**Core Concept**
The sympathetic nervous system innervates the heart through the thoracic sympathetic trunk, which is responsible for regulating heart rate, contractility, and vascular tone. This innervation is mediated by the sympathetic nerves that arise from the thoracic spinal cord segments.

**Why the Correct Answer is Right**
The sympathetic nerves that innervate the heart arise from the thoracic spinal cord segments T1-T5. These nerves release the neurotransmitter norepinephrine, which stimulates the beta-1 adrenergic receptors on the cardiac myocytes, leading to an increase in heart rate and contractility. The sympathetic innervation also stimulates the release of renin, which increases blood pressure and cardiac output.

**Why Each Wrong Option is Incorrect**
**Option A:** T1-T3 is incorrect because the sympathetic innervation of the heart extends to T5, not just T3.
**Option C:** T3-T7 is incorrect because the sympathetic innervation of the heart is primarily mediated by the thoracic spinal cord segments T1-T5, not T3-T7.
**Option D:** L1-L5 is incorrect because the sympathetic innervation of the heart is mediated by the thoracic spinal cord segments, not the lumbar spinal cord segments.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the sympathetic innervation of the heart is primarily mediated by the thoracic spinal cord segments T1-T5, which is responsible for regulating heart rate, contractility, and vascular tone.

**✓ Correct Answer: B. T1 - T5**