The sympathetic innervation of the heart originates from the thoracic portion (T1-T4 or T5) of the spinal cord. These presynaptic fibers first travel to either the cervical (superior, middle or inferior) or the thoracic ganglia of the sympathetic chain, where they synapse. The postsynaptic fibers emerging from the ganglia will then travel to the heart in small cervical or thoracic cardiac nerves (a.k.a. thoracic visceral nerves) to innervate it. Stimulation of the sympathetic nervous system within this target organ causes an INCREASE in heart rate and contractility.

Why do the cardiac nerves bother to ascend into the neck, simply to descend back down into the chest? It's because the heart initially develops in the general vicinity of the neck, and then descends into the chest, drawing its nerves down with it. This is a recurrent theme in anatomy - when the adult anatomy seems illogical, look to embryology to help explain it.