Question: Where are the group of cells that control the natural resting heart rate found?

Disambiguated questions and answers:
context: The sinoatrial node (also known as the SA node or the sinus node) is a group of cells located in the wall of the right atrium of the heart. These cells have the ability to spontaneously produce an electrical impulse (action potential; see below for more details), that travels through the heart via the electrical conduction system (see figure 1) causing it to contract. In a healthy heart, the SA node continuously produces action potential, setting the rhythm of the heart and so is known as the heart's natural pacemaker. The rate of action potential production (and therefore the heart rate) is influenced by nerves that supply it.
question: Where are the group of  muscle cells that control the natural resting heart rate found?
short_answers: atria

Right Atrium
wikipage: Sinoatrial node

context: If the SA node does not function, or the impulse generated in the SA node is blocked before it travels down the electrical conduction system, a group of cells further down the heart will become its pacemaker. This center is typically represented by cells inside the atrioventricular node (AV node), which is an area between the atria and ventricles, within the atrial septum. If the AV node also fails, Purkinje fibers are occasionally capable of acting as the default or "escape" pacemaker. The reason Purkinje cells do not normally control the heart rate is that they generate action potentials at a lower frequency than the AV or SA nodes.
question: Where are the cells that control the resting heart rate found if the SA node is damaged?
short_answers: Atrioventricular node

AV node
wikipage: Cardiac pacemaker

context: The main role of a sinoatrial node cell is to initiate action potentials of the heart, so that it can pass throughout the heart and cause contraction. An action potential is a change in voltage (membrane potential) across the membrane of the cell, produced by the movement of charged atoms (ions). Non-pacemaker cells (including the ventricular and atrial cells) have a period, immediately after an action potential, where the membrane potential remains relatively constant; this is known as a resting membrane potential. This resting phase (see cardiac action potential, phase 4) ends when another action potential reaches the cell. This produces a positive change in membrane potential (known as depolarisation), which initiates the start of the next action potential. Pacemaker cells, however, don’t have this resting phase. Instead, immediately after one action potential, the membrane potential of these cells begins to depolarise again automatically, this is known as the pacemaker potential. Once the pacemaker potential reaches a set value, known as the threshold value, it then produces an action potential. Other cells within the heart (including the purkinje fibers and atrioventricular node; AVN) can also initiate action potentials; however, they do so at a slower rate and therefore, if the SA node is working, it usually beats the AVN to it.
question: Where are the cells that control the resting heart rate found?
short_answers: SA node

sinoatrial node
wikipage: Sinoatrial node

Long answers:
long_answer: The main role of a sinoatrial node cell, or SA node, is to initiate action potentials of the heart, so that it can pass throughout the heart and cause contraction. If the SA node does not function, or the impulse generated in the SA node is blocked before it travels down the electrical conduction system, a group of cells further down the heart will become its pacemaker. This center is typically represented by cells inside the atrioventricular node (AV node), which is an area between the atria and ventricles, within the atrial septum.

knowledge: content: The normal resting heart rate is based on the at-rest firing rate of the heart's sinoatrial node, where the faster pacemaker cells driving the self-generated rhythmic firing and responsible for the heart's autorhythmicity are located.[24]
wikipage: Heart rate Resting heart rate
long_answer: The sinoatrial node (also known as the SA node or the sinus node) is a group of cells located in the wall of the right atrium of the heart. In a healthy heart, the SA node continuously produces action potential, setting the rhythm of the heart and so is known as the heart's natural pacemaker. The normal resting heart rate is based on the at-rest firing rate of the heart's sinoatrial node. If the SA node does not function, or the impulse generated in the SA node is blocked before it travels down the electrical conduction system, a group of cells further down the heart will become its pacemaker. This center is typically represented by cells inside the atrioventricular node (AV node), which is an area between the atria and ventricles, within the atrial septum.