USMLE Step 1 Physiology Review 55 05 Heart Function & Heartbeat (1_2)

USMLE Step 1 Physiology Review 55 05 Heart Function & Heartbeat (1_2)

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Play USMLE Audio MP3 55 05 Heart Function & Heartbeat (1_2) Below

Begin 55 05 Heart Function & Heartbeat (1_2) Transcription

What is the key concept behind heart function?

  • Both SA and AV nodes have slow upstroke and slow depolarization phases.

What do SA and AV stand for?

  • Sino-Atrial and Atrio-Ventricular.

What initiates the heart beat?

  •  The SA node.

Where specifically is the SA node located?

  • At the superior cristae terminalis in the right atrial wall.

What is its arteriole supply?

  • The nodal artery from the right coronary artery.

What do SA cells lack?

  •  SA cells do not have fast voltage gated Sodium channels

What result does this lack of fast voltage gated Sodium channels have on cardiac action potential?

  • Phase zero has a slow upstroke.

What is actually opening in phase zero slow upstroke?

  •  Slower voltage gated Calcium channels are opening.

Student please pause the tape and summarize the information discussed on Heart function and Heartbeat.

  • The key concept behind heart function is that both SA and AV nodes have slow upstroke and slow depolarization phases.  SA and AV stand for Sino-Atrial and Atrio-Ventricular.  The SA node which is located at the superior cristae terminalis in the right atrial wall initiates the heartbeat.  The nodal artery from the right coronary artery supplies the SA node.  SA cells do not have fast voltage gated Sodium channels which leads to a very slow upstroke in phase zero. Slower voltage gated Calcium channels are opening in phase zero.

What is the intrinsic pace dependent upon?

  • Phase four diastolic spontaneous depolarization.

What does the phase four diastolic spontaneous depolarization depend on?

  • It depends on the increase of membrane permeability to Potassium+

What does the slope of depolarization equal?

  • The pace

If the slope of depolarization is steeper, what happens to the pace?

  • The pace speeds up

How does the autonomic nervous system affect the firing rate of the SA node?

  • The sympathetic speeds it up, the parasympathetic slows it down.

Specifically, what compounds cause the slope of depolarization to become steeper?

  • Catecholamines

What compound causes it to be less steep?

  • Acetylcholine

Student please pause the tape and summarize the information discussed on Heart function discussed since the last summary.

  • The first question asked about what the intrinsic pace depended on. The intrinsic phase depends upon the phase four diastolic spontaneous depolarization which in turn depends on an increased membrane permeability to Potassium+. The slope of depolarization equals the pace. If the slope of depolarization is steeper, the pace speeds up. The sympathetic nervous system speeds up the firing rate of the SA while the parasympathetic slows it down. Catecholamines cause the slope of depolarization to become steeper while acetylcholine causes it to be less steep.

****END OF TRANSCRIPTION****

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