USMLE Step 1 Physiology Review 54 04 Smooth muscle (2_3)

USMLE Step 1 Physiology Review 54 04 Smooth muscle (2_3)

 

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Let’s start our USMLE Review with Anatomy from the Gold Standard USMLE Step 1 Audio Review program.

Play USMLE Audio MP3 54 04 Smooth muscle (2_3) Below

Begin 54 04 Smooth muscle (2_3) Transcription

Okay, now for some more questions.

Regardless of the mechanism of excitation, whether it is pharmaco-mechanical or electric-mechanical, what is required to activate contraction?

  • An increase in cytosolic calcium.

What are the two potential sources of calcium for the activation of contraction in smooth muscle?

  • The two sources of calcium are extracellular and the sarcoplasmic reticulum.

The amount of calcium that comes from the sarcoplasmic reticulum and the amount that comes from  extra cellular space when raising cytosolic calcium depends on the type of smooth muscle tissue and the type of stimuli.

Through what kinds of channels does extra cellular calcium enter the cell in electro-chemical coupling?

  • There is a hint in the name of the coupling mechanism. Calcium enters through the voltage sensitive channels in electro-chemical coupling.

Through what kinds of channels does extracellular calcium enter the cell in pharmaco-mechanical coupling? Again there is a hint the name of the coupling mechanism.

  • Receptor operator channels.

Student Doctor, please pause the tape and summarize the information discussed thus far on role of calcium and the excitation coupling of smooth muscle.

  • Regardless of the mechanism of excitation, whether it is pharmaco-mechanical or electric-mechanical, an increase in cytosolic calcium is required to activate contraction. The two sources of calcium are extracellular and the sarcoplasmic reticulum. The amount of the calcium that comes from the sarcoplasmic reticulum and the amount that comes from  the extracellular space when raising cytosolic calcium depends on the type of smooth muscle tissue as well as the type of stimuli. Calcium enters through the voltage sensitive channels in electro-chemical coupling. Extra cellular calcium enters the cell via receptor operator channels in pharmaco-mechanical coupling.

Okay, now let’s talk about contraction and smooth muscle.

The contractile proteins are the same in smooth muscle as they are in skeletal muscle, but there is an important difference in the two types of muscle with regard to the ration of actin to myosin. What is the difference?

  • The molar ratio of actin to myosin is much higher in smooth muscle compared to skeletal muscle.

What is the ratio of actin to myosin in the smooth muscle?

  • Thirty to one.

What is the ratio of actin to myosin in skeletal muscle?

  •  Three to one.

Does smooth muscle contain troponin?

  •   No.

Is the actin-myosin ATPase in smooth muscle lower or higher than it is in skeletal muscle?

  • The actin-myosin ATPase activity is significantly lower in smooth muscle.

Student Doctor, please pause the tape and summarize the information discussed thus far on contraction of smooth muscle. We started out by talking about the ratio of actin to myosin.

  • The contractile proteins are the same in smooth muscle as they are in skeletal muscle, but the molar ratio of actin to myosin is much higher in smooth muscle compared to skeletal muscle. The ratio of actin to myosin in the smooth muscle is thirty to one, whereas the ratio of actin to myosin in skeletal muscle is three to one. Smooth muscle contains no troponin. The actin-myosin ATPase activity in smooth muscle is significantly lower than it is in skeletal muscle.

Okay, now for some more questions.

The mechanism in initiating contraction in smooth muscle involves a cytosolic protein and an enzyme.

What is this cytosolic protein?

  • Calmodulin.

What is the enzyme?

  • Myosin light chain kinase, abbreviated as MLCK.

What ion, when sufficiently concentrated in the cytosol binds calmodulin?

  • Calcium.

What activates MLCK or  Myosin light chain kinase?

  • The calcium calmodulin complex.

Once activated what does MLCK do?

  • Myosin light chain kinase phosphorylates myosin light chains.

Once they are phosphorylated, what do the myosin light chains do?

  • Myosin light chains interact with actin thin filaments.

So, when calcium concentration is high enough calcium binds calmodulin, which activates myosin light chain kinase, which in turn phosphorylates myosin light chains and contraction takes place. Please repeat what I just said.

Now you can pause the tape and summarize the information about the mechanism for initiating contraction of smooth muscle.

The mechanism for initiating contraction in smooth muscle involves a cytosolic protein, calmodulin, and myosin light chain kinase, abbreviated as MLCK. Calcium, when sufficiently concentrated in the cytosol binds calmodulin. The calcium calmodulin complex activates MLCK. Once activated, myosin light chain kinase phosphorylates myosin light chains. Once they are phosphorylated the myosin light chains interact with actin thin filaments. So, when calcium concentration is high enough calcium binds calmodulin which activates the myosin light chain kinase, which phosphorylates myosin light chains and contraction takes place.

 

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54-03 Smooth Muscle

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