Final answer:
Sinus rhythm is the normal rhythmical beating of the heart controlled by the sinoatrial node. Electrical signals regulate heart contractions, influenced by autonomic nervous system activation. PVC couplets are two consecutive premature ventricular contractions.
Step-by-step explanation:
The term sinus rhythm refers to the normal, rhythmical beating of the heart that is established by the pacemaker cells located in the sinoatrial (SA) node. These cells generate and propagate electrical signals, leading to the synchronized contraction of the heart muscle. These signals first cause the atria to contract and then proceed to the atrioventricular (AV) node and the ventricles. The sinus venosus, from which the posterior portion of the right atrium, the SA node, and the coronary sinus develop, plays a crucial role in creating this pattern. Moreover, the heart can be influenced by the autonomic nervous system through parasympathetic and sympathetic stimulation, which can decrease or increase the heart rate, respectively. Premature ventricular contractions (PVCs) are a form of arrhythmia where ectopic beats originate in the ventricles; a PVC couplet is two consecutive PVCs. A cardiac cycle encompasses the period from atrial contraction through ventricular relaxation.