Final answer:
Normal sinus rhythm is the standard pattern of heartbeats initiated by the SA node and can vary with nervous system influences. Arrhythmias disrupt this rhythm, potentially leading to severe health complications, and the absence of electrical activity would significantly affect the heart rate.
Step-by-step explanation:
Normal sinus rhythm is the normal contractile pattern of the heart set by the sinoatrial (SA) node, the heart's natural pacemaker. This rhythm ensures that electrical signals are correctly initiated and propagated through the heart, causing the atria to contract first, followed by the ventricles. The sinus rhythm can be influenced by the autonomous nervous system; the parasympathetic nervous system can decrease the heart rate, while sympathetic stimulation can increase it up to approximately 100 beats per minute (bpm).
When the heart experiences an arrhythmia, which refers to any deviation from the normal sinus rhythm, it might beat too quickly (tachycardia) or too slowly (bradycardia). The consequences of arrhythmias can range from benign to severe, potentially leading to complications like stroke or heart failure. With the absence of normal electrical activity, one would expect a significant alteration or cessation of the heart rate (pulse).