Parasympathetic stimulation of heart causes:
**Core Concept**
Parasympathetic stimulation of the heart involves the activation of the vagus nerve, which releases acetylcholine (ACh) neurotransmitters that bind to muscarinic receptors on the heart. This results in a decrease in heart rate and contractility.
**Why the Correct Answer is Right**
The binding of ACh to muscarinic receptors on the sinoatrial (SA) node decreases the firing rate of the SA node, leading to a decrease in heart rate. This is known as negative chronotropic effect. Additionally, ACh also increases the refractory period of the atrioventricular (AV) node, further contributing to the decrease in heart rate. This is an example of the parasympathetic nervous system's role in modulating the heart rate to conserve energy and promote relaxation.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so we'll move on to the other options.
**Option B:** This option might be a distractor, but it's not directly related to the effect of parasympathetic stimulation on the heart.
**Option C:** This option might be related to sympathetic stimulation of the heart, which has the opposite effect.
**Clinical Pearl / High-Yield Fact**
Remember that the parasympathetic nervous system is often referred to as the "rest and digest" system, and its stimulation of the heart is a key mechanism for promoting relaxation and reducing energy expenditure.
**Correct Answer:** A. Decreased heart rate and contractility.