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MCAT Mini-Tutorial: Regulation of Heart Rate

4 months ago
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4 months ago
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$${\bf{SA~Node:}}$$ - region in the right atrium that initiates and maintains regular contraction of the heart - action potential can be mapped with 3 phases (phase 0, phase 3, phase 4) |dw:1563750142789:dw| Phase 4: resting potential (notice that it is not a flat line, but rather increases in potential because of sodium leak channels that leak positive charge into the cell once the potential reaches the threshold, the Ca2+ channels open and phase 0 begins Phase 0: depolarization caused by the influx of calcium ions, up to equilibrium potential note: calcium channels are slower than sodium channels which helps explain why the action potential of a muscle cell is slower than the action potential of a neuron Phase 3: Ca2+ channels close, K+ channels open, letting out positive charge and returning the cell back to resting potential Special Note: the SA node depolarizes first so it "sets" the heart rate, but other parts of the heart can also act as a pacemaker if the SA node is inhibited or obstructed

4 months ago
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$${\bf{Cardiac~Cell~Potential:}}$$ this is the typical pattern for a cardiac cell other than the SA node |dw:1563750738105:dw| Phase 0: The action potential from the previous cell makes the consecutive cell reach the threshold required for Na+ channels to open, letting Na+ into the cell and causing depolarization Phase 1: Na+ channels close, K+ channels and Ca2+ open, letting out K+ and lowering the cell potential. influx of Ca2+ increases cell potential so a temporary equilibrium is established (phase 2) Phase 3: Ca2+ channels close, K+ channels remain open, causing repolarization Phase 4: return to resting membrane potential, K+ channels close

4 months ago
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$${\bf{Conduction~Pathway:}}$$ SA node --> atra & AV node --> AV bundle (bundle of His) --> bundle branches --> ventricles $${\bf{Other~Regulation:}}$$ - vagus nerve innervates SA node and release ACh (which is usually a stimulator but in this case binds to inhibitory receptors) - sympathetic nerves release norepinephrine in "fight/flight" situations - adrenal medulla secretes epinephrine and increases heart rate & contraction - baroreceptors in aorta and carotid arteries react to blood pressure and relay the information to the CNS, which raises or lowers vagus nerve activity and sympathetic nerve activity as needed

4 months ago
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Source material is Chapter 9.3 of MCAT: Biology Review, 2nd edition by Princeton Review

4 months ago