USMLE Step 1 Physiology Review 55 03 Cardiac Muscle

USMLE Step 1 Physiology Review 55 03 Cardiac Muscle

 

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Begin 55 03 Cardiac Muscle Transcription

Okay, now for some questions about the structure of cardiac muscle. List two types of cardiac muscle:

  •  First, atrial muscle fibers; second, ventricular muscle fibers.

In additions to these two, there also cardiac muscle fibers specialized for something other than generating force. What are they specialized for?

  • These other muscle fibers are specialized for initiation and conduction of action potentials.

Cardiac muscle cells like skeletal muscle cells contain two contractile proteins and two regulatory proteins. List the two contractile proteins.

  • Actin and Myosin

What is actin?

  • Actin is a thin rod like protein that interacts with myosin during contraction.

What portion of the myosin molecule directly interacts with actin?

  • The head mirror myosin.

What is the terminology for the interactive bond formed between actin and myosin?

  •  Cross-bridging

List the two regulatory proteins in cardiac muscle cells, both start with the letter ‘T”.

  •  Tropomyosin and Troponin

Are these regulatory molecules more closely associated with the actin molecule or the myosin molecule?

  • The actin molecule.

Which one of these regulatory proteins, tropomyosin or troponin intermittently rest in the F actin groove thus blocking myosin binding site?

  • Tropomyosin

What is the purpose of the troponin molecule? (Pause the tape.)

  •  It regulates the positioning of tropomyosin in the interaction between actin and myosin.

What ion regulates the action of the troponin molecule?

  • Calcium

Student doctor, please pause the tape and summarize the information discussed thus far about the structure of cardiac muscle.

  • Two types of cardiac muscle are atrial muscle fibers and ventricular muscle fibers. In addition to these two, there are also cardiac muscle fibers specialized for the initiation and conduction of action potentials. Cardiac muscle cells like skeletal muscle cells contain the two contractile proteins, actin and myosin and the two regulatory proteins, tropomyosin and troponin.  Actin is a thin rod like protein that interacts with myosin during contraction. The heavy mirror myosin portion of the myosin molecule directly interacts with actin. The terminology for the interactive bond formed between actin and myosin is cross-bridging. Tropomyosin intermittently rest in the F actin groove thus blocking myosin binding sites. The troponin molecule regulates the positioning of tropomyosin and the interaction between actin and myosin.  Calcium regulates the action of troponin molecule.

Okay, now for some more information about the structure of cardiac muscle. What organelle is located in large numbers close myofibrils?

  •  Elongated mitochondria are located in large numbers close to the myofibrils

What structures aid in the conducting of electrical activation toward the interior region of cells?

  • Transverse tubules

What serves as an intracellular store of Calcium?

  • An extensive sarcoplasmic reticulum.

What is the specialized intracellular attachment of cardiac muscle made up of gap junctions, fascia adherens, and occasionally desmosomes?

  • Intercalated disk

Adjacent cardiac muscle cells are adjoined at the what, by intercalated disk to form cardiac muscle fibers?

  •  Adjacent muscle fibers are adjoined at the sarcolemma membrane by the intercalated disk to form cardiac muscle fibers.

What structures provide strong mechanical connection between cardiac muscle cells?

  • Intercalated disk provide this connection.

This strong mechanic connection provided by intercalated disk, allows what kinds of forces to be translated from one cell to another?

  • Axial mechanical forces.

Student doctor, please pause the tape and summarize the information discussed about the structure of cardiac muscle since the last summary.

  • Elongated mitochondria are located in large numbers close to the myofibrils. Electrical activation is conducted toward the interior regions of cells via transverse tubules. An extensive sarcoplasmic reticulum serves as an intracellular store of Calcium. An intercalated disk is a specialized intracellular attachment of cardiac muscle made up of gap junctions, fascia adherens, and occasionally desmosomes. Adjacent muscle fibers are adjoined at the sarcolemma membrane by the intercalated disk to form cardiac muscle fibers. A strong mechanical connection between cardiac muscle cells by the intercalated disk, thus allowing axial mechanical forces to be transmitted from one cell to another.

Is there protoplasmic continuity between cardiac cells?

  • No, there is no protoplasmic continuity between cardiac cells.

Although there is no protoplasmic continuity between the cells, the spread of electrical excitation from one cardiac muscle cells to the next is made possible by what, in the interacalated disk?

  • Gap junctions in the intercalated disk make the spread of electrical excitation from one cell to the next possible.

How does this gap junction make it possible for electrical excitation to spread from one cell to another?

  • By providing low resistant pathway

A syncytium is a multinucleated protoplasmic mass formed by the secondary union of originally separate cells. In what way does a cardiac muscle act as a functional syncytium?

  • Action potentials initiated in one cell spread to all other cells.

Explain the all or nothing principle as applied to cardiac cells?

  •  Sufficient stimulus anywhere in the heart always spread throughout the entire heart.

Student doctor, please pause the tape and summarize the information discussed about the structure of cardiac muscle since the last summary.

  • The first question asked about protoplasmic continuity between cardiac muscle cells. Although there is no protoplasmic continuity between the cells, the spread of electrical excitation between the one cardiac muscle cells to the next is made possible by gap junctions in the intercalated disk. These gap junctions provided low resistant pathway which allow electrical excitation to spread from one cell to another. A syncytium is a multinucleated protoplasmic mass formed by the secondary union of originally separate cells. Cardiac muscle behaves as a functional syncytium because action potential initiated in one cell spread to all other cells. The all or nothing principle as applied to cardiac cells means that sufficient stimulus anywhere in the heart always spread throughout the entire heart.

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