USMLE Step 1 Physiology Review 53 10 Skeletal Muscle (2 of 3)

USMLE Step 1 Physiology Review 53 10 Skeletal Muscle (2 of 3)

 

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Begin 53 10 Skeletal Muscle (2 of 3) Transcription

Okay, now for some more questions about the excitation contraction coupling in skeletal muscle.

What is the major regulatory protein in addition to tropomyosin, affecting the contraction of skeletal muscle?

  • Troponin.

What are the three major polypeptides that make up troponin?  List them alphabetically.

  • First, troponin C.  Second, troponin I.  Third, troponin T.

What is the function of troponin T?

  • It holds the tropomyosin in the f-actin group.

What is the function of troponin I?

  • It inhibits interaction between actin and myosin.

What is the function of troponin C?

  • It binds calcium and takes part in the contraction of muscle.

What happens when troponin C binds calcium?

  • The troponin molecule changes shape.     

What happens when the troponin molecule changes shape?

  • When the troponin molecule changes shape, it moves away from its blocking position, thus exposing the series of myosin binding sites on the actin molecule.

ATP then binds to what part of the myosin molecule?

  • ATP binds to the heavy meromyosin.

What happens next to the ATP?

  • Then the ATPase within the heavy meromyosin cleaves the ATP into ADP and energy.

What purpose does this liberated energy serve?

  • The energy liberated from the cleavage of ATP energizes, or cocks, the heavy meromyosin.

Student Doctor, please pause the tape and summarize the information discussed on excitation contraction coupling since the last summary.  We started talking about the troponin molecule.

  • Troponin is the other major regulatory protein affecting the contraction of skeletal muscle.  Troponin C, troponin I, and troponin T are the three major polypeptides that make up troponin.  Troponin T holds the tropomyosin in the f-actin group.  Troponin I inhibits interaction between the actin and myosin.  Troponin C binds calcium and takes part in the contraction of muscle.  When the troponin C moiety binds calcium the troponin molecule changes shape.  When the troponin molecule changes shape, it moves away from its blocking position, thus exposing the series of myosin binding sites on the actin molecule.  The ATP binds to the heavy meromyosin, then the ATPase within the heavy meromyosin cleaves the ATP into ADP and energy.  The energy liberated from the cleavage of ATP energizes, or cocks, the heavy meromyosin.

Okay, now for some more questions.

What happens next to the cocked heavy meromyosin?

  • It binds to the actin molecule.

What does the act of binding to the actin molecule do to the cocked heavy meromyosin?

  • The binding causes the heavy meromyosin to return to its unenergized steady state position, thus constituting the power stroke of the actin-myosin complex.

What happens to the actin-myosin complex because of this power stroke?

  • The actin molecule ratchets past the myosin molecule.

How far does this actin molecule ratchet past the myosin molecule?

  • The actin slides past the myosin molecule to the next myosin binding site on the actin molecule, and then the process repeats.

This phenomena happens in concert to all the actin-myosin complexes in all of the myofibrils in a single muscle fiber.  This phenomena constitutes contraction. When does contraction end?

  • It ends when the calcium concentration in the cytoplasm is reduced. 

To what level must calcium concentration be reduced to end contraction?

  • It must be as low as ten to the minus eight molar (10^-8 M).

Where does the calcium go?

  • It is taken back up by the sarcoplasmic reticulum.

What happens when there is such a low concentration of calcium?

  • Calcium dissociates from troponin C.

What happens when calcium dissociates from troponin C?

  • Troponin changes shape, which permits the tropomyosin molecule to move back into its blocking position in the f-actin groove.

Student Doctor, please pause the tape and summarize what has been discussed on excitation contraction coupling since the last summary.  The first event in this past section was the cocked heavy meromyosin binding to the actin molecule.

  • The act of the heavy meromyosin binding to the actin molecule causes the heavy meromyosin to return to its unenergized or steady state position, thus constituting the power stroke of the actin-myosin complex, and the actin molecule ratchets past the myosin molecule.  The actin slides past the myosin molecule to the next myosin binding site on the actin molecule, and then the process repeats.  This phenomena happens in concert to all of the actin-myosin complexes in all of the myofibrils in a single muscle fiber.  This phenomena constitutes contraction.  Contraction ends when the calcium concentration in the cytoplasm is reduced as low as ten to the minus eight molar (10^-8 M).  The calcium is taken back up by the sarcoplasmic reticulum.  When there is such a low concentration of calcium, it dissociates from troponin C.  When calcium dissociates from troponin C, the troponin molecule changes shape which permits the tropomyosin molecule to move back into its blocking position in the f-actin groove.

****END OF TRANSCRIPTION****

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