# Parasympathetic vs Sympathetic Nervous System The parasympathetic nervous system calms and restores the body while the sympathetic nervous system mobilizes it for action, and the quickest rule of thumb is that parasympathetic fibers travel with the vagus and sacral nerves to slow the heart and drive digestion, whereas sympathetic fibers arise from the thoracolumbar cord to speed the heart, dilate the pupils, and prepare the body for fight or flight. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## The Two Divisions of the Autonomic Nervous System

The autonomic nervous system (ANS) controls the involuntary functions of the body: heart rate, blood pressure, digestion, pupil size, glandular secretion, and bladder emptying. It operates without conscious input and is divided into two major motor outflows, the parasympathetic nervous system (PSNS) and the sympathetic nervous system (SNS) [1]. Both divisions are built from a two-neuron chain. A preganglionic neuron sits inside the central nervous system and sends its axon out to a ganglion, where it synapses on a postganglionic neuron. The postganglionic axon then reaches the target organ. A common teaching claim is that the parasympathetic system has a low ratio of preganglionic to postganglionic neurons (one-to-few) while the sympathetic system has a high ratio (one-to-many). A review of the original observations behind this "ratio rule" found that the difference was overstated and that the two divisions are not as cleanly separated by neuron ratios as once taught [1]. For clinical purposes, the more reliable distinctions are the site of origin, the neurotransmitters released, and the receptor subtypes on the target organs. ### Where Each Division Begins The sympathetic outflow is thoracolumbar. Preganglionic cell bodies sit in the intermediolateral cell column of the spinal cord gray matter from the first thoracic segment to about the third lumbar segment. Their axons leave through the ventral roots and pass into the sympathetic chain, a paired set of ganglia running alongside the vertebral column. Comparative anatomy work in sheep fetuses shows that the cervical sympathetic trunk and its ganglia vary in number and position with age and species, and that sheep anatomy differs substantially from human anatomy, which is one reason veterinary students cannot simply borrow human diagrams [2]. The parasympathetic outflow is craniosacral. Preganglionic neurons sit in the brainstem nuclei associated with cranial nerves III, VII, IX, and X, and in the sacral spinal cord. The vagus nerve (cranial nerve X) is the largest parasympathetic nerve in the body. Its efferent fibers arise in the dorsal motor nucleus of the vagus in the caudal brainstem and innervate the heart, lungs, and gut from the esophagus to the proximal colon [3][4]. Vagal afferent fibers, which carry sensory information back to the brain, sit in the nodose ganglia and pick up mechanical and chemical signals from the stomach wall [3]. ### Ganglia and the Two-Neuron Chain Sympathetic ganglia lie close to the spinal cord in the sympathetic chain, so sympathetic preganglionic fibers are short and postganglionic fibers are long. Parasympathetic ganglia lie near or inside the target organ (for example, the myenteric plexus of the gut), so parasympathetic preganglionic fibers are long and postganglionic fibers are short. This anatomical difference explains why sympathetic activation can produce widespread, coordinated effects while parasympathetic activation tends to be more organ-specific. ## Side-by-Side Comparison Table The table below summarizes the key differences. Receptor subtypes are named using standard pharmacology: nicotinic receptors respond to acetylcholine at ganglia, muscarinic receptors respond to acetylcholine at target organs, and alpha and beta adrenergic receptors respond to norepinephrine and epinephrine at sympathetic targets. | Feature | Parasympathetic | Sympathetic | |--|--|--| | Origin | Craniosacral: brainstem (CN III, VII, IX, X) and sacral cord | Thoracolumbar: T1 to L3 intermediolateral column | | Main nerve | Vagus (CN X) plus pelvic nerves | Sympathetic chain and splanchnic nerves | | Ganglion location | Near or within target organ | Near spinal cord (paravertebral chain) | | Preganglionic neurotransmitter | Acetylcholine | Acetylcholine | | Preganglionic receptor | Nicotinic (Nn) | Nicotinic (Nn) | | Postganglionic neurotransmitter | Acetylcholine | Norepinephrine (except sweat glands in horses, which are cholinergic) | | Postganglionic receptor | Muscarinic (M2, M3) | Alpha-1, alpha-2, beta-1, beta-2 | | Heart rate | Slows (vagal brake) | Speeds | | Cardiac contractility | Minor direct effect | Increases | | Pupils | Constricts (miosis) | Dilates (mydriasis) | | Gut motility | Increases | Decreases | | Gut secretion | Increases | Decreases | | Bladder detrusor | Contracts (emptying) | Relaxes (filling) | | Internal urethral sphincter | Relaxes | Contracts | | Airways | Bronchoconstriction, increased secretion | Bronchodilation, decreased secretion | | Salivary glands | Thin, watery saliva | Thick, viscous saliva | | Sweat glands | Not innervated | Cholinergic in horses, adrenergic in most other species | | Overall role | Rest, digest, restore | Fight, flight, stress | ## Neurotransmitters and Receptors in Detail ### Cholinergic Transmission Acetylcholine (ACh) is the neurotransmitter at every autonomic ganglion, both sympathetic and parasympathetic, and at every parasympathetic postganglionic ending. The receptor at the ganglion is nicotinic, a ligand-gated ion channel that produces fast depolarization. The receptor at the target organ is muscarinic, a G protein-coupled receptor. Muscarinic M2 receptors dominate in the heart and slow the rate by reducing the slope of the pacemaker potential. Muscarinic M3 receptors dominate in smooth muscle and glands and drive contraction and secretion. Vagal control of the stomach illustrates how this works in practice. Vagal cholinergic efferent neurons in the dorsal motor nucleus of the vagus do not synapse directly on muscle cells. They synapse on enteric neurons in the myenteric plexus, which act as muscle motor neurons and pattern generators that regulate specific motility and secretion programs [3]. This arrangement gives the gut local control while still allowing the brain to modulate activity through the vagus. ### Adrenergic Transmission Most sympathetic postganglionic neurons release norepinephrine (noradrenaline), which acts on alpha and beta adrenergic receptors. Alpha-1 receptors constrict blood vessels and dilate pupils. Alpha-2 receptors sit on presynaptic terminals and provide negative feedback. Beta-1 receptors dominate in the heart and increase rate and force. Beta-2 receptors relax bronchial smooth muscle and dilate airways. A small number of sympathetic postganglionic neurons are cholinergic, meaning they release acetylcholine instead of norepinephrine. The classic veterinary example is the sweat glands of the horse, which are innervated by sympathetic cholinergic fibers. In most other domestic species, sweat glands are either adrenergic or respond to circulating epinephrine. This species difference matters when interpreting responses to drugs that block muscarinic or adrenergic receptors. The adrenal medulla is a special case. It is a modified sympathetic ganglion that releases epinephrine and some norepinephrine directly into the bloodstream, producing a body-wide adrenergic effect that lasts longer than direct nerve stimulation. ## Organ-by-Organ Effects ### Heart The heart receives dual innervation. The right vagus nerve is essential for cardiac homeostasis, and its intrathoracic resection can lead to postoperative cardiac complications [5]. In a minipig model, reconnecting the right vagus nerve with a nerve guidance conduit preserved cardiac mechanical function and restored myocardial parasympathetic fibers, showing that even partial vagal input protects the heart from remodeling [5]. Sympathetic and vagal activity are not simply opposites. In dogs with premature ventricular contractions, exercise triggered an increase in both sympathetic nerve activity and vagal nerve activity, followed by a late withdrawal of vagal tone. When premature ventricular contractions were present, the sympathetic surge was magnified and the late vagal withdrawal was blunted. After cardiomyopathy developed, resting sympathetic activity was increased but failed to augment properly during exercise, and vagal recovery was impaired [6]. This shows that sympathovagal balance is dynamic and that disease can disrupt both limbs. ### Pupils Sympathetic activation dilates the pupil through alpha-1 receptors on the dilator pupillae muscle. Parasympathetic activation constricts the pupil through muscarinic receptors on the sphincter pupillae muscle, carried by the oculomotor nerve (CN III). In a frightened dog or cat, the pupils dilate because sympathetic tone dominates. In a dog under deep anesthesia or with high vagal tone, the pupils may constrict. ### Gut The vagus nerve innervates the gut from the esophagus to the proximal colon, while the sacral nerves innervate the distal colon and rectum [4]. Vagal stimulation in rats increased rectal volume, indicating relaxation, and accelerated distal colon transit. The relaxation was abolished by a nitric oxide synthase inhibitor, showing that nitrergic (nitric oxide-releasing) neurons mediate part of the effect [4]. This means the parasympathetic control of the lower gut is not purely cholinergic. The sympathetic system inhibits gut motility and secretion. In gastroesophageal reflux disease, a decrease in vagal tone and an increase in sympathetic activity have been reported, with autonomic balance shifted toward the sympathetic system [7]. While this is a human clinical observation, the underlying physiology applies across mammals. ### Bladder The bladder is a classic example of dual innervation with opposite effects. Parasympathetic fibers from the sacral cord cause the detrusor muscle to contract and the internal urethral sphincter to relax, producing micturition. Sympathetic fibers from the lumbar cord keep the detrusor relaxed and the internal sphincter closed, allowing the bladder to fill without leakage. Increased bladder volume raises sympathetic tone and constricts the sphincter, a response that can be detected as a shift in heart rate variability in healthy men [8]. The urethral outflow tract is more complex than a simple sphincter. It contains striated and smooth muscle and is controlled by somatic and autonomic nerves. It generates sustained tone to prevent leakage during filling, transient reflex pressure increases when abdominal pressure rises, and relaxation before micturition [9]. The external urethral sphincter is striated muscle under somatic (pudendal) control, while the internal sphincter is smooth muscle under autonomic control. ### Airways Parasympathetic fibers to the airways cause bronchoconstriction and increase mucus secretion through muscarinic M3 receptors. Sympathetic fibers cause bronchodilation through beta-2 receptors. In cats, the sympathetic response is prominent, and beta-2 receptors are a major target for bronchodilator drugs in [feline asthma](/knowledge/veterinary-medicine/clinical-methods/feline-asthma-diagnosis-management). In horses, the vagus dominates airway tone, and vagal-mediated bronchoconstriction is a key feature of recurrent airway obstruction. ### Liver and Metabolism The liver receives both sympathetic and parasympathetic innervation. Parasympathetic cholinergic neurons from the dorsal motor nucleus of the vagus innervate a subset of hepatocytes and cholangiocytes. Eliminating these neurons in mice prevented hepatic steatosis on a high-fat diet and promoted browning of inguinal white adipose tissue [10]. This shows that the parasympathetic system is not only a digestive regulator but also a metabolic one. ## Comparative Species Notes ### Dogs and Horses: Vagal Dominance In dogs and horses, the vagus nerve exerts strong resting control over the heart, a state sometimes called vagal dominance. This is why these species can develop vagally mediated bradycardia and why atropine, a muscarinic blocker, produces a marked increase in heart rate. In horses, the vagus also dominates airway tone and sweat gland control, with sympathetic cholinergic fibers driving sweating during exercise. ### Cats: Prominent Sympathetic Response Cats show a prominent sympathetic response, which is why they are classic models for studying fight-or-flight behavior. Pupillary dilation, piloerection, and tachycardia are pronounced. Feline asthma involves both parasympathetic bronchoconstriction and sympathetic beta-2-mediated bronchodilation, making beta-2 agonists effective. ### Ruminants: Species-Specific Responses Ruminants show species-specific autonomic responses tied to their digestive anatomy. The vagus nerve plays a major role in rumen motility and eructation. Sheep have a cervical sympathetic trunk with variable ganglion numbers and positions that differ from humans and other species [2]. These anatomical differences affect how sympathetic and parasympathetic signals reach the gut and other organs. ### Sweat Glands: A Key Exception Sweat glands are sympathetic cholinergic in horses but adrenergic in most other species. This means that in horses, sweating is driven by acetylcholine acting on muscarinic receptors, while in dogs and cats, sweating is minimal and adrenergic mechanisms dominate where present. This exception is a favorite exam question and a practical point when using anticholinergic drugs in horses. ## Worked Clinical Scenarios ### Scenario 1: A Dog with Acute Heart Failure A dog presents with tachycardia, pale mucous membranes, and weak pulses. The sympathetic system is maximally activated, increasing heart rate and constricting peripheral vessels to maintain blood pressure. Beta-1 receptors in the heart are stimulated, and alpha-1 receptors constrict arterioles. The parasympathetic system is suppressed. Treatment aims to reduce sympathetic drive and support the heart, not to stimulate the vagus further. ### Scenario 2: A Horse with Colic A horse with colic may have reduced gut motility from sympathetic dominance. Pain and stress increase sympathetic tone, which inhibits gut motility and secretion. The vagus normally promotes motility, but sympathetic activation overrides it. This is why colic management includes pain control and why vagal stimulation is not a primary treatment. ### Scenario 3: A Cat with Asthma A cat with asthma has bronchoconstriction from parasympathetic muscarinic activation and reduced beta-2-mediated bronchodilation. Beta-2 agonists relax the airways, while muscarinic antagonists reduce vagal tone. The prominent sympathetic response in cats means beta-2 receptors are a key drug target. ### Scenario 4: A Cow with Bloat A cow with bloat has impaired eructation, often from vagal dysfunction. The vagus nerve controls rumen motility and the opening of the esophageal groove. If vagal tone is lost, the rumen stops contracting and gas accumulates. This is a species-specific example of parasympathetic control of a digestive function that has no direct equivalent in dogs or cats. ## Clinical Relevance, Limitations and Common Mistakes The autonomic nervous system is a frequent source of confusion because its effects are organ-specific and species-specific. The most common mistake is assuming that all sympathetic effects are excitatory and all parasympathetic effects are inhibitory. In reality, parasympathetic activation contracts the bladder detrusor and slows the heart, while sympathetic activation contracts the internal urethral sphincter and dilates the pupil. The direction of effect depends on the receptor, not the division. A second mistake is ignoring species differences. Sweat glands are sympathetic cholinergic in horses but adrenergic in most other species. Vagal tone dominates in dogs and horses, while cats show a prominent sympathetic response. Ruminants have unique vagal control of rumen motility. These differences mean that drug responses and clinical signs vary across species. A third mistake is treating sympathovagal balance as a fixed set point. In dogs with premature ventricular contractions, exercise increased both sympathetic and vagal activity, and disease changed the pattern of both [6]. Balance is dynamic and disease-dependent. A fourth mistake is forgetting that the parasympathetic system has metabolic roles beyond digestion. Vagal cholinergic neurons innervate the liver and influence lipid metabolism [10]. This is an active area of research and a reminder that the autonomic system touches nearly every organ. Individual animals vary, and clinical decisions require a veterinarian who can assess the whole patient. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## Frequently Asked Questions ### What is the main difference between the parasympathetic and sympathetic nervous systems? The parasympathetic system originates from the craniosacral region and promotes rest and digestion, while the sympathetic system originates from the thoracolumbar region and promotes fight-or-flight responses. ### Which neurotransmitter is used by both divisions at the ganglion? Acetylcholine is the neurotransmitter at every autonomic ganglion, acting on nicotinic receptors in both the parasympathetic and sympathetic divisions. ### Why do horses sweat differently from dogs? Horse sweat glands are innervated by sympathetic cholinergic fibers that release acetylcholine, while most other species have adrenergic sweat gland control or minimal sweating. ### Which division controls bladder emptying? The parasympathetic system controls bladder emptying by contracting the detrusor muscle and relaxing the internal urethral sphincter. ### Do cats have a stronger sympathetic response than dogs? Cats show a prominent sympathetic response, which is why they are classic models for fight-or-flight behavior, while dogs often show stronger vagal tone at rest. ### What happens to the heart when the vagus nerve is cut? Cutting the vagus nerve removes the parasympathetic brake on the heart, leading to increased heart rate and, in some cases, cardiac remodeling and fibrosis. ## Related Articles - [Viral Nervous Necrosis Virus (Betanodavirus)](/knowledge/viruses/aquatic-viruses/viral-nervous-necrosis-virus) - [Differential Diagnosis of Nervous Signs in Cattle: From Metabolic to Infectious](/knowledge/veterinary-medicine/food-animal-medicine/differential-diagnosis-nervous-signs-cattle-metabolic-infectious) - [Coenurus cerebralis in Sheep: Gid or Sturdy, Taenia multiceps Larval Cestode Infection, and Nervous Signs](/knowledge/parasites/livestock-parasites/coenurus-cerebralis-gid-sturdy-taenia-multiceps-sheep-nervous-larval-cestode) - [Veterinary Computer Communication Systems: An Overview](/knowledge/veterinary-medicine/clinical-skills-training/veterinary-computer-communication-systems) - [Canine Respiratory System: Anatomy and Physiology](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/canine-respiratory-system-anatomy-physiology) - [Canine Lymphatic System: Anatomy and Function](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/canine-lymphatic-system-anatomy-function) - [Function of Nerves: How the Nervous System Works](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/function-of-nerves-how-the-nervous-system-works) - [Somatic Nervous System: Structure and Function](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/somatic-nervous-system-structure-and-function) - [Brain Ventricles: Anatomy and Function of the Ventricular System](/blog/guides/brain-ventricles-anatomy-and-function-of-the-ventricular-system) ## Sources 1. 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