Fig. 3.3.

Pathways for control of efferent sympathetic outflow. Major hypothalamic and brainstem sites involved in regulation of preganglionic sympathetic nerve activity in the spinal cord are shown. The direction of changes in hormones or inputs in pregnancy (P) are indicated by vertical gray up and down arrows. Circulating angiotensin II (AngII) and relaxin are known to activate paraventricular nucleus (PVN) projecting neurons in the subfornical organ (SFO), and insulin activates α-MSH neurons in the arcuate nucleus, which then project to and excite neurons in the PVN. PVN neurons excite preganglionic sympathetic neurons in the intermediolateral cell column of the spinal cord (IML) through either direct or indirect (synapse in rostral ventrolateral medulla, RVLM) projections. The arterial baroreflex pathway is shown in the bottom half of the diagram. Afferent baroreceptor input activates nucleus tractus solitarius (NTS) neurons, which in turn activate barosensitive GABAergic neurons in the caudal ventrolateral medulla (CVLM) projecting to and inhibiting ongoing activity in presympathetic RVLM neurons. In addition, there is a baroreceptor-independent inhibitory input to RVLM neurons arising from the CVLM that receives excitatory inputs from unknown brain regions. 3α-Hydroxy-dihydroprogesterone (3α-OH-DHP, a metabolite of progesterone) potentiates GABAergic inhibition of the RVLM. “+” indicates an excitatory effect and “−” indicates an inhibitory effect. Horizontal arrows indicate pathways where experimental results support (blue) or refute (gray) a possible role in adaptations during pregnancy.