**Core Concept**
Pacemaker cells are specialized cardiac cells that generate electrical impulses, initiating the heartbeat. These cells are responsible for the generation of the basic rhythm (BER) in the heart, known as the "pacemaker potential." The pacemaker cells are primarily located in the sinoatrial (SA) node, but they also exist in other parts of the heart, including the atrioventricular (AV) node and the bundle of His.

**Why the Correct Answer is Right**
The pacemaker cells contain a unique set of ion channels that allow them to generate a rhythmic electrical discharge. The pacemaker current, also known as the funny current (If), is an outward flow of potassium ions (K+) that contributes to the depolarization of the pacemaker cells. The slow depolarization of the pacemaker cells is due to the closure of potassium channels and the opening of sodium channels (Na+), allowing an influx of sodium ions. This process is mediated by the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, which are specific to the pacemaker cells.

**Why Each Wrong Option is Incorrect**
**Option A:** The cells in the sinoatrial (SA) node are indeed pacemaker cells, but this option is incomplete and does not specify the relationship with "BER."

**Option B:** The atrial myocytes are not typically referred to as pacemaker cells, as they primarily follow the electrical impulses generated by the SA node.

**Option C:** The ventricular myocytes do not function as pacemaker cells, as they are primarily responsible for contraction in response to electrical impulses from the AV node and the bundle of His.

**Clinical Pearl / High-Yield Fact**
The pacemaker cells in the SA node fire at a rate of approximately 60-100 beats per minute (bpm) at body temperature, and this rate can be influenced by various factors, including temperature, pH, and the presence of certain medications.

**Correct Answer:** C.