USMLE Step 1 Physiology Review 55 04 Cardiac Action Potentials

USMLE Step 1 Physiology Review 55 04 Cardiac Action Potentials

 

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Begin 55 04 Cardiac Action Potentials Transcription

There are five stages that characterize action potential. List them from stage zero to stage four, giving a brief descriptive phrase for each phase.

  •  Stage zero (upstroke of depolarization), Stage one (early slight repolarization), Stage two (plateau of depolarization), stage three repolarization, Stage four resting potential

What is the approximate value of the resting membrane potential of atrial and ventricular cell?

  •  -80mV

Is this value more or less than the value of resting membrane potential for skeletal muscle?

  • The resting membrane for cardiac muscle is slightly less than resting membrane potential for skeletal muscle

Are cardiac action potential longer than or shorter than those in skeletal muscle?

  •  Cardiac action potential are 20 to 50 times longer than action potentials of skeletal muscle.

Okay, now for some questions about excitation contraction complex. From where are calcium ion released to initiate the contractile process?

  •  Calcium ions are released from the sarcoplasmic reticulum.

During what phase of the action potential do calcium ions enter the cardiac muscle cells?

  • During the plateau phase

Once calcium enters  the cardiac muscle cells, what is triggered?

  •  Calcium entering triggers the release of internal stores of calcium.

Student doctor please pause the tape and summarize the information discussed thus far on cardiac action potentials and excitation contraction coupling.

  • There are five phases of cardiac action potential, they are as follows; Stage zero or the upstroke of depolarization, Stage one or the early slight repolarization, Stage two or the plateau of depolarization, stage three or the repolarization, Stage four or the resting potential. The approximate value of the resting membrane potential of atrial and ventricular muscle cells is -80mV.  Which is slightly less than resting membrane potential for skeletal muscle. Cardiac action potential are 20 to 50 times longer than action potentials of skeletal muscle. Calcium ions are released from the sarcoplasmic reticulum to initiate the contractile process. Calcium ions enter the cardiac muscle cells during the plateau phase. Calcium entering triggers the release of internal stores of calcium.

Okay, now for some more questions about excitation contraction coupling. The mechanical properties of cardiac muscle are similar to the mechanical properties of skeletal muscle except for two important differences. What are the key concepts behind these two difference?

  •  Tetanic contraction and passive tension

The cardiac action potential last for about what percentage of the duration mechanical contraction?

  •  about 70%

The duration of the action potential extends beyond the time when peak contractile forces is generated.  How does this create a situation in which cardiac muscle cannot undergo tetanic contraction? (Please pause the tape)

  • The muscle cells are refractory to further stimulation for a period long enough to preclude tetanic contraction.

Student please pause the tape and summarize the information discussed this far on excitation contraction coupling.

  • The mechanical properties of cardiac muscle are similar to the mechanical properties of skeletal muscle except for two important differences. The key concepts behind these two differences are Tetanic contraction and passive tension. The cardiac action potential last for about 70% percentage of the duration of the mechanical contraction. The duration of the cardiac action potential extends beyond the time when peak contractile forces is generated. So, the muscle cells are refractory to further stimulation for a period long enough to preclude tetanic contraction.

Okay now let’s talk about the second difference between the mechanical properties of cardiac muscle and those of skeletal muscle. The second difference between mechanical properties of cardiac muscle and those of skeletal muscle has to do with passive tension. Compared to skeletal muscle, Why does cardiac muscle have so much passive tension at normal resting let because there are more elastic components in cardiac muscle

In skeletal muscle what determine the force of contraction?

  • The number of fibers that contract determines the force of contraction in skeletal muscle.

Is the force of contraction in myocardial cells determined by intercellular phenomena, that is phenomena that takes place between the cell or is the force of contraction determined by Intracellular phenomena that is phenomena within the cell?

  • Intra- or within cell phenomena

What determines strength of contraction of cardiac muscle?

  •  Strength of contraction in cardiac muscle is determined by the direct adjustment of the intracellular contractile mechanism.

There is another factor that determines the strength of contraction of cardiac muscle. What is that factor?

  •  The length of cardiac fibers upon initiation of contraction affects the force of contraction.

Student please pause the tape and summarize the information discussed this far on excitation contraction coupling since the last summary.

The first question talked about the difference between the mechanical properties of cardiac muscle and those of skeletal muscle. The second difference between mechanical properties of cardiac muscle and those of skeletal muscle has to do with passive tension. Cardiac muscle have so much passive tension at normal resting let because there are more elastic components in cardiac muscle. The number of fibers that contract determines the force of contraction in skeletal muscle. The force of contraction in myocardial cells is determined by intracellular phenomena that is the phenomena within the cell. Strength of contraction in cardiac muscle is determined by the direct adjustment of the intracellular contractile mechanism. In addition, the length of cardiac fibers upon initiation of contraction affects the force of contraction.

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