Term
| T/F Smooth muscle fibers are arranged in sarcomeres. |
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Definition
| False--only skeletal and cardiac muscles are arranged in sarcomeres. |
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Term
| Which type of muscle cells does contraction last longer in skeletal or smooth? |
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Definition
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Term
| What are the different types of smooth muscle cells? |
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Definition
| Single unit smooth muscle cells and multi-unit smooth muscle cells. |
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Term
| What are 4 characteristics of single unit smooth muscle cells? |
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Definition
-cells are electrically connected via gap junctions
-function in synctytial fashion
-electrical stimulation of one cell leads to stimulation of adjacent cells through the gap junctions
-connections through gap junctions lead to waves of contraction |
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Term
| What are some characteristics of multi-unit smooth muscle cells? |
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Definition
-cells are not electrically connected
-each cell has to be individually stimulated |
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Term
| Where are actin and myosin mainly located in smooth muscle cells? |
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Definition
| Around the periphery of the cell |
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Term
| T/F Actin myosin ratio is great in skeletal muscle than in smooth muscle. |
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Definition
| False. There is greater actin to myosin in smooth muscle cell |
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Term
| Are there T-tubules in smooth muscles cells? |
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Definition
| No there is sarcoplasmic reticulum, but no T-tublules. |
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Term
| What is required to initiate contraction in smooth muscle cells? What is the mechanism for this? |
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Definition
An increase in intracellular calcium.
Mechanism:
-Ca can enter cell through voltage gated and ligand gated calcium channels
-Calcium can be released from SR when IP3 channel is activated |
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Term
| How is the IP3 channel activated? What does activation lead to? |
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Definition
| The IP3 channel is activated when a hormone or neurotransmitter binds to a G protein coupled receptor. This G protein activated PLC which converts PIP2 into IP3. IP3 binds to the IP3 channel and calcium is released from the SR into the cytoplasm. |
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Term
| If calcium is removed from a smooth muscle cell what happens? |
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Definition
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Term
| What is the mechanism for contraction in smooth muscle cells? |
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Definition
-Intracellular calcium concentrations increase
-Ca binds to calmodulin forming Ca-Calmodulin
-Ca-calmodulin activates myosin light chain kinase (MLCK)
-MLCK phosphorylates light chains in myosin heads and increases myosin ATPase activity
-Active myosin crossbridges slide along actin and create muscle tension. |
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Term
| Describe the mechanism for cross bridge cycling in smooth muscle cells? |
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Definition
-ATP binds to myosin, releasing actin
-ATPase on globular head of myosin hydrolyzes ATP to ADP and Pi
-the release of ADP and Pi cuases the myosin head to turn causing a ratcheting movement
-cycling will continue as long as MLC is phosphorylated and Ca is high enough |
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Term
| How is smooth muscle cell contraction regulated? How does this differ from skeletal muscle contraction? |
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Definition
| Smooth muscle contraction is thick filament regulated. Skeletal muscle contraction is thin filament regulated |
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Term
| Describe the mechanism for relaxation of smooth muscle cells? |
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Definition
-SERCA pumps calcium back into the SR
-calcium pump and Na-Ca exchanger in the plasma membrane pumps calcium out of cell
-decrease in intracellular calcium decreases the amount of ca-calmodulin.
-Decrease in ca-calmodulin means less active MLCK to phosphorylate
-Myosin phosphatase removes the phosphate from myosin, leading to a drease in myosin ATPase activity
-Myosin can no longer bind to actin and thus no cross bridge cycling leading to a decrease in muscle tension |
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Term
| Does dephosphorylation of MLCK automatically lead to relaxation? |
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Definition
| No, could simply be sustained contraction due to the latch bridge or other mechanisms |
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Term
| Define phasic contractions. |
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Definition
| Phasic contractions are those that contract rhythmically or intermittently |
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Term
| Define tonic contractions. |
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Definition
| Tonic contractions contract continuously |
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Term
| Describe, mechanistically, the differences between phasic and tonic contractions? |
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Definition
| During phasic contractions calcium levels, cross bridge phosphorylation, and force reach peak and then return to baselline. During tonic contractions intracellular calcium and cross-bridge phosphorylation decline after initial peak, but they do not return to baseline. |
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Term
| Describe the latch state of smooth muscle cells. |
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Definition
| The latch state occurs during tonic muscle contractions and is when myosin stays attached to actin and force continues to be generated though cross bridge cycling is not occuring. |
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Term
| What are three ways smooth muscle contraction is regulated? |
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Definition
-slow wave-which are rhythmic changes in membrane potential, contraction occurs when threshold is crossed
-pacemaker- regular depolarizations leading to regular rhythms of contraction
-phrmacomechanical
-change in tension without change in potential |
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Term
| What are various ways to induce contraction is smooth muscle cells? |
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Definition
-increase the activity of calcium channels
-inhibit MLC phosphatase
-add ACh-makes the membrane potential more positive |
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Term
| What are various ways to induce relaxation of smooth muscle cells? |
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Definition
-block calcium channels
-open potassium channels
-add epinephrine/norepinephrine to make the memrane potential more negative
-increase levels of cGMP
-increase levels of cAMP |
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Term
| How are levels of cGMP increased and what effects does increasing cGMP have on smooth muscle cells? |
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Definition
| Certain hormones, drugs, or activation of the autonomic nervous system increase levels of cGMP. An increase in cGMP leads to activations of kinases. Increased cGMP may also activate MLC phosphatase and/or reduce intracellular calcium levels |
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Term
| How are cAMP levels increased and what does this lead to? |
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Definition
| Increase of cAMP occurs through stimulation of b adrenergic or adenosine receptors. The increase in cAMP then leads to cAMP dependent kinases phosphorylating MLCK. This leads to reduction in calcium dependent increase in MLCK activity. If MLC cannot be phosphorylated then it cannot contract. Increase in cAMP my also reduce intracellular calcium. |
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Term
| What is the difference between the relationship of length and tension in smooth muscle cells and skeletal muscle cells. |
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Definition
| For skeletal muscle L0 will generate the most force, however smooth muscles exhibit length adaptation which allows smooth muscle to generate maximal tension at different lengths. |
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Term
| Mechanistically, how does does length adaptation occur in smooth muscle cells? |
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Definition
| Length adaptation is though to involve a change in the number of contractile units. |
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Term
| How are force and velocity related in smooth muscle cells? |
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Definition
| Force and velocity is dependent on load--an increase in force a decrease in velocity. |
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Term
| Which contracts faster slow skeletal muscle or smooth muscle? |
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Definition
| Slow skeletal muscle contracts faster than smooth muscle. |
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Term
| How are smooth muscle velocities regulated? |
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Definition
| By phosphorylation of myosin light chain, increased phosphoryation leads to more actin-myosin interactions and more force generated. |
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