Smooth Muscle Contraction

Question 1

what smooth muscle structural features?

Answer 1

• thick and thin filaments are not organised into myofibrils, and there are NO sarcomeres

Question 2

by what mechanism does smooth muscular contraction occur?

Answer 2

sliding-filament

Question 3

where is smooth muscle found?

Answer 3

surrounding hollow structures and organs that undergo changes in volume with accompanying changes in lengths of the smooth muscle fibres in their walls

Question 4

what are features of smooth muscle cells?

Answer 4

• absence of cross striations indicates that smooth muscle lacks myofibrillar structure
• contain only 10% of actin and myosin found in striated muscle
• actin:myosin ratio 15:1

Question 5

how is the cross-bridge activated/smooth muscle contracted?

Answer 5

• cross-bridge cycling in smooth muscle is controlled by Ca2+ regulated enzyme that phosphorylates myosin
• only phosphorylated form of smooth muscle myosin can bind to actin and undergo cross-bridge cycling
• this is done by myosin light chain kinase (MLCK)

Question 6

how is contracted smooth muscle made to relax, in relation to cross-bridge?

Answer 6

• myosin must be dephosphorylated because dephosphorylated myosin is unable to bind to actin
• the dephosphorylation is mediated by enzyme myosin light-chain phosphatase (MLCP)

Question 7

what are the steps in smooth muscle contraction?

Answer 7

1. initiated by calcium from ECF or SR
2. calcium binds to calmodulin (instead of troponin as in skeletal muscle)
3. ca-calmodulin-MLCK complex leads to phosphorylation of MLC (requires 1 ATP)
4. MLC is part of myosin head
5. phosphorylated myosin head binds to actin and power stroke occurs automatically
6. a second ATP is required to release myosin head from actin
7. calcium concentration dips below critical level due to being pumped out of cell into SR
8. calcium is released from calmodulin
9. MLCP removes phosphate from MLC, causing detachment of myosin head from actin filament, causing relaxation

Question 8

what are the two sources of cytosolic Ca2+ that initiate smooth muscle contraction?

Answer 8

• sarcoplasmic reticulum

- extracellular Ca2+ entering the cell through plasma-membrane Ca2+ channels

Question 9

how does smooth muscle relax, in relation to cytosolic Ca2+?

Answer 9

Ca2+ has to be removed either to the SR or back to extracellular fluid

Question 10

what inputs influence smooth muscle contractile ability?

Answer 10

• spontaneous electrical activity in plasma membrane of cell
• NT released by autonomic neurons
• hormones
• locally induced changes in chemical compositionof the extracellular fluid surrounding cell (paracrine factors, acidity, oxygen, osmolarity, ion concentration)
• stretch

Question 11

what is spontaneous electrical activity?

Answer 11

when some types of smooth muscle generate action potentials spontaneously in absence of any neural or hormonal input

Question 12

what is the pacemaker potential?

Answer 12

the membrane potential change occurring during the spontaneous depolariza/on to threshold