Chapter 16 Continues

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Chapter 16 Continues
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Regulation by Ca2

• Nerve has action potential that will elicit the
  muscle contraction
• Nerve contacts at neuromuscular junction
• Axon terminals contain acetylcholine
• Muscle membrane under the axon is the motor end
  plate and contains acetylcholine receptors that
  are ligand gated channels
• Message spreads by way of the transverse (T)
  tubule system that causes the SR to release Ca2
• Muscle contraction

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SR in Ca2 Release and Uptake

• SR has 2 components medial element and terminal
  cisternae
• Terminal cisternae releases Ca2 and also has
  high concentration of ATP-dependent Ca2 pumps
• T tubule and terminal cisternae make up the triad
  connected by junctional complex
• Ca2 released when action potential moves thru
  the T tubule system

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Ca2 Release
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Cardiac Muscle

• Similar in structure to skeletal muscle
• Difference 1 ATP energy comes from fatty acids
  rather than glucose, carried by serum albumin
• Difference 2 not multinucleated, cells joined
  end to end by intercalated disc, electric impulse
  moves thru gap junctions to continue contraction

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Smooth Muscle

• Involuntary contraction
• Slower than skeletal/cardiac contraction, lasts
  longer
• Structure no striations, no Z-line, contains
  dense bodies

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Smooth Muscle Contraction

• Dense bodies are on the cell membrane and in the
  cytoplasm
• Intermediate filaments and actin attach to the
  dense bodies and crisscross the length of the
  cell

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Contraction

• Increase in Ca2 leads to cascade of events
• activates calmodulin and the Ca2-calmodulin
  complex activates myosin light chain kinase
  (MLCK)
• MLCK phosphorylates myosin light chain
  (regulatory protein)
• myosin makes filaments when phosphorylated
• also activates myosin to interact with actin

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MLCK Activation
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MLCK Regulation

• Autoinhibition
• MLCK has a pseudo-substrate which prevents it
  from phosphorylating myosin light chain
• Ca2-calmodulin complex prevents the
  pseudo-substrate from binding MLCK and therefore
  get phosphorylation of myosin light chain
• Decreasing Ca2 levels activate myosin light
  chain phosphatase to remove the PO4 and again
  allowing the pseudo-substrate to bind

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Actin-Based Motility in Non-Muscle Cells

• Cell migration
• Amoeboid motion
• Ctyoplasmic streaming
• Chemotaxis

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Events of Cell Migration

• Extension of protrusions on leading edge
• 2 types
• lamellapodia thin sheets of cytoplasm
• filopodia thin pointed structures
• Attach protrusions to substrate
• Generation of tension to pull cell forward
• Release attachment and retract tail

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Migration
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Lamellapodia Formation

• Retrograde flow of F-actin bulk of movement of
  MFs is towards the rear of protrusion
• 2 simultaneous processes
• actin assembly towards protrusion
• rearward translocation of actin to back of
  protrusion
• Non-muscle myosin motor involved

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Amoeboid Movement

• Pseudopodia involved
• Gelation and solation of actin cytoskeleton
• 2 layers
• outer gelatinous ectoplasm which is gel state
• inner, more fluid endoplasm which is the sol
  state
• Transition between gel or sol state

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Cytoplasmic Streaming

• Actomyosin-dependent movement
• actin, also some specific myosins, ATP dependent
• Happens in cells not doing amoeboid movement
• Plants do cyclosis stream around central
  vacuole circulate and mix cell contents

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Chemotaxis

• Occurs usually on one side of the cell
• Migrating cells move towards an increase or
  decrease in the concentration of diffusible
  substances
• move to increase concentration is a
  chemoattractant move to high concentration
• move to decrease concentration is a
  chemorepellant move away from high concentration