The pacemaker is under autonomic (involuntary) control from the brain, specifically the medulla oblongata (brain stem)

Two nerves connected to the medulla regulate heart rate by either speeding it up or slowing it down:

The sympathetic nerve releases the neurotransmitter noradrenaline (a.k.a. norepinephrine) to increase heart rate
The parasympathetic nerve (vagus nerve) releases the neurotransmitter acetylcholine to decrease heart rate