USMLE Step 1 Physiology Review 53 12 Cardiac Muscle

USMLE Step 1 Physiology Review 53 12 Cardiac Muscle

 

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Begin 53 12 Cardiac Muscle Transcription

Okay, now we are going to talk about the electrophysiology of cardiac muscle and we’ll start with action potentials.

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

  • Stage zero: upstroke depolarization.  Stage one: early slight repolarization.  Stage two: plateau of depolarization.  Stage three: repolarization.  Stage four: resting potential.

In stage zero, what produces the upstroke of depolarization?

  •  An increase in sodium conductance. 

What triggers the increase in sodium conductance?

  • The increase in sodium conductance is triggered by a regenerative depolarization.

In stage one, what causes the early slight repolarization?

  • A brief increase in the conductance of chlorine.

What two conditions maintain the plateau of depolarization in stage two?

  • An increased conductance of calcium relative to its resting value, and a decreased conductance of potassium relative to its resting value.

In stage three, what two conditions cause the repolarization?

  • Both calcium conductance and potassium conductance returning to their resting values.

In stage four, what condition maintains the resting potential in diastole?

  • High potassium conductance.

Student Doctor, please pause the tape and summarize the general information on the stages of cardiac muscle action potentials.

  • There are five stages that characterize cardiac action potentials.  Stage zero is the upstroke of depolarization that is produced by a regenerative depolarization triggered increase in sodium conductance.  The increase in sodium conductance is inactivated after a few milliseconds.  Stage one is the early slight repolarization resulting from a brief increase in conductance of chlorine.  Stage two is the plateau of depolarization that is maintained by increased calcium conductance from its resting value, and a decrease in potassium conductance from its resting value.  Stage three is a repolarization caused by calcium conductance and potassium conductance returning to their resting values.  Stage four is the resting potential in diastole maintained by high potassium conductance.

Okay, now for some more questions about cardiac action potentials.

During what stage is the membrane potential for cardiac muscle cells the lowest?

  • Stage four.

What is the approximate value of the membrane potential for cardiac muscle during stage four?

  • A minus eighty millivolts.

What is the approximate value for membrane potential for cardiac muscle during stage one?

  • Forty millivolts.

During what stage is the membrane potential for cardiac muscle cells the highest?

  • Stage one.

During what stage is the membrane potential of the pacemaker cell the highest, or most positive?

  • During stage two, the plateau of depolarization.

During what stage is the membrane potential of the pacemaker cell the lowest, or most negative?

  •  During stage four, the resting potential in diastole.

What are the two possible reasons for stage four depolarization of cardiac pacemaker cells?

  • First, spontaneously increasing calcium conductance, and second, a spontaneous decrease in potassium conductance.

Student Doctor, please pause the tape and summarize the information discussed on cardiac muscle action potentials since the last summary.  The first question was about how the membrane potential is the lowest during stage four.

  • During stage four the membrane potential for cardiac muscle cells dips to its lowest value of eighty millivolts.  During stage one the membrane potential for the cardiac muscle cells climbs to its highest value of forty millivolts.  The membrane potential of the pacemaker cell is the highest or most positive during stage two, the plateau of depolarization.  The membrane potential of the pacemaker cell is the lowest or most negative during stage four, the resting potential in diastole.  Stage four depolarization of cardiac pacemaker cells is caused by spontaneously increasing calcium conductance, and possibly a spontaneous decrease in potassium conductance.

Okay, now for some more questions.

What is the approximate value for the membrane potential for the pacemaker cell during stages zero and three?

  • Minus twenty millivolts. (-20 mV)

Would acetylcholine increase or decrease the rate of depolarization?

  •  Decrease.

Agents like acetylcholine that decrease the rate of depolarization increase potassium conductance hyperpolarization in stage four.

What ion conductances are moderated by agents that increase the rate of spontaneous depolarization?

  • Calcium or sodium.

How are the conductance of calcium and the conductance of sodium moderated to increase the rate of spontaneous depolarization?

  • Calcium conductance and sodium conductance are increased.

What neurotransmitter located in the locus coeruleus in the brain and the ventromedial nucleus of the hypothalamus can act as an agent to increase the rate of spontaneous depolarization?

  •  Norepinephrine.

What effect does aging have on the phases in the action potential and in coupling to contraction?

  • The phases are prolonged with aging.

In terms of ion movement, why does aging lead to prolonged phases?

  • The movement of calcium slows with aging.

Student Doctor, please pause the tape and summarize the information discussed on cardiac action potentials since the last summary.  The first question asked about the value for the membrane potential for the pacemaker cell during stages zero and three.

  • Negative twenty millivolts (-20 mV) is the approximate value for the membrane potential of the pacemaker cell during stages zero and three.  Agents that decrease the rate of depolarization, like acetylcholine, increase potassium conductance hyperpolarization in stage four.  Agents that increase the rate of spontaneous depolarization increase calcium conductance and sodium conductance.  Norepinephrine can act as an agent to increase the rate of spontaneous depolarization.  The movement of calcium slows with aging so the phases in the action potential and in coupling to contraction are prolonged.

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