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Autonomic Nervous System Overview

Autonomic Nervous System: Sympathetic vs Parasympathetic Physiology Sympathetic Nervous System (SNS) Parasympathetic Nervous System (PNS) Short preganglionic SNS neurons originate in thoracolumbar spinal cord (T1-L2) Long preganglionic PNS neurons originate in brainstem & sacral spinal cord (S2-S4) Authors: Aradhana Jacob Reviewers Allesha Eman Emily J. Doucette Jean Mah* * MD at time of publication Preganglionic neurons release excitatory neurotransmitter Acetylcholine (ACh) ACh binds nicotinic receptors on long postganglionic SNS neurons at the prevertebral ganglia (midline) or paravertebral ganglia (sympathetic chain) Postganglionic SNS neurons release norepinephrine (NE) NE binds α1- & β-adrenergic receptors in target organs NE binds α1 receptors on apocrine sweat glands Emotional sweating NE binds α1 receptors on radial muscle of the iris Contraction of radial muscle (dilator pupillae) Mydriasis (pupil dilation) NE binds α1 & β receptors on salivary gland acinar cells ↓ Saliva volume & ↑ protein content Xerostomia (dry mouth) NE binds β2 receptors on bronchial smooth muscle Inhibition of smooth muscle contraction Bronchodilation NE binds α1 & β2 receptors on hepatocytes ↑ Gluconeogenesis & ↑ glycogenolysis ↑ Blood glucose NE binds β1 receptors on the sinoatrial node in the heart ↑ Action potential frequency ↑ Heart rate NE binds α1 receptors on vascular smooth muscle Vascular smooth muscle contraction Vasoconstriction NE binds α1 receptors on smooth muscle of vas deferens, seminal vesicles, prostate, & internal urethral sphincter Smooth muscle contraction Seminal emission NE binds β2 receptors on GI smooth muscle GI smooth muscle relaxation ↓ GI motility ACh binds nicotinic receptors on short postganglionic PNS neurons located near or within target organs Postganglionic PNS neurons release ACh ACh binds muscarinic receptors in target organs ACh binds M3 receptors on sphincter pupillae muscle of iris Contraction of iris sphincter Myosis (pupil constriction) ACh binds M3 receptors on salivary gland acinar cells ↑ Saliva volume & ↑ salivary amylase ↑ Watery saliva ACh binds M3 receptors on Activation of smooth bronchial smooth muscle Bronchoconstriction muscle contraction ACh binds M3 receptors on Pancreatic β-cells ↑ Insulin secretion ↓ Blood glucose ACh binds M2 muscarinic receptors on the sinoatrial node in the heart ↓ Action potential frequency ↓ Heart rate ACh binds endothelial M3 receptors on blood vessels Vascular smooth muscle relaxation Vasodilation ↑ Nitric oxide production Smooth muscle relaxation in corpora cavernosa & vasodilation penile arteries Tumescence (penile erection) ACh binds M3 receptors on GI smooth muscle & glands ↑ Peristalsis (wave-like smooth muscle contractions) ↑GI motility Legend: Complications Published Aug 4, 2025 on www.thecalgaryguide.com Pathophysiology Mechanism Sign/Symptom/Lab Finding