Final answer:
The correct answer is option 3. Parasympathetic stimulation to the SA node decreases heart rate by releasing acetylcholine, which opens potassium channels and slows depolarization, thus extending the repolarization period.
Step-by-step explanation:
Parasympathetic stimulation to the SA node has the effect of decreasing heart rate. This occurs through the action of the vagus nerve, which releases the neurotransmitter acetylcholine (ACh) at the neuromuscular junction. ACh then opens potassium ion channels which slows the rate of spontaneous depolarization, leading to an extended repolarization period, and thus, increases the time before the next spontaneous depolarization.
With the SA node setting a sinus rhythm of about 100 bpm naturally, the resting heart rate is typically lower due to the dominant parasympathetic influence. Decreasing parasympathetic stimulation reduces ACh release, allowing the heart rate to rise back towards 100 bpm, but any increase beyond this would necessitate sympathetic stimulation. Therefore, the correct option relevant to the effect of parasympathetic stimulation on the SA node is that it decreases heart rate.