Chapter 17: Nervous System: Autonomic Division

The anatomical organization of the ANS differs significantly from the SNS, utilizing a two-neuron efferent pathway: a preganglionic fiber originating in the CNS synapses with a peripherally located ganglionic neuron, whose postganglionic fiber then innervates the visceral effector organ. The ANS is functionally divided into the sympathetic division (the thoracolumbar or "fight or flight" system) and the parasympathetic division (the craniosacral or "rest and repose" system), which frequently exhibit opposing effects on target organs. The sympathetic division, originating in spinal segments T1–L2, is characterized by short preganglionic fibers that display extensive divergence before synapsing in either sympathetic chain ganglia (paravertebral/lateral ganglia) or collateral ganglia (prevertebral ganglia), such as the celiac, superior mesenteric, and inferior mesenteric ganglia. Sympathetic activation, which prepares the body for crisis by increasing alertness, mobilizing energy reserves, and increasing cardiovascular activity, is significantly enhanced by specialized neurons in the suprarenal medullae that release epinephrine (E) and norepinephrine (NE) into the circulation as hormones, producing widespread and prolonged effects mediated through alpha and beta receptors. In contrast, the parasympathetic division, which conserves energy and promotes sedentary activities like digestion and urination, originates in the brain stem (Cranial Nerves III, VII, IX, X) and sacral segments S2–S4, featuring long preganglionic fibers that synapse on ganglionic neurons located in terminal ganglia or intramural ganglia near or within the target organs. Neurotransmitter activity is consistent across the parasympathetic division, where all neurons release acetylcholine (ACh), with stimulation effects determined by whether the target cell possesses nicotinic receptors (always excitatory) or muscarinic receptors (excitatory or inhibitory). Most vital organs exhibit dual innervation, receiving instruction from both divisions, which intermingle in nerve networks called plexuses (e.g., cardiac, pulmonary, celiac) to coordinate antagonistic functions. Finally, the chapter defines visceral reflexes as the polysynaptic functional units of the ANS, distinguishing between long reflexes involving the CNS and short reflexes that bypass the CNS entirely.