heads (in low E during
  rest)
• provides heads with ATPase activity
• cleave ATP ?back to high energy state
• binding with actin
• power stroke