Initial heart development occurs independently of the nerves; however, there are three separate origins of the nerve fibres that innervate the heart. First, there are the sympathetic nerve fibres (adrenergic) which are derived from trunk neural crest act to speed up the heartbeat. Then there are the parasympathetic (cholinergic) nerve fibres which are derived from the cardiac component of the cranial neural crest. From the neural crest there are neurons of the cardiac ganglia which are the second order of parasympathetic neurons and migrate directly to the heart. These synapse with axons of the first order parasympathetic neurons, which gain access to the heart via the vagus nerve. Additionally, sensory innervation supplies the heart via the vagus nerve; however, the sensory neurons originate from placodal ectoderm.
Figure.7: innervation of the heart (Carlson, 2009).
Conducting system of the heart:
The sinuatrial node is formed when the pacemaker initially lies in the caudal part of the left cardiac tube. when the sinus venosus is incorporated into the right atrium,the pacemaker lies near the opening of the superior vena cava. Moreover, there are two sources that help to form the atrioventricular node and bundle (bundle of His). The first are the cells in the left wall of the sinus venosus, and the second are the cells from the atrioventricular canal. These cells lie in their position at the base of the interatrial septum when the sinus venosus is incorporated into the right atrium.