Anatomy of the Autonomic Nervous System
The ANS, both central and peripheral nervous system components, is crucial to proper cardiovascular control, and is affected by SCI (Furlan & Fehlings 2008). The ANS controls involuntary responses relaying information from the central nervous system to target organs. The ANS is comprised of two opposing components, the parasympathetic nervous system (PNS) and sympathetic nervous system (SNS), which provide balanced regulation. 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.
The heart receives dual innervation from both the PNS and SNS. Parasympathetic cardiac innervation originates from the cardiovascular nuclei within the medulla oblongata located in the brainstem and innervates the heart via the vagus nerve (CN X) and then through the sinoatrial node; it reduces heart rate (HR) and heart contractility. Sympathetic cardiac innervation stems from the upper thoracic segments of the spinal cord (T1-T5) and has a stimulating effect on these same functions of the heart. The sympathetic division of the ANS is also responsible for controlling the smooth muscle of peripheral blood vessels by inducing vasoconstriction, while there exists no corresponding parasympathetic innervation of the 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 detect changes in blood oxygen and carbon dioxide concentrations, and relay the information to help establish hemodynamic homeostasis. Together, the PNS and SNS interact to allow 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).