Final answer:
The brain controls the heart's primary pacemaker through the autonomic nervous system, which includes sympathetic stimulation to increase heart rate and parasympathetic stimulation to decrease it, the latter primarily through the vagus nerve. The correct option is 4) autonomic nervous system
Step-by-step explanation:
The electrical stimulus that originates in the heart's primary pacemaker is controlled by impulses from the brain that arrive by way of the autonomic nervous system.
This system can be divided into the sympathetic and parasympathetic nervous systems, which have opposite effects on heart rate. The sympathetic nerves increase heart rate, while the parasympathetic nerves, particularly the vagus nerve (cranial nerve X), decrease it.
The pacemaker cells within the sinoatrial (SA) node of the heart, which is responsible for initiating the heartbeat, are modulated by these autonomic inputs to maintain appropriate heart rates that match the body's requirements for oxygen and nutrients. The correct option is 4) autonomic nervous system