1: Smooth muscle

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Created by
Karlo Catabian

Cards (10)

  • Smooth muscle will usually either surround...
    • a hollow organ or tube (GI tract, bladder, uterus), wherein contraction generates a pressure to propel the contents, or...
    • many tubes (blood vessels, airways), wherein contraction changes diameter to increase or decrease resistance to flow.
  • There are multiple input types for smooth muscle, which can innervate muscle simultaneously:
    • Autonomic neurons: sympathetic and/or parasympathtic
    • Circulating hormones
    • Paracrine factors
  • Smooth muscle has a slow myosin ATPase (rate?).
  • Contraction in smooth muscle starts via phosphorylation of regulatory light chain, i.e. thick filaments.
    1. Ca2+ binds to calmodulin (CM)
    2. Ca2+ CM complex activates myosin light-chain kinase
    3. MLCK activity increases
    4. MLC phosphorylation increases
    5. More cross-bridge cycling = force increases
    6. Ca2+ decreases; MLC phosphatase activity increases relative to MLCK
  • Relaxation: Ca2+ is removed from the cytosol by…
    • Ca2+ ATPase on plasma membrane
    • Ca2+ ATPase on SR membrane (SERCA)
    • Na+/Ca2+ exchanger on the plasma membrane
  • Control of smooth muscle Ca2+ influx into the cytosol:
    1. Voltage-gated Ca2+ channels; open due to AP
    2. Stretch-sensitive Ca2+ channels
    3. Chemical messengers interacting with G-protein receptors; open IP3R channels on the SR membrane (no AP needed)
    4. Receptor operated channels; open RyR channels on SR membrane alongside Ca2+ influx from plasma membrane channel (no AP needed)
  • Pacemaker potentials:
    1. Ca2+ dependent K+ channels close, depolarization to threshold
    2. Voltage-gated Ca2+ channels open, AP occurs
    3. Ca2+ dependent K+ channels open, repolarization
    4. Voltage-gated Ca2+ channels close
  • The depolarizing phase of a smooth muscle action potential is due to Ca2+ influx through V-gated Ca2+ channels.
  • Single unit smooth muscle allows a whole tissue to act synchronously, where one input affects all cells in the unit; it contains pacemaker cells, cell connections through gap junctions, and can be initiated by stretch.
  • Multi unit smooth muscle does not contain pacemaker cells, and activity is usually innervated by second messengers (neurotransmitters or hormones) or by graded potentials.