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Phys Exam 1: Ch 8

Contraction and Excitation of Smooth Muscle

QuestionAnswer
Compare and contrast the architecture of smooth vs. skeletal muscle (also review slides) Skeletal: well organized, MUCH larger than smooth muscle, I-band, Z-line, A-band, M-line… Smooth: can be organized or very irregular, can be multi-unit or unitary; dense bodies = Z-line (anchor actin filaments, can be bonded w/adjacent cells
Describe the structure, locations, function, and nervous innervation of multi-unit smooth muscle - single fibers w/o gap jxns - innervated by single nerve ending - covered by thin basement membrane of collagen & glycoprotein - Ciliary muscles – focus eye lens - Iris muscles – contraction & dilation of iris - Piloerector muscles of hair follicles
Describe the structure, locations, function, and nervous innervation of unitary smooth muscle -sheets/bundles of fibers connected by protein adhesions -Gap jxns allow ion flow -visceral organ walls, GI tract, bile ducts, ureters, uterus, blood vessels -regulates arterial pressure & local blood flow, Peristalsis, Micturition reflex, Parturition
TRUE or FALSE: Unitary smooth muscle fibers are single-independent fibers covered by a thin basement membrane FALSE: multi-unit; unitary – sheets/bundles of fibers connected by protein adhesions
TRUE or FALSE: Multi-unit smooth muscle fibers are more likely to play a role in fine adjustments, e.g. focusing of the eye lens for close-up vision TRUE
TRUE or FALSE: Unitary smooth muscle fibers are often connected by gap junctions allowing easy passage of ions from cell-to cell TRUE
TRUE or FALSE: Unitary smooth muscle fibers receive no innervation FALSE – they receive nervous excitation at varicosities
TRUE or FALSE: Each smooth muscle fiber, whether unitary or multi-unit is directly innervated at a neuromuscular junction FALSE – (1) each muscle fiber in multi-unit smooth muscle is directly innervated, not each unitary muscle fiber is; (2) Smooth muscle receives innervation at varicosities, not NMJ
TRUE or FALSE: ACh is responsible for all nervous excitation of smooth muscle FALSE – smooth muscle receives excitation 3 ways: (1) Nervous @ varicosities (ACh, Norepi, others); (2) Hormonal (endocrine) signals (epi, Ang II, endothelin, ADH); (3) Mechanical (stretch)
A lower ATP requirement for sustained muscle contraction Does this describe SMOOTH muscle, SKELETAL muscle, or both? SMOOTH: 1/10 to 1/300 as much ATP is needed to sustain tension compared to skeletal
A faster rate of cross bridge cycling Does this describe SMOOTH muscle, SKELETAL muscle, or both? SKELETAL: smooth is 1/10 to 1/300 slower b/c myosin cross-bridges remain attached to actin longer
A requirement for a rise in cytoplasmic calcium to initiate contraction Does this describe SMOOTH muscle, SKELETAL muscle, or both? BOTH
Myosin heads with high-intrinsic ATPase activity Does this describe SMOOTH muscle, SKELETAL muscle, or both? SKELETAL
Phosphorylation of regulatory light chains of myosin heads increases ATPase activity Does this describe SMOOTH muscle, SKELETAL muscle, or both? SMOOTH
Be able to give the step-wise mechanism of smooth muscle contraction from initiation by Ca --> termination Ca inc -> bind CaM -> activate MLCK -> phosphorylates light chains of myosin, inc ATPase activity -> ATP hydrolysis -> Actin cross-bridge forms -> power stroke -> ATP released, new ATP binds -> repeats until Ca is cleared and light chains dephosphorylated
Why are both Ca2+ clearance from the cytoplasm and myosin phosphatase required for termination of smooth muscle contraction? Ca clearance required b/c Ca activates the contraction mechanism . Clearing Ca prevents calmodulin from activating MLCK. Myosin phosphatase required b/c will dephosphorylate myosin head, dec ATPase activity, preventing ATP hydrolysis & cross-bridging
Name 3 mechanisms for Ca2+ clearance from the cytosol that participate in termination of smooth muscle contraction 1. Ca returns to the SR via a Ca-ATPase pump (SERCA pump) 2. Ca goes out of the cell via a Na-Ca exchanger 3. Ca goes out of the cell via a Plasma Membrane Ca-ATPase pump (PMCA)
TRUE or FALSE: Excitation of smooth muscle can occur without an action potential TRUE – pacemaker potential
What are the 3 patterns of smooth muscle APs? Spike Potentials, Slow Wave (Pacemaker) Potentials, Plateau Potentials
What is the mechanism of depolarization & repolarization for spike and plateau potentials? Depolarization: requires opening CaV -Open/close slower than VG Na channels -APs in skeletal muscle: fast (<1.0ms) -APs in smooth muscle are slower: Spike: 10-50ms; Plateau: 0.3-1.0s Repolarization: require opening VG-K channels & Ca-gated K channels
Why does the plateau potential have the plateau? opening of these K channels is delayed
What is the main theory behind Pacemaker (slow wave) Potentials? a resting membrane potential of -50 to -60mV, some CaV have increased open probability
Discuss the mechanism by which Angiotensin II stimulates contraction of vascular smooth muscle Ang II binds receptor, causing vasoconstriction -Gq-coupled GPRCs (α1-adrenergic) -αq subunit dissociates, activates PLC -PLC activation causes release of DAG, IP3 -DAG activates PKC, IP3 signals Ca release from SR -Ca release causes contraction
Discuss the mechanism by which albuterol (a B2-adrenergic receptor stimulating drug) would be expected to relax bronchiolar smooth muscle in an asthmatic patient Binds G2-coupled GPCR receptors (B2-adrenergic): -α subunit dissociates from the G-protein complex, activates AC -Activation of AC causes up-regulation of cAMP -Incr cAMP activates PKA -PKA phosphorylates MLCK, dec MLCK activity, result: relaxation
A signal that elevates cAMP levels in smooth muscle cells Would this be expected to cause contraction or relaxation of smooth muscle? RELAX
A signal that activates myosin light chain kinase Would this be expected to cause contraction or relaxation of smooth muscle? CONTRACT
A signal that increases levels of IP3 in smooth muscle cells Would this be expected to cause contraction or relaxation of smooth muscle? CONTRACT
A signal that increases cytoplasmic Ca2+ levels in smooth muscle cells Would this be expected to cause contraction or relaxation of smooth muscle? CONTRACT
A signal that increases outward K+ current in smooth muscle cells Would this be expected to cause contraction or relaxation of smooth muscle? RELAX
A signal that stimulates a Gq-coupled GPCR: would this be expected to cause contraction or relaxation of smooth muscle? CONTRACT
A signal that stimulates a Gs-coupled GPCR: would this be expected to cause contraction or relaxation of smooth muscle? RELAX
A signal that stimulates production of nitric oxide Would this be expected to cause contraction or relaxation of smooth muscle? RELAX
A signal that inhibits the activity of myosin light-chain phosphatase Would this be expected to cause contraction or relaxation of smooth muscle? CONTRACT
Created by: hclark86