Sympathetic supply to hea
**Core Concept**
The sympathetic nervous system supplies the heart through the cardiac plexus, which is a complex network of nerves that regulate heart rate, contractility, and conduction. This sympathetic innervation is mediated by the release of neurotransmitters such as norepinephrine, which acts on beta-1 adrenergic receptors to increase heart rate and contractility.
**Why the Correct Answer is Right**
The cardiac plexus receives sympathetic input from the thoracic splanchnic nerves, specifically T1-T5, which carry preganglionic fibers from the thoracic sympathetic trunk. These fibers synapse with postganglionic neurons in the cardiac plexus, which then release norepinephrine to act on beta-1 adrenergic receptors in the heart. This results in increased heart rate, contractility, and cardiac output, which is particularly important during times of stress or increased demand.
**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because the vagus nerve, not the sympathetic nervous system, is responsible for parasympathetic innervation of the heart.
**Option B:** This is incorrect because the cardiac plexus receives sympathetic input, not parasympathetic input, from the thoracic splanchnic nerves.
**Option C:** This is incorrect because the sympathetic nervous system does not innervate the heart through the phrenic nerve, which is primarily responsible for diaphragmatic innervation.
**Clinical Pearl / High-Yield Fact**
A key clinical correlation is that beta-1 adrenergic receptor blockers, such as atenolol, can be used to treat conditions such as hypertension and heart failure by blocking the sympathetic stimulation of the heart.
**Correct Answer:** C.