USMLE Step 1 Physiology Review 56 07 Pacemaker action potentials

USMLE Step 1 Physiology Review 56 07 Pacemaker action potentials

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Play USMLE Audio MP3 56 07 Pacemaker action potentials Below

Begin 56 07 Pacemaker action potentials Transcription

Okay now for some question about pacemaker action potentials. Where are pacemaker tissue in the heart, there are three places?

  • The SA node, the AV node and Purkinje fibers

What happens in phase four of the action potential that is peculiar to pacemaker tissue?

  • Slow diastolic depolarization

What does this slow diastolic depolarization in phase four indicate the presence of?

  • Automaticity

Define Automaticity?

  • The ability of the heart to generate its own beat

Where does the generation of a signal for the SA node reside?

  •  Within the node

The trigerred for the signal for the AV node is the depolarization wave that travels through the myocardium, where does the wave originate?

  •  The SA node

Where is the normal pacemaker of the heart?

  •  The SA node

Why?

  • Because the SA node has the highest automaticity, that is the steepest loop of phase four 

Elimination of the SA node activity causes a pacemaker outside the SA node to initiate the depolarization. What is the term for this new pacemaker?

  • Ectopic pacemaker because it is outside the normal place.

Which phases of the pacemaker action potential is similar to the myocytic action potential?

  • Phases 1, 2 and 3

Which phases differ?

  •  Phases 0 and 4

Describe the pacemaker potential in phase zero and how it differs from the action potential in myocyte. (Pause the tape)

  • It differs from myocytic action potential with the slow upstroke linked to voltage gated Calcium channels opening without fast voltage gated Sodium channels with a decreased velocity of conduction that enables a longer period of transmission from atrial to ventricles.

Describe phase four pacemaker action potential. (You may want to pause the tape here)

  • There is a simultaneously fallen Potassium permeability and membrane depolarization. SA node diastolic rate regulates heart rate which varies as the phase four slope increases with decreased acetylcholine and increased catecholamines or vice versa.

Student doctor please pause the tape and summarize the information on pacemaker action potentials.

  • Pacemaker tissues in the heart are the SA node, the AV node and Purkinje fibers. In phase four of the action potential that is a slow diastolic depolarization. This slow diastolic depolarization in phase four indicates the presence of Automaticity which is the ability of the heart to generate its own beat. The generation of a signal for the SA node resides within the node. The trigger for the signal for the AV node is the depolarization wave that travels through the myocardium. The wave originates in the SA node. The normal pacemaker of the heart is the SA node because the SA node has the highest automaticity that is the steepest loop of phase four. Elimination of the SA node activity causes a pacemaker outside the SA node to initiate the depolarization. The term for this new pacemaker is the Ectopic pacemaker because it is outside the normal place. The pacemaker action potential is similar to the myocytic action potential in Phases 1, 2 and 3. It is different in Phases 0 and 4. In phase zero of the pacemaker action potential, there is a slow upstroke linked to voltage gated Calcium channels opening without fast voltage gated Sodium channels with a decreased velocity of conduction that enables a longer period of transmission from atrial to ventricles. In phase four, there is a simultaneously fallen Potassium permeability and membrane depolarization. SA node diastolic rate regulates heart rate which varies as the phase four slope increases with decreased acetylcholine and increased catecholamines or vice versa.

****END OF TRANSCRIPTION****

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