The ANS is divided into the sympathetic nervous system (SNS; adrenergic system) and the parasympathetic nervous system (PNS; cholinergic system) (Fig. 15-1: Schematic distribution of the craniosacral (parasympathetic) and thoracolumbar (sympathetic) nervous systems). The SNS and PNS produce complementary effects on the activity of various organ systems (Table 15-1: Homeostatic Balance Between Divisions of the Autonomic Nervous System).
- Central Autonomic Organization. The principal site of ANS integration (blood pressure control, temperature regulation, stress responses) is the hypothalamus. Vital centers for hemodynamic and ventilatory control are located in the medulla oblongata and pons. ANS hyperreflexia is an example of spinal cord mediation of ANS reflexes without integration of function from higher inhibitory centers.
- Peripheral Autonomic Nervous System Organization (Fig. 15-2: Schematic diagram of the efferent autonomic nervous system)
- The cell body of the preganglionic neuron originates in the central nervous system (CNS) and synapses in an autonomic ganglion. The adrenal medulla is an exception. Preganglionic fibers are myelinated (rapid conducting).
- Postganglionic neurons arise from the autonomic ganglia and are distributed to effector organs. Postganglionic fibers are unmyelinated (slow conducting).
- The 22 pairs of SNS (paravertebral) ganglia are located closer to the spinal cord than to the innervated organ.
- The PNS ganglia are located in or near the innervated organ.
- Whereas activation of the SNS produces a diffuse physiologic response (mass reflex), activation of the PNS produces more discrete responses. For example, vagal stimulation may produce bradycardia with no effect on intestinal motility.
- Autonomic Innervation
- Heart. SNS and PNS innervation of the heart (via the stellate ganglion) influences heart rate (chronotropism), the strength of contraction (inotropism), and coronary blood flow.
- The PNS cardiac vagal fibers are distributed mainly to the sinoatrial (SA) and atrioventricular (AV) nodes, such that the main effect of cardiac vagal stimulation is chronotropic. Strong vagal stimulation can arrest SA node firing and block impulse conduction to the ventricles.
- The SNS has the same supraventricular distribution as the PNS but with stronger distribution to the ventricles. Normal SNS tone maintains contractility about 20% above that in the absence of SNS stimulation.
- Peripheral circulation. The SNS is the most important regulator of the peripheral circulation. Basal ANS tone maintains arteriolar diameter at about 50% of maximum, thus permitting the potential for further vasoconstriction or vasodilation. By functioning as a reservoir for about 80% of the blood volume, small changes in venous capacitance produced by SNS-mediated venoconstriction produce large changes in venous return.