non-striated muscle i. locations of non-striated muscle in vertebrate body gi tract (including...
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Non-striated Muscle I
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Locations of non-striated muscle in vertebrate body
• GI tract (including internal, but not external sphincters)
• Blood vessels and lymphatics (except capillaries)
• Pulmonary airway
• Genitourinary tract
• Piloerectors attached to hairs
• Iris of eye
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Questions to consider
• Structure/function of the contractile machinery
• Control of sliding filaments
• Excitation-contraction coupling
• Control systems involving hormones and neurotransmitters
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Dense plaque or dense band attaches actin to sarcolemma
Organization of the contractile machinery in smooth muscle
Dense body made of alpha actinin attaches actin to actin
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Calciosomes
Dense plaque
This is rat ileal smooth muscle. The SR takes the form of small vesicles (calciosomes) near the sarcolemma.
Dense body
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On-off regulation of cross-bridges in smooth muscle is primarily based on the thick
filament, with secondary force control on the thin filament.
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Control of myofilaments: thin filament vs thick filament based control systems
• The classical thin filament based system: troponin and tropomyosin present and bound to thin filament, as in heart and skeletal muscle. The control by Ca++ cannot be removed by washing the filaments.
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Thick filament based: troponin is absent; control by Ca++ is lost when the filaments are washed and can be restored by adding back soluble factors, which are now recognized as calmodulin (CM) and myosin light chain kinase (MLCK)
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The sequence of events in thick filament control
• Ca++ binds to calmodulin
• Ca++-calmodulin binds to MLCK
• MLCK-CM-Ca++ phosphorylates myosin regulatory light chain
• Head associates with actin and begins to cycle
• Cycling continues until Pi is removed by myosin light chain phosphatase (MLCP)
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Thick-filament-based regulation through myosin light-chain kinase (MLCK)
MLCK phosphorylates the myosin regulatory light chain at site x - this allows the myosin head to cycle
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A Ca++-independent mechanism of relaxation -
Protein Kinase C phosphorylates myosin light chain at site y - this antagonizes activation by phosphorylation at site x.
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Single Unit vs Multiunit
• Multiunit muscle is not connected by gap junctions - each cell must receive innervation. Multiunit muscle shows little spontaneous activity and is controlled by its dual autonomic innervation..
• Single Unit (or unitary) muscle is joined by extensive gap junctions so multiple fibers act as a unit. Spontaneous activity is typical of such muscle; innervation modulates this activity.