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Smooth Muscle - Powell
Smooth muscle

Additional Physiology Flashcards




The dense bodies of smooth muscle are analogous to what structures in skeletal muscle?

Z disks

What are the different types of action potentials in smooth muscle?

Spike (typical): generated by an external stimulus (electrical, hormone, stretch, spontaneous)


Plateau: characteristic of more prolonged contractions (uterus, ureter, vascular smooth muscle)


Self-excitatory: generated without external stimuli (intestinal walls, gut)


Slow waves: aka pacemaker waves; low oscillation of RMP - don't reach threshold unless strong enough

    • if threshold is reached (-35mV), train of action potentials occurs - rythmical contractions
    • Interstitial cells of cajal: pacemakers for the electrical and contractile activity of smooth muscle
Describe excitation-contraction coupling in smooth muscle
  • Action potential is carried out throughout the fiber
  • calcium enters the cell from the ECF via voltage-gated Ca2+ channels
  • calcium binds to calmodulin and activates it
  • calmodulin then activates myosin kinase, which then phosphorylates myosin
  • phosphorylated myosin is then able to bind to actin
Describe multiunit and unitary smooth muscle
  • Multiunit: electrical isolation of cells allows finer motor control; each fiber contracts independently of each other
    • action potentials not common b/c fibers are too small
    • ANS stimulation w/ ACh and NE; local depolarization spreads over fibers causing contraction
    • found in Iris, ciliary muscle of eye, vas deferens, piloerector muscles
  • Unitary (visceral): gap junctions b/t cells allows coordinated contractions
    • action potentials normall occur
    • 30-40 fibers must depolarize b4 action potential occurs
    • found in GI tract, bile duct, bladder, ureters, uterus, blood vessels
Describe Phasic and Tonic contractions

Phasic: characterized by twitch-like shortening of the muscle, followed by relaxation


Tonic: characterized by ability of muscle to sustain force of contraction with dec'd number of activated crossbridges, greatly reducing levels of energy consumption (ATP turnover); Calcium level doesn't drop back to intial level (unlike phasic) - they remain constant level above normal


List the ways calcium enters the smooth muscle during an action potential

  1. Voltage-gated calcium channels open with AP depolarization (direct entry - main)
  2. Ligand-gated channels are receptor-mediated by hormones and neurotransmitters (direct entry- main)
  3. IP3-gated SR channels are receptor mediated and by hormones and neurotransmitters (indirect entry by 2nd messenger- main)
  4. Leak channels (direct entry)
Describe Raynaud's Phenomenom

skin discoloration due to abnormal spasms of vascular smooth muscle of the peripheral tissues. Leads to diminished blood supply to local tissues


Discoloration is usually due to temperature or emotional changes: initially digits turn white due to dec'd blood supply; then they turn blue due to prolonged lack of oxygen; finally blood vessels reopen causing rush of blood "flushing", which turn digits red


Can ultimately lead to digital ulcerations

In what ways is cardiac muscle different from skeletal muscle?
  • underdeveloped SR
  • T tubules run in both transverse and longitudinal directions
  • cells function as syncytium (one unit)
  • cells joined at intercalated disks
  • cells held together by desmosomes
  • cells connected by gap junctions
  • cells are branched to form network
  • fibers are morphologically different in different areas of the heart
What is meant by calcium-induced calcium release?
The influx of calcium into the cell from the ECF (L-type channels) during an action potential causes more calcium to be released from the sarcoplasmic recticulum
Describe Starling's Law of the heart and the length/tension relationship

The greater the end diastolic volume (preload), the greater the ventricular pressure that can be developed


The resting length of the muscle fiber corresponds to the period of end diastolic pressure

In cardiac muscle, how do you change the strength of contraction?
Change end-diastolic volume or change cytosolic calcium = cardiac contractility

Describe contractility


How do you change contractility in cardiac muscle?


Contractility: the ability of myocardial cells to develop force at a given muscle length - correlates directly with calcium (internal)


Changing the strength of contraction without changing the length


To change: use Neurohumoral factors such as NE which increases sarcolemmal permeability to calcium; increase the frequency of contractons - can cause treppe, but not tetany due to long refractory period

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