Muscular System

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Muscular System

• Anatomy Physiology

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Muscles

• From the Latin mus meaning little mouse (Flexing
  muscles looked like mice scurrying under the
  skin)
• Have ability to transform ATP into mechanical
  energy
• Muscles can only pull, never push, which allows
  them to exert a force

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ATP --gt ADP P Energy
ADP P Energy ---gt ATP
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Muscle Types
Actin
Myosin

• Skeletal - Slow to Fast twitch
• Cardiac - Fast twitch
• Smooth - Slow twitch
• Twitch contraction
• Skeletal smooth muscle cells are elongated
  called fibers
• All have contractile myofilaments actin myosin

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Muscle Fxns

• -Produce movement
• Skeletal locomotion manipulation in response
  to the environment
• Cardiac moves blood
• Smooth propels (squeezes) stuff through the
  digestive, urinary, circulatory, and reproductive
  systems
• -Maintaining posture
• -Stabilizing joints
• -Generating heat (40 of your body heat)

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Functional Characteristics of Muscle

• Excitability (Irritability) the ability to
  respond to a stimulus
• Contractility the ability to shorten forcibly
  when adequately stimulated
• Extensibility the ability to be stretched or
  extended
• Elasticity the ability of a muscle fiber to
  recoil resume its resting length after being
  stretched

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Skeletal Muscle

• Striated
• Multinucleate
• Voluntary muscles
• Can generate great power but fatigue quickly
• Non-rhythmic contraction

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Gross Anatomy of Skeletal Muscle

• Epimysium outermost layer of dense irregular
  connective tissue (Surrounds the whole muscle)
• Fascicle bundle of muscle fibers
• Perimysium fibrous C.T. which surrounds the
  fascicle
• Endomysium each muscle fiber is surrounded by
  reticular C.T.

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Sarcoplasmic Reticulum
Sarcolemma (Plasma membrane)
Sarcoplasm (Cytoplasm with lots of glycogen
stored)
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Sarcomere
Functional unit of muscle
Protein Titan
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Nerve Blood Supply

• Each muscle is served by
• 1 nerve
• 1 artery
• 1 or more veins

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Skeletal Muscle Attachment

• Skeletal muscles attach to bones in at least 2
  places
• When the muscle contracts, the moveable bone
  (Insertion), moves toward the immovable or
  less-moveable bone (Origin)
• Direct muscle attachment epimysium fused to
  periosteum
• Indirect muscle attachment tendon or
  aponeurosis (flat, sheet-like tendon)

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Skeletal Muscle Contraction

• Sarcomere - contractile unit (z-line to z-line)
• Myosin - thick filaments (contain ATPase which is
  used to split ATP to power muscle contraction)
  found in the dark A-band. Myosin heads form
  cross-bridges when attached to actin
• Actin - thin filaments found in the light I-band
  which are anchored to the z-line

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Sliding Filament Theory

• Hugh Huxley 1954 proposed that during contraction
  actin will slide past myosin which result in
  overlapping filaments
• Cross bridge attachment
• Power stroke (Myosin head pivots pulling actin)
• Cross bridge detachment (ATP binds to myosin head
  loosening the bond to actin)
• Cocking the myosin head ATPase hydrolyzes ATP
  to ADP Pi returning the myosin head to its
  cocked position

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Tropomyosin Troponin

• Tropomyosin stiffen the actin protein block
  myosin binding sites in relaxed muscle fibers,
  preventing myosin actin from forming a
  cross-bridge
• Troponin regulates cross-bridge formation. In
  the presence of Ca2 troponin moves tropomyosin,
  thereby exposing the myosin binding sites

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Z- Line - Defines each end of the sacromere. Thin
filaments of adjacent sarcomeres are linked
together here. A band - Consists of overlapping
thin and thick filaments. I band - Only thin
filaments. H zone - Only thick filaments. This
also shortens during contraction.
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Thin Fibers - Each thin filament is made of three
different proteins. Actin - Actin filaments are
made of subunits called G-actin these are
globular proteins which are linked together to
form a filament. Myosin binding site - Each
G-actin contains a binding site for myosin head
groups (part of the thick filament) Troponin -
This protein is associated with actin and it
binds calcium. Tropomyosin -The third protein of
the group when the muscle fiber is not
contracting, tropomyosin covers the myosin
binding site, preventing the myosin head groups
from binding to actin.
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Thick Fibers Consists of a bundle of proteins
called myosin Myosin Tails Each tail has two head
groups at the same end and each has two binding
sites. heads
form cross bridges Actin binding site - This
binds to the myosin binding site on actin, when
it is exposed as tropomyosin moves. ATP-binding
site - Binds to ATP splits the molecule and the
released energy is used to drive the movement of
the myosin head groups.
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Power Stroke
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Calcium

• Sarcoplasmic reticulum regulates intracellular
  Ca2 by storing releasing Ca2 when a stimulus
  causes the muscle to contract
• T (Transverse) Tubules extensions of the
  sarcolemma which allow for rapid impulse
  transmission through the muscle which ensures a
  single muscle contraction

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Regulation of Contraction

• Muscle contraction is stimulated by an action
  potential from a nerve
• The neuromuscular junction (motor end plate) in
  skeletal muscle is regulated by acetylcholine
  (ACh)
• Ach needs to be broken down as soon as it is
  used the enzyme acetlycholinesterase serves this
  function.

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• ACH released
• AP propagated along membrane and at T-tubules
• 3. Ca released from SR voltage gated Ca channel
  opens
• 4. Ca binds to Troponin-C conformation changes
  favor tropomyosin opens actin sites
• 5. myosin cross-bridges attach-detach from
  actin...pulls filament toward M-line
• 6. Ca removed (uptake by SR)
• 7. tropomyosin blocks actin sites relaxation

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Generating an Action Potential

• Depolarization (Na channels open)
• Repolarization (Na channels close K channels
  open)
• Refractory Period (K channels close)
• Na/K pump

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Homeostatic Imbalance

• Myasthenia gravis autoimmune disease where ACh
  receptors are broken down by ACh antibodies
  resulting in drooping eyelids and general muscle
  fatigue
• Curare arrowhead poison used in South America
  which blocks ACh receptors resulting in
  respiratory arrest death
• Cobra venom same as curare
• Botulinum toxin prevents ACh release
• Black widow spider venom releases all Ach
• Nerve gasses inhibit AChase which keeps cleft
  flooded with ACh

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ACh destruction

• After ACh initiates the action potential the ACh
  is broken down by acetylcholinesterase
• This prevents continued muscle contraction in
  the absence of additional nerve stimuli

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Mysostatins control muscle growth Absence of
results in enlarged muscle development
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Energy for muscle contraction

• ATP is needed for
• 1) contraction - Cocking and detachment of the
  myosin head.
• 2) calsequestrin - Pumping calcium into the SR of
  the sarcoplasm.
• 3) Na/K-ATPase - Needed for impulse conduction..

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ATP Production

• ATP lasts - Only a few seconds during active
  muscle contraction as ATP stores are used up.
• ATP is quickly reconstituted -There are several
  mechanisms that replenish the ATP stores.
• Sources of energy for ATP production
• 1) phosphocreatine (creatine phosphate, CP)
  ATP is produced from another high energy molecule
  called phosphocreatine.
• Creatine kinase
• Breaks down phosphocreatine, releasing a
  phosphate and energy. The energy is used to make
  new ATP.
• This store of energy lasts about 8-15 seconds.

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ATP Production

• 2) Anaerobic Respiration Glycolysis
• Glycogen - A polymer of glucose.
• For the muscle to recover the lactic acid needs
  to be removed quickly a well vascularized muscle
  serves this purpose.
• lasts about 2 minutes.
• Lactic acid/pyruvic acid
• The product of anaerobic respiration is pyruvate
  as no oxygen is available the pyruvate is
  converted to lactic acid.
• As lactic acid builds up in the muscles it
  changes the pH of the tissues in the muscle,
  which causes a decrease in the efficiency of
  proteins and enzymes.
• This leads to soreness and fatigue.

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ATP Production

• Oxidative metabolism a.k.a. Aerobic Respiration -
  Oxygen used by mitochondria to produce 36 net
  ATPs (2 from glycolysis and 34 from Krebs and
  ETC)
• Can produce ATP indefinitely as long as you have
  oxygen and energy stores (fat, proteins or
  glucose.)