FIGURE 2.

The mid sagittal section of the brain and the spinal cord used here for schematic illustration of the functional anatomy of postural orthostatic tachycardia. Electrocardiogram (ECG) and the arterial blood pressure waveform (BP waveform), both of which are generated by the heart, are the only two cardiovascular biometrics used to derive all the cardiovascular parameters displayed on the NeuroScope screen (see Figure 12 and main text for details). The Roman numerals show the ninth (IX) and tenth (X) cranial nerves. These are the most important nerves in the generation of central parasympathetic cardiovascular regulation and they provide the only tonic parasympathetic supply for the whole body in humans. These two cranial nerves communicate with the bradycardia area (Bra) in the caudal part of medulla oblongata and the tachycardia area (Tac) in the rostral part of the medulla (RVLM). Skeletal muscles vascular bed provides the most of the windkessel vascular resistance, while the visceral (or splanchnic) vascular bed provides the most of the venous vascular capacitance, but the sympathetic supply to the skin is largely suppressed during orthostasis (see main text for more details). These three vascular beds are important for arterial blood pressure defense during orthostasis. Animal studies show that there are organotopic neurons, which are discrete and separate groups of neurons in the RVLM that specifically regulate sympathetic supplies to the skin (Skn), skeletal muscles (Mcl), and the visceral (Gut) vascular beds. There are also organotopic neurons within the RVLM regulating catecholamine release (Cat) and sympathetic supply to the kidney (Ren). The Arabic numerals (1–14) within the brain and the brainstem show the central autonomic regulation sites (CARS) that have been described during functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) studies of central autonomic functions. These are large sites in the brain that regulate several autonomic functions and some of the connections shown between the different CARS sites are based on animal studies (see main text for more details). The Arabic numerals (1–12) beside the spinal cord indicate the respective thoracic segments. The tonic vasomotor supplies to the three major vascular beds come out of the spinal between T1 and T5