Contractile mechanism in smooth muscle.

Question 1

mechanism - step 1

Answer 1

ARRIVAL OF AP

• AP travels along axon to reach smooth muscle
• smooth muscle receives this stimulus causing start of process

Question 2

mechanism - step 2

Answer 2

DEPOLARISATION

• voltage gated calcium channels
• influx of Ca2+ ions
• from ECF/ down electrochemical membrane

making membrane potential inside more positive

Question 3

mechanism - step 3

Answer 3

calcium-calmodulin

• influx of calcium ions causes detection by calmodulin
• calmodulin binds to Ca2+ forming calcium calmodulin complex
• calcium calmodulin complex activating MLCK
• phosphorylating myosin head to interact with actin = ADP + Pi from myosin pull actin towards centre of sarcomere = muscle shortening = power stroke

after power stroke then ADP is released, new ATP binds to myosin, actin detaches from myosin

ATP is hydrolysed into ADP+PI which reenergise next myosin head

Question 4

mechanism - step 4

Answer 4

LATCH STATE

actin clings onto dephosphorylated myosin w/o ATP consumption

functions to maintain constant tension with minimal energy expenditure

Question 5

mechanism - step 5

Answer 5

RELAXATION

1 - Ca2+ is taken away by Ca2+/ATPase pump actively back into SR

3 - no calmodulin-calcium complex

4 - MLCK inhibited by PKA/PKG

4 - MLCP dephosphorylates MLC = less myosin ATP-ase activity