The sympathetic cardiac nerves supply the heart with sympathetic fibers. It means that these nerves increase the heart rate, accelerating the ventricular contraction. There three pairs: 1) the superior cardiac nerve, which arises from the superior cervical ganglion; 2) the middle cardiac nerve, which originates from the middle cervical ganglion; 3) the inferior cardiac nerve, which extends from the stellate ganglion. They are postganglionic, non-myelinated fibers that are distributed in the heart, making up, along with the parasympathetic vagal fibers, the cardiac plexus.
The sympathetic cardiac nerves originate from the neurons located in the lateral horns of the spinal cord grey matter, known as tratus intermedium lateralis, from T1 to T5. The axons of these nerve cells come out of the spinal cord, following the anterior motor roots of the first four thoracic pairs.
There is also a thin sympathetic bundle of cardiac accelerator fibers that arises from the medulla oblongata. This runs along parallel to the vagal nerve.