Comprehensive Osteopathic Medical Licensing Examination (COMLEX USA) Practice

What is the SNS effect on heart rate and contractility?

1. Decrease in rate and contractility

2. No significant effect

3. Increased rate and contractility

4. Irregular beat patterns

The correct answer is: Increased rate and contractility

The sympathetic nervous system (SNS) has a well-established role in modulating heart function, primarily through the release of neurotransmitters such as norepinephrine. When the SNS is activated, it leads to an increase in heart rate (positive chronotropic effect) and an increase in the force of contraction of the heart muscle (positive inotropic effect). This physiological response is part of the body's "fight or flight" mechanism, preparing the organism for increased physical activity, stress, or emergencies. Increased heart rate allows for more frequent pumping of blood, facilitating enhanced oxygen delivery to tissues. Simultaneously, increased contractility means each heartbeat is more effective in ejecting blood from the heart, thus improving cardiac output. Other options don’t reflect the primary actions of the SNS. A decrease in heart rate and contractility would be more characteristic of parasympathetic activity. There would be no situation where the SNS has no significant effect, as its activation inherently influences heart function. Lastly, irregular beat patterns pertain to arrhythmias, which can have many causes unrelated to straightforward SNS activity. Therefore, the positive effects of increased rate and contractility accurately describe the SNS's impact on heart function.