Skeletal Muscles

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Skeletal Muscles
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Anatomy and innervation of skeletal muscle tissue

• Connective tissue components
• Fascia (bandage) sheet or band of fibrous C.T.
  under the skin or around organs
• Superficial fascia (subcutaneous fascia)
• Areolar C.T. and adipose tissue
• Stores water and fat
• Reduces heat loss (insulates)
• Protects against trauma
• Framework for nerves and blood vessels

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• Deep fascia
• Dense irregular C.T. holds muscles together and
  separates them into groups
• 3 layers
• Epimysium surrounds the whole muscle
• Perimysium separates muscle into bundles of
  muscle fibers fascicles
• Endomysium covers individual fibers

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• These three layers come together to form cords of
  dense, regular connective tissue called tendons.
  Tendons attach muscle to the periosteum of bones.
• When the connective tissues form a broad, flat
  layer, the tendon is called an aponeurosis.

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Microscopic Anatomy

• Muscle cells are called muscle fibers or
  myofibers
• Plasma membrane sarcolemma
• Cytoplasm sarcoplasm
• Myoblasts fuse to form one myofiber several
  nuclei
• Myofibrils run lengthwise

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Myofibrils are made of filaments

• Thin filaments primarily actin
• Thick filament myosin
• Elastic filaments
• Sarcomeres are the basic, functional units of
  striated muscle fibers.

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Thick filaments

• Made of about 200 molecules of myosin
• Each myosin molecule has two heads
• Each head has an actin binding site and an ATP
  binding site
• The ATP site splits ATP and transfers energy to
  myosin head which remains charged (cocked)
  until contraction.

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Thin Filaments

• Actin molecules form a helix
• Each actin molecule has a myosin binding site
• Other proteins
• Tropomyosin long, filamentous protein, it wraps
  around the actin and covers the myosin binding
  sites
• Troponin a smaller molecule bound to
  tropomyosin, it has calcium binding sites.

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Sarcoplasmic reticulum

• Specialized smooth E. R.
• Tubes fuse to form cisternae
• In a relaxed muscle, S.R. stores Ca (Ca
  active transport pumps)
• When stimulated, Ca leaves through Ca release
  channels.

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Transverse tubules (T-tubules)

• Infoldings of sarcolemma that penetrate into
  muscle fiber at right angles to filaments. They
  are filled with extracellular fluid.
• T-tubules and the cisternae on either side form a
  triad.

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Blood and nerve supply

• Muscle contraction uses a lot of ATP
• To generate ATP, muscles need oxygen
• Each muscle fiber is in close contact with one or
  more capillaries
• Motor neurons originate in brain and spinal
  cord cause muscle contraction

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Motor unit

• A Motor Unit is made of one motor neuron and all
  the muscle fibers it innervates.
• These cells all contract together.
• A single motor unit can have 2 2,000 muscle
  fibers.
• Precise movements are controlled by small motor
  units, and large movements by large motor units.

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Neuromuscular Junction (NMJ)

• Nerves communicate with muscles and other organs
  at structures called synapses.
• Synaptic cleft gap between neuron and
  sarcolemma
• Axon releases a chemical called a
  neurotransmitter Acetylcholine (Ach)
• Axon branches into axon terminals.
• At the end of each axon terminal is a swelling
  called the synaptic end bulb.

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• The region across the synaptic cleft from the
  synaptic end bulb is called the motor end plate.
• The sarcolemma of the motor end plate is folded
  and contains many receptors for ACh .
• When a nerve impulse reaches the synaptic end
  bulbs, it causes synaptic vesicles to fuse with
  the membrane and release ACh by exocytosis.

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• Acetylcholine diffuses across the synaptic cleft,
  and binds with receptors on the motor end plate.
• This binding causes the receptor to change shape,
  and opens Na channels in the membrane.
• When enough Na channels are opened, an
  electrical current is generated and is carried
  along the sarcolemma. This is called a muscle
  impulse or muscle action potential. This
  electrical activity can be recorded in an
  electromyogram.

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Sliding Filament Mechanism

• When a nerve impulse reaches an axon terminal,
  the synaptic vesicles release
• acetylcholine (ACh)
• ACh crosses the synaptic cleft and binds with
• receptors on the motor end plate.
• This binding opens channels that allow
• sodium to rush in, beginning a muscle action
  potential in the sarcolemma.

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• The action potential or impulse travels down the
  sarcolemma and into the T-tubules, causing the
  sarcoplasmic reticulum to release Ca into the
  sarcoplasm.
• The Ca binds to the troponin, which changes
  shape, pulling the tropomyosin away from the
  myosin binding sites on the actin.
• The activated myosin attaches to the actin,
  forming actin/myosin crossbridges.
• The myosin head moves toward the center of the
  sarcomere, pulling the actin filaments past the
  myosin. This is called a power stroke.

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• When the myosin heads turn, they release ADP, and
  ATP binds to the heads.
• When ATP binds, it causes the myosin to release
  the actin.
• ATP is split, and the myosin heads again bind to
  the actin, but further down the filament.
• The myosin again pulls the actin.
• This action is repeated many times.
• The Z lines (discs) get closer together as the
  actin and myosin filaments slide past each other,
  and the muscle fiber shortens.

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Relaxation

• ACh is broken down by an enzyme called
  acetylcholinesterase.
• Action potentials are no longer generated, so the
  Ca release channels in the S.R. close.
• Ca active transport pumps take Ca out of the
  sarcoplasm and into the S.R. where it binds to a
  protein called calsequestrin.

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• As the Ca levels in the sarcoplasm fall,
  troponin releases tropomyosin, which falls back
  and covers the myosin binding sites on the actin.
• The thin filaments slip back into their relaxed
  positions.

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Rigor mortis

• After death, muscle cells begin autolysis, and
  Ca leaks out of the S.R.
• This causes muscles to begin to contract.
• Since the body is dead, no more ATP is produced.
• Without the ATP to recharge the myosin heads,
  they remain linked to the actin, and neither
  relax nor contract any further.
• After about 24 - 72 hours it disappears as the
  tissues begin to disintegrate.

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Origin and Insertions

• Origin the attachment of a muscle to the less
  movable part (torso, etc.)
• Insertion the attachment of a muscle to the
  more movable part

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Interactions of muscles

• Prime mover the muscle primarily responsible
  for a movement
• Synergist stabilizes or assists prime mover
• Antagonist opposes action of prime mover and
  must relax for prime mover to contract completely