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Phys Exam 1: Ch 8
Contraction and Excitation of Smooth Muscle
| Question | Answer |
|---|---|
| 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 |
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hclark86
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