Final answer:
Premature Atrial Contraction (PAC) occurs when an area of the heart other than the SA node initiates an impulse that leads to a premature contraction. Conditions like localized ischemia, drug exposure, or autonomic nervous system stimulation can stimulate the ectopic focus. Pacemaker cells are self-excitable cells that transfer depolarization to other cardiac muscle fibers, coordinating the heart's contraction.
Step-by-step explanation:
Premature Atrial Contraction (PAC) occurs when an area of the heart other than the SA node initiates an impulse that is followed by a premature contraction. This area, known as an ectopic focus or ectopic pacemaker, may be stimulated by various factors such as localized ischemia, certain drugs, or elevated stimulation by the autonomic nervous system. Chronic PAC can lead to arrhythmias or fibrillation, which are deviations from the normal pattern of impulse conduction and contraction.
The Pacemaker cells, which are self-excitable and able to depolarize on their own, play a crucial role in the coordinated contraction of the heart. These cells transfer the depolarization to other cardiac muscle fibers through gap junctions, allowing the heart to contract as a synchronized unit.