Where is the nervous tissue located in the heart?

Sympathetic innervation of the heart is from cardiac accelerator nerves descending from the uppermost thoracic sympathetic ganglia and the cervical sympathetic ganglia. Parasympathetic innervation comes from the vagus nerve (Cranial Nerve X). These nerves continue into the walls of the heart and provide autonomic innervation, which can only modify the rate and contractility of the heart.

Inside the heart, intrinsic pacemaking comes from the sinoatrial node, which automatically paces the heart due to the existence of "funny currents" (seriously, that's what they're called) that depolarize the node. The SA node is located in the right atrium and the depolarization follows tracks to the left atrium and the atrioventricular node. The AV node can automatically depolarize, but it does so at a lower rate than the SA node, so the SA node overrides the AV nodal rhythm.

From the AV node, the electrical signal goes through Bundles of His and Purkinje Fibers to deliver a wave of depolarization to the cardiac muscle. These cells are modified cardiac myocytes between the endocardium and the myocardium specialized for the transmission of electrical impulse. Technically, these are not nerves, though they function similar to nerves.

Beyond that, there are gap junctions between cardiac myocytes in the intercalated disks that allow for the coordinated contraction of cardiac muscle. In other Words, the cells are arranged like a net and the depolarization and contraction of one cell leads to the depolarization and contraction of the cells next to it that have not yet been depolarized.