9/11/2023 0 Comments Chordotonal neuron dendrite iav![]() ![]() 4, 8 The majority of post-ganglionic sympathetic fibers, however, synapse directly onto the myocardium. 7 In addition, these fibers provide input to the numerous ganglionated subplexuses interspersed throughout bilateral atria and ventricles. 5, 6 Therefore, sympathetic innervation is particularly dense around the sinus node and coronary sinus, with decreasing in density from the base of the ventricle to the apex. Postganglionic fibers then originate from these ganglia and travel along epicardial vascular structures as dictated by embryological growth cues of endothelin-1 and nerve growth factor (NGF) released by vascular smooth muscle cells, particularly along coronary veins and then arteries. 4 These preganglionic fibers leave the spinal cord at the level of T1 to T4 and synapse in the right and left stellate ganglia, T2–T4 thoracic, and middle cervical ganglia. The journey of cardiac sympathetic preganglionic fibers originates in the central nervous system (CNS) primarily in the brainstem with modulation by higher centers such as the subthalamic and periaqueductal grey as well as rostral ventrolateral medulla. Finally, although sympathetic efferent fibers originate in the thoracic ganglia and parasympathetic preganglionic fibers travel in the vagal trunk, it is important to note that there is significant intermixing of these fibers in the thorax so that most nerves reaching the heart in the mediastinum have mixed (sympathetic and parasympathetic) fibers. In addition, direct vagal afferent fibers originate from the myocardium and synapse via pseudo-unipolar neurons of the nodose ganglia in the nucleus tractus solitarius of the brainstem. At each level, afferent neurotransmission feeds back information to neurons that in turn affect efferent control of the heart, completing an independent neural circuit that modulates cardiac function. Some of this information is then sent to the brainstem and higher centers. Finally, sympathetic afferent information passes through the dorsal root ganglia and reaches the spinal cord where additional neural processing can take place. In the thorax, the extra-cardiac but intrathoracic ganglia such as the stellate ganglia, the middle cervical ganglia, and the thoracic ganglia of T2–T4 also process neural information, controlling sympathetic outflow to the heart. The intrinsic cardiac ganglia reside on the epicardium and receive post-ganglionic sympathetic and pre-ganglionic parasympathetic connections. Neural processing occurs at several levels, figure 1. The autonomic nervous system consists of sympathetic and parasympathetic branches. ![]()
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