Smooth Muscle Flashcards
Where are the smooth (S) mm found?
although the variety and function of S mm is amazing. It mostly surrounds hollow organs (blood vessels, bronchi, gut, reproductive and urinary tracts) and are arranged in sheets
What are some of the functions of S mm?
Maintains pressure-circular layer in airways, blood vessels
Mix and Propel contents-circular and longitudinal layers in intestinal wall
Regulates internal flow-rectangular layer in bladder, rectum, small testicular duct
In terms of contractile elements, what are the 2 structures missing in S mm but present in C mm, and SK mm?
Troponin- instead they have Calponin and Caldesmon
T-Tubules
What are other unique characteristics of S mm?
- Have calponin and caldesmon instead of troponin
- Lack T-tubule
- contain less developed SR
- Dense areas (points of mechanical coupling) analogs to Z lines in striated mm
- Gap junction (points of electrical coupling)
- Thick and thin filaments organized diagonally
Where does Ca need for contraction come from?
Ca needed for contraction come from the extracellular space and SR unlike in SK mm where it solely comes from SR.
What is the difference b/n tonic and phasic contraction?
Tonic: maintain a continuous level of partial contraction (tone). Ex: walls of blood vessels, airways of the lungs, sphincters. NO AP needed to initiate contraction but affected by Em, not at elevated Ca, sustained stimulation
Phasic: contract rhythmically or intermittently in GI, reproductive, and urinary systems. NEED AP to initiate contraction, short stimulation
Label and understand Tonic and Phasic contraction figures.
pp. 6
T/F Unlike in cardiac mm, S mm need extracellular Ca for contraction.
F. Both need extracellular Ca influx that causes CICR from SR. But S mm also uses the release of IP3.
How does IP3 result in cytoplasmic [Ca] increase?
- Agonist bind to Ca channels and activate a pathway that results in IP3 release.
- IP3 goes to SR to stimulate IP3 R
- Ca released from SR
Label/Study Sources of Ca for S mm contraction Figure
pp7
If S mm lack troponin, how does Ca control contraction in S mm?
Ca bind to calmodulin-Ca-calmodulin complex activates myosin light chain kinase (MLCK)-MLCK hydrolyzes ATP to phosphorylate inactive myosin-trigger crossbridge formation (Figure on pp. 8).
What is the Ca switch that turns on the contractile apparatus in S mm, and striated mm?
S mm: Ca dependent phosphorylation of MLC
Striated (C mm and SK mm): Ca binding to troponin
What is the rate-limiting step in S mm contraction?
Phosphorylation of the myosin head. Unlike SK mm, in S mm regulation of cross-bridge occurs on the thick filament.
Understand Figure showing rate limiting step.
pp. 9
T/F Myosin ATPase of S mm splits ATP at a much slower rate than SK mm.
T. S mm can generate more tension per unit of cross sectional area (Figure pp 10).
