Nothing. It would still continue to send the electrical impulses. I have read of some pacemaker cases where the nerve to the heart was severed so that the brain's command to pump would not interfer with the signal from the pacemaker.

1) The heart's function is normally not controlled by the brain, but by the sinoatrial node, or "cardiac pacemaker" (the body's *natural* pacemaker). However, the rate of contraction can be changed by nervous or hormonal influences, exercise and emotions. The cardiovascular centre is a part of the human brain responsible for this regulation. By a defect of the body's own, natural pacemaker, an artificial pacemaker can be used. I could not find any information about what happens to the natural function of the sinoatrial node and the cardiovascular centre when an artificial pacemaker is used. I assume that they still try to regulate the heart as they would normally do, but that their pacemaker potential is overridden by the artificial pacemaker. 2) "Cardiac muscle has automaticity, which means that it is self-exciting. This is in contrast with skeletal muscle, which requires either conscious or reflex nervous stimuli for excitation. The heart's rhythmic contractions occur spontaneously, although the rate of contraction can be changed by nervous or hormonal influences, exercise and emotions. For example, the sympathetic nerves to heart accelerate heart rate and the vagus nerve decelerates heart rate. The rhythmic sequence of contractions is coordinated by the sinoatrial (SA) and atrioventricular (AV) nodes. The sinoatrial node, often known as the cardiac pacemaker, is located in the upper wall of the right atrium and is responsible for the wave of electrical stimulation that initiates atrial contraction by creating an action potential. Once the wave reaches the AV node, situated in the lower right atrium, it is delayed there before being conducted through the bundles of His and back up the Purkinje fibers, leading to a contraction of the ventricles. The delay at the AV node allows enough time for all of the blood in the atria to fill their respective ventricles. In the event of severe pathology, the AV node can also act as a pacemaker; this is usually not the case because their rate of spontaneous firing is considerably lower than that of the pacemaker cells in the SA node and hence is overridden." Source and further information: http://en.wikipedia.org/wiki/Cardiac_cycle 3) "The cardiovascular centre is a part of the human brain responsible for the regulation of the rate at which the heart beats. It is found in the medulla. Normally, the heart beats without nervous control, but in some situations (e.g., exercise, body trauma), the cardiovascular centre is responsible for altering the rate at which the heart beats. The cardiovascular centre effects changes to the heart rate via sympathetic fibres (to speed up the heart rate) and the vagus nerve (to slow down the heart rate). The accelerator and vagus nerves both connect to the sinoatrial node (SAN). The cardiovascular centre also increases the stroke volume of the heart (that is, the amount of blood it pumps). These two changes help to regulate the cardiac output, so that a sufficient amount of blood reaches tissue. Hormones like adrenaline can affect the cardiovascular centre and cause it to increase the rate of impulses sent to the sinoatrial node, or "cardiac pacemaker", thus increasing the rate of the heart beat. Chemoreceptors may also prompt this regulation." Source and further information: http://en.wikipedia.org/wiki/Cardiovascular_centre 4) "The contractions of the heart are controlled by chemical impulses, which fire at a rate which controls the beat of the heart. The cells that create these rhythmical impulses are called pacemaker cells, and they directly control the heart rate. Artificial devices also called pacemakers can be used after damage to the body's intrinsic conduction system to produce these impulses synthetically." "Although all of the heart's cells possess the ability to generate these electrical impulses (or action potentials), a specialised portion of the heart, called the sinoatrial node, is responsible for the whole heart's beat." Source and further information: http://en.wikipedia.org/wiki/Cardiac_pacemaker Further information: http://en.wikipedia.org/wiki/Sinoatrial_node 5) "One unrealized advancement in pacemaker function could mimic nature by utilizing various bodily input parameters such as CO2 - O2 at in arterial-vein system, body temperature, ATP levels, Adrenaline, etc. Instead of producing a static, predetermined heart rate, or intermittent control, a Dynamic Pacemaker could compensate for both actual respiratory loading and potentially anticipated respiratory loading. A Dynamic Pacemaker would require sensory technology for which heart-rate regulation parameters must first be acutely identified. Dynamic Pacemaking technology could also be applied to future artificial hearts. Advances in transitional tissue welding would support this and other artificial organ/joint/tissue replacement efforts. Stem cells may or may not be of interest to transitional tissue welding. When first invented, pacemakers controlled only the rate at which the heart's two largest chambers, the ventricles, beat. Many advancements have been made to enhance the control of the pacemaker once implanted. Many of these enhancements have been made possible by the transition to microprocessor controlled pacemakers. Pacemakers that control not only the ventricles but the atria as well have become common. Pacemakers that control both the atria and ventricles are called dual-chamber pacemakers. Although these dual-chamber models are usually more expensive, timing the contractions of the atria to precede that of the ventricles improves the pumping efficiency of the heart and can be useful in congestive heart failure. Rate responsive pacing allows the device to sense the physical activity of the patient and respond appropriately by increasing or decreasing the base pacing rate via rate response algorithms. The DAVID trials have shown that unnecessary pacing of the right ventricle can lead to heart failure and an increased incidence of atrial fibrillation. The newer dual chamber devices can keep the amount of right ventricle pacing to a minimum and thus prevent worsening of the heart disease." Source and further information: http://en.wikipedia.org/wiki/Artificial_pacemaker