Question

MCAT Mini-Tutorial: Regulation of Heart Rate

Answer 1

\({\bf{SA~Node:}}\)
- 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)



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

Answer 2

\({\bf{Cardiac~Cell~Potential:}}\) this is the typical pattern for a cardiac cell other than the SA node



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

Answer 3

\({\bf{Conduction~Pathway:}}\) 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

Answer 4

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