Depolarization of the SA node, Action potentials of the myocardium & ANS effects

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• A typical contractile cell in the myocardium has a resting membrane potential. • The resting membrane potential of cells in the SA node is not fixed, and is known as the pacemaker potential. • The action potential of a healthy SA nodal cell has three parts: • Pacemaker potential: the membrane potential spontaneously and gradually becomes more positive until threshold is reached • Rapid depolarization is triggered • The SA node's rate of firing, combined with modifications from the autonomic nervous system (ANS), determines the heart rate. • The action potential of a healthy ventricular cell has four major parts: • Resting membrane potential: the membrane potential remains steady until some stimulus causes it to exceed threshold • Rapid depolarization is triggered • A plateau follows, during which the cell remains depolarized • Repolarization follows, and the return of the cell to the resting potential. • The SA node's rate of firing, combined with modifications from the autonomic nervous system (ANS), determines the heart rate. • The autonomic nervous system (ANS) can modify the heart's rate in the SA node in two ways: • Acetylcholine (ACh), secreted from parasympathetic neurons, decreases heart rate. • Norepinephrine (NE), secreted from sympathetic neurons, increases heart rate. • NE also causes the heart to beat more strongly.

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