The most important level of integration of sympathetic nervous system efferent activities to the cardiovascular system resides in the dorsolateral reticular formation of the medulla. The hypothalamus modifies the activity of the medullary centers and is important in stimulating cardiovascular responses to emotion and stress. The motor outflow of the sympathetic nervous system is formed by 2 serially connected sets of neurons. The first set (pre-ganglionic neurons) originates in the brain stem or the spinal cord, and the second set (post-ganglionic neurons) lies outside the central nervous system in collections of nerve cells called sympathetic ganglia. The sympathetic pre-ganglionic neurons originate in the lateral horns of the 12 thoracic and the first 2 or 3 lumbar segments of the spinal cord (thoracolumbar outflow). The axons of these neurons (short, myelinated) exit the spinal cord in the ventral roots and then synapse on either sympathetic ganglion cells or chromaffin cells in the adrenal gland that release epinephrine (EPI). The sympathetic ganglia can be divided into 2 major groups: 1) the paravertebral (3 in the cervical region including the right and left stellate ganglia, 10 to 11 in the thoracic region, 4 in the lumbar region, 4 in the sacral region, and a single, unpaired ganglion lying in front of the coccyx), which lie on each side of the vertebrae and are connected to form the sympathetic chain or trunk; and 2) the pre-vertebral (pre-aortic), which provide axons that are distributed with the 3 major gastrointestinal arteries arising from the aorta. The predominant neurotransmitter of the sympathetic pre-ganglionic neurons is acetylcholine, whereas the predominant neurotransmitter of most sympathetic post-ganglionic neurons is norepinephrine. Sympathetic activity is attenuated (–) by the arterial baroreflex and the cardiopulmonary reflex and increased (+) by the cardiac sympathetic afferent reflex (CSAR) and the arterial chemoreceptor reflex. H = hypothalamus; M = medulla.

The major intracellular effect of the sympathetic transmitters norepinephrine and epinephrine is mediated by formation of 3',5'-cyclic monophosphate (cAMP), which increases the activity of the cAMP-dependent protein kinase A (PKA). PKA mediates a series of phosphorylations in diverse intracellular substrates, including the L-type Ca⁺⁺ channels (LTTC), hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, sarcoplasmic ryanodine receptors (RyR), phospholamban (PLB), myofibrillar proteins troponin I (TnI), cardiac myosin-binding protein C (MyBPC), and phospholemman (PLM). AC = adenylyl cyclase; AR = adrenergic receptor; ATP = adenosine triphosphate; CNBD = cyclic nucleotide-binding domain; G_alpha-i and G_alpha-s = G protein alpha-subunit subtypes; SERCA = sarcoendoplasmic reticulum.

Agonist-induced stimulation of alpha₁-ARs activates G_q and phospholipase C_b (PLC_b), resulting in hydrolysis of phosphatidylinositol bisphosphate (PIP₂), to generate inositol trisphosphate (IP₃) and diacylglycerol (DAG). DAG, in turn, activates protein kinase C (PKC) to initiate a series of phosphorylations that alter channel activity and induce transcriptional changes. Moreover, IP₃ interacts with perinuclear inositol trisphosphate receptors (IP₃R) disinhibiting growth-related gene transcription. Both PIP₂ and DAG increase the permeability of the transient receptor potential (Trp) channel to Ca²⁺, which enters the cell and activates calcineurin to initiate downstream growth signaling pathways. Ca²⁺ entry through transient receptor potential channels may also act on myofilaments enhancing contractile responses. The alpha₁-AR also transactivates epithelial growth factor receptors, resulting in formation of phosphoinositide 3-kinase (PI₃K) and phosphatidylinositol trisphosphate (PIP₃), activation of the Akt pathway, and initiation of cell-survival signaling pathways. Abbreviations as in Figure 2.

(1) An insult causes cardiac dysfunction and decreases cardiac output. (2) Attenuation of inhibitory sympathetic cardiovascular reflexes and augmentation of excitatory sympathetic cardiovascular reflexes is associated with increased sympathetic input in the central nervous system. (3 and 4) Central facilitation of the augmented cardiovascular sympathetic afferent reflex mediated by an increase in angiotensin II and cytokines and a decrease in nitric oxide (NO) contributes to tonic increases in sympathetic output. (5) The chronic increase in sympathetic output is associated with structural and functional changes in the cardiomyocytes and the interstitium leading to left ventricular (LV) dilation and systolic dysfunction (LV remodeling). m = medulla; RAAS = renin-angiotensin-aldosterone system.