The ANS, as part of the peripheral nervous system, is crucial to proper cardiovascular control. The organization of autonomic innervation of the cardiovascular system is important for the understanding of complications that arise as a result of autonomic dysfunction following SCI. The ANS controls involuntary responses, and is responsible for relaying information from the central nervous system to target organs. It is comprised of two opposing systems, the parasympathetic nervous system (PNS) and sympathetic nervous system (SNS), which can act almost opposite to each other according to various needs to provide balanced regulation. There are two populations of neurons comprising efferent pathways involved in the transmission of information from the central nervous system to target organs. Preganglionic neurons reside centrally in brainstem or spinal cord gray matter and synapse onto peripheral postganglionic neurons, which are directly connected to and project onto the target organ. Parasympathetic preganglionic neurons are located within the nuclei of the four cranial nerves (CN III, VII, IX, X) as well as the sacral spine segments (S2-S4); therefore, parasympathetic outflow is both cranial and sacral. Sympathetic outflow is thoracolumbar, as preganglionic neurons are located within the thoracic (T1-T12) and upper lumbar (L1-L2) segments of the spinal cord.
Specific to cardiovascular function, the heart receives dual innervation from both the PNS and SNS. Parasympathetic cardiac innervations originates from the cardiovascular nuclei within the medulla oblongata located in teh brain stem and reaches the heart via the vagus nerve (CN X), innervating the heart through the sinoatrial node and reducing heart rate (HR) and contractility. Sympathetic cardiac innervation stems from the upper thoracic segments of the spinal cord (T1-T5) and has a stimulating effect on these functions of the heart. The sympathetic divisioni of the ANS is also responsible for controlling the smooth muscle of peripheral blood vessels by inducing vasoconstriction, while there exists no parasympathetic innervation of peripheral vasculature. Modulation of sympathetic and parasympathetic activity is determined by the baroreflex system, which monitors changes in arterial blood pressure (BP) by means of baroreceptors located in the aortic arch, carotid sinus and coronary arteries. Further, chemoreceptors in the carotid bodies deteect changes in blood oxygen and carbon dioxide concentrations, and relay the information via negative feedback to establish hemodynamic homeostasis. Together, the PNS and SNS interact to produce proper regulation of HR, BP and vagal tone (Furlan & Fehlings, 2008; Hagen et al. 2012; Krassioukov, 2009; Occhi et al. 2002; Popa et al. 2010; Sampson et al. 2000).