USMLE Step 1 Physiology Review 54 02 Cardiac Muscle (2 of 2)

USMLE Step 1 Physiology Review 54 02 Cardiac Muscle (2 of 2)

 

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Play USMLE Audio MP3 54 02 Cardiac Muscle (2 of 2) Below

Begin 54 02 Cardiac Muscle (2 of 2) Transcription

Now for some questions about the excitation contraction coupling and cardiac muscle.

What ion induces the release of calcium that is stored in the lateral sacs of the sarcoplasmic reticulum into the cytoplasm?

  • Calcium.

Is it extra or intercellular calcium that induces the release of calcium that in stored in the lateral sacs?

  •  Extracellular calcium.

So, the excitation contraction coupling of cardiac muscle requires both extracellular calcium and calcium that is stored in the sacroplasmic reticulum. What happens to calcium after it is released from the lateral sacs into the cytoplasm?

  •  The calcium binds to troponin C.

What happens next?

  •  The troponin molecule changes shape and allows the tropomyosin molecule to move away from the myosin binding sights on the acting molecule.

Next, actin and myosin can interact via cross bridges and contraction can follow. What terminates contraction?

  • A rapid decline of cytoplasmic calcium

How is this rapid decrease of cytoplasmic calcium achieved?

  • Some calcium is taken back up by the sarcoplasmic reticulum and some is released into the extracellular space.

Student Doctor, please pause the tape and summarize the information given on excitation contraction coupling of cardiac muscle.

  •  The excitation contraction coupling of cardiac muscle requires both extracellular calcium and calcium that is stored in the sarcoplasmic reticulum. Extracellular induces the release of calcium that in stored in the lateral sacs of the sarcoplasmic reticulum into the cytoplasm. After the calcium is released from the lateral sacs into the cytoplasm it binds to troponin C. Then the troponin molecule changes shape and allows the tropomyosin molecule to move away from the myosin binding sights on the acting molecule. Next, actin and myosin can interact via cross bridges and contraction can follow.  A rapid decline of cytoplasmic calcium terminates contraction. Some calcium is taken back up by the sarcoplasmic reticulum and some is released into the extracellular space.

Now for some questions about the mechanics of electrophysiology of cardiac muscle.

Do myocardium and skeletal muscle have the same molecular mechanism of contraction?

  • Yes

Can a tetanus be produced of myocardium like it can in skeletal muscle?

  • No

What is it about a cardiac action potential that precludes cardiac muscle from being tetanized?

  •  The duration of the action potential is too long.

Why does an action potential of long duration preclude tetanus and myocardium?

  • The long action potential is accompanied by a long refractory period which precludes muscle stimulation.

During what period is the mechanical response of cardiac muscle completed?

  • The refractory period.

What two changes alter the force of cardiac contraction?

  •  A change in preload and a change in contractility.

Explain a change in preload in terms of muscle length.

  • A change in preload is a change in the initial starting length of the muscle.

What is the relationship between preload and force of contraction?

  • Within limits, a longer initial muscle length correlates with a stronger force of contraction.

What is contractility?

  •  Contractility is the ability of muscle to contract with increasing force even though the preload remains constant.

Student Doctor, please pause the tape and summarize the information on the mechanics of cardiac muscle.

  • Myocardium and skeletal muscle have the same molecular mechanism of contraction. However, tetanus cannot be produced in myocardium like it can in skeletal muscle because the duration of the action potential in cardiac muscle is too long. The mechanic response of cardiac muscle is completed during the refractory period. A change in preload and a change in contractility can alter the force of contraction. A change in preload is a change is the initial starting length of the muscle. Within limits, a longer initial muscle length correlates with a stronger force of contraction. Contractility is the ability of muscle to contract with increasing force even though the preload remains constant.

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