Single most important factor in control of automatic contractility of the heart is
**Core Concept**
The automatic contractility of the heart is controlled by various factors, but the single most important factor is the intracellular concentration of calcium ions (Ca2+). This concept is crucial in understanding the mechanisms of cardiac contractility and the regulation of heart function.
**Why the Correct Answer is Right**
The intracellular concentration of calcium ions plays a pivotal role in regulating cardiac contractility through the process of excitation-contraction coupling. When an action potential reaches the cardiac muscle cell, voltage-gated calcium channels open, allowing an influx of calcium ions into the cell. This increase in intracellular calcium triggers the release of more calcium from the sarcoplasmic reticulum, leading to muscle contraction. The level of intracellular calcium directly influences the force of contraction, making it the most important factor in controlling automatic contractility.
**Why Each Wrong Option is Incorrect**
* **Option A:** While the sympathetic nervous system does play a role in regulating heart rate and contractility, it is not the single most important factor controlling automatic contractility.
* **Option B:** The parasympathetic nervous system, primarily through the release of acetylcholine, has a negative chronotropic effect on the heart but does not directly control contractility.
* **Option D:** The endothelium-derived relaxing factor, now known as nitric oxide (NO), primarily affects vascular tone and is not directly involved in regulating cardiac contractility.
**Clinical Pearl / High-Yield Fact**
The concept of excitation-contraction coupling and the importance of intracellular calcium in regulating cardiac contractility is critical in understanding the pathophysiology of heart failure, where decreased contractility is a hallmark of the disease.
**Correct Answer: C. Intracellular calcium ions.**