Parasympathetic activation of the heart produces______________?
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Answer: A. SA node decreases firing
Explanation
Parasympathetic vagal stimulation releases acetylcholine which hyperpolarizes the sinoatrial node and decreases firing. This inhibitory action on pacemaker cells slows down the spontaneous rate of diastolic depolarization. Consequently the heart rate drops as fewer action potentials are generated at the sinoatrial junction. The autonomic nervous system regulates heart rate through opposing sympathetic and parasympathetic nerve influences. Acetylcholine binds to muscarinic M2 receptors on nodal cells increasing potassium permeability and slowing pacemakers. Vagal tone exerts a constant resting inhibitory influence that keeps the intrinsic heart rate at lower levels.
Parasympathetic activation is essential for lowering heart rate during rest and recovery periods after exertion. Excessive vagal tone can lead to pathological bradycardia and temporary fainting spells in susceptible individuals. Athletes often exhibit resting bradycardia due to enhanced vagal tone and strong cardiovascular conditioning. Decreased AV node excitability accompanies vagal stimulation but the primary pacemaker rate reduction occurs at the SA node. Direct effects on ventricular contraction strength are primarily mediated by sympathetic innervation rather than parasympathetic. Tachycardia describes an elevated heart rate which results from sympathetic activation rather than parasympathetic firing.
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