CVS Practice Test

The sympathetic nervous system plays a crucial role in influencing the sinoatrial (SA) node, which is the heart's natural pacemaker. This influence is primarily through the cardioaccelerator fibers, which release neurotransmitters such as norepinephrine. When stimulated by the sympathetic nervous system, the SA node increases its rate of depolarization, leading to an increased heart rate. This mechanism is part of the body's "fight or flight" response, where adrenaline prepares the body for quick, acute actions, requiring an elevated heart rate to supply more oxygen and nutrients to muscles. In contrast, the parasympathetic nervous system generally slows down the heart rate through its influence on the SA node by releasing acetylcholine, creating a balance with the sympathetic system. The somatic nervous system mainly controls voluntary movements and does not influence the heart directly. Lastly, while the central nervous system integrates signals and processes higher functions, it does not directly alter the heart rate; however, it can influence both the sympathetic and parasympathetic nervous systems indirectly.