Difficulty: Intermediate | Prerequisites: Nervous system divisions (CNS, PNS, somatic vs autonomic), basic understanding of neurotransmitters and synaptic transmission.
Tags: autonomic nervous system, ANS, sympathetic, parasympathetic, fight or flight, rest and digest, noradrenaline, norepinephrine, acetylcholine, dual innervation, antagonistic control, organ effects, heart rate, bronchodilation, bronchoconstriction, pupil dilation, pupil constriction, homeostasis, adrenergic, cholinergic
The autonomic nervous system (ANS) runs the body's background operations: heart rate, digestion, breathing rate, pupil size, glandular secretion, and more. None of this requires conscious effort. The ANS has two main branches that mostly work in opposition to each other, the sympathetic division (mobilises resources during stress) and the parasympathetic division (conserves resources during rest).
You should already know the high-level nervous system hierarchy (CNS → PNS → somatic vs autonomic) and have a working understanding of how neurotransmitters act at synapses. If those are not solid, revisit the earlier notes on nervous system branches and synaptic transmission.
This topic is clinically important because many common medications (beta-blockers, bronchodilators, atropine) act on sympathetic or parasympathetic receptors. Understanding the two divisions and their organ-level effects is the foundation for that pharmacology.
The sympathetic division activates during stress, increasing heart rate, dilating airways, dilating pupils, and diverting blood to skeletal muscles. The parasympathetic division dominates at rest, slowing the heart, constricting pupils, and stimulating digestion. Most organs receive input from both divisions, and the balance between them maintains homeostasis.
Autonomic nervous system (ANS)
The involuntary division of the PNS that controls smooth muscle, cardiac muscle, and glands. In simple terms, the ANS handles everything you do not have to think about.
Sympathetic division ("fight or flight")
The branch of the ANS that prepares the body for physical exertion, stress, or danger. It increases heart rate, redirects blood to muscles, and raises blood glucose.
Parasympathetic division ("rest and digest")
The branch of the ANS that conserves energy and maintains routine body functions. It slows the heart, promotes digestion, and stimulates secretion from glands.
Dual innervation
The principle that most organs receive nerve fibres from both sympathetic and parasympathetic divisions. The two divisions typically produce opposite effects on the same organ, and the balance between them determines the organ's activity level at any given moment.
Noradrenaline (norepinephrine)
The primary neurotransmitter released by postganglionic sympathetic neurons at target organs. It binds adrenergic receptors. Think of it as the chemical messenger of the "fight or flight" response at the organ level.
Acetylcholine (ACh)
The neurotransmitter used by all preganglionic autonomic neurons (both divisions), all postganglionic parasympathetic neurons, and also the somatic motor system. At parasympathetic target organs, it binds muscarinic receptors.
Adrenergic receptors
Receptors that respond to noradrenaline (and adrenaline). Found on target organs of sympathetic postganglionic neurons. Major subtypes include alpha (α) and beta (β) receptors, each producing different effects depending on the tissue.
Cholinergic receptors
Receptors that respond to acetylcholine. Nicotinic receptors are found at autonomic ganglia and the neuromuscular junction. Muscarinic receptors are found at parasympathetic target organs.
Homeostasis
The maintenance of a stable internal environment. The antagonistic action of the sympathetic and parasympathetic divisions is a primary mechanism for achieving this.
The sympathetic division activates when the body perceives a threat or needs to perform. Its effects are widespread and rapid:
Heart – increases rate and force of contraction (gets more blood to muscles).
Lungs – dilates bronchi (opens the airways to increase oxygen intake).
Eyes – dilates pupils (mydriasis, lets more light in for better vision).
Digestive organs – reduces activity (blood is redirected away from digestion toward muscles).
Liver – stimulates glycogenolysis (releases stored glucose into the blood for quick energy).
Blood vessels – constricts vessels to non-essential organs, dilates vessels to skeletal muscle.
Sweat glands – increases secretion (cooling during exertion).
Adrenal medulla – stimulates release of adrenaline (epinephrine) and noradrenaline into the bloodstream, amplifying and prolonging the sympathetic effects.
The neurotransmitter at the target organ is noradrenaline (with the exception of sweat glands, which use acetylcholine). All preganglionic fibres use acetylcholine.
The parasympathetic division dominates during calm, routine conditions. Its effects are essentially the reverse of sympathetic activation:
Heart – slows rate (reduces cardiac output when exertion is not needed).
Lungs – constricts bronchi (airways narrow slightly when maximum airflow is not required).
Eyes – constricts pupils (miosis, reduces light entry in bright conditions) and accommodates the lens for near vision.
Digestive organs – increases motility and secretion (promotes digestion and absorption of nutrients).
Salivary glands – increases saliva production (preparing food for digestion).
Bladder – contracts the detrusor muscle, promotes urination.
Glands generally – increases secretion.
The neurotransmitter at the target organ is acetylcholine (acting on muscarinic receptors). As with the sympathetic division, preganglionic fibres also use acetylcholine (acting on nicotinic receptors at the ganglion).
Most organs receive both sympathetic and parasympathetic fibres. The two divisions act like a seesaw:
At rest, parasympathetic tone dominates. The heart beats at a moderate resting rate, digestion proceeds, and pupils are at normal diameter.
During a stressful event, sympathetic activity increases and parasympathetic activity decreases. Heart rate rises, digestion slows, airways open, and pupils dilate.
After the stress passes, sympathetic activity drops and parasympathetic tone returns. The body settles back into maintenance mode.
This continuous push-and-pull is what maintains homeostasis. It is not an all-or-nothing switch; both systems are always active to some degree, and the balance shifts depending on circumstances.
Heart rate – Sympathetic increases; parasympathetic decreases.
Bronchi – Sympathetic dilates; parasympathetic constricts.
Pupils – Sympathetic dilates (mydriasis); parasympathetic constricts (miosis).
Digestive activity – Sympathetic inhibits; parasympathetic stimulates.
Salivary glands – Sympathetic produces thick, viscous secretion; parasympathetic produces watery secretion.
Bladder – Sympathetic relaxes detrusor (holds urine); parasympathetic contracts detrusor (promotes urination).
Blood glucose – Sympathetic raises (glycogenolysis); parasympathetic has minimal direct effect.
Beta-blockers (e.g. propranolol, atenolol) are among the most widely prescribed medications in the world. They block sympathetic beta-adrenergic receptors on the heart, reducing heart rate and blood pressure. This is a direct clinical application of understanding sympathetic effects on the heart.
Atropine, used in emergency medicine and ophthalmology, blocks muscarinic (parasympathetic) receptors. It increases heart rate (blocks parasympathetic slowing) and dilates pupils (blocks parasympathetic constriction). Every effect follows logically from blocking the "rest and digest" division.
Students often assume the sympathetic and parasympathetic systems are either fully "on" or fully "off." In reality, both are always partially active (tonic activity), and homeostasis results from the balance between them.
Confusing the neurotransmitter at the target organ: sympathetic postganglionic neurons mostly release noradrenaline, but preganglionic neurons (both divisions) release acetylcholine. The parasympathetic division uses acetylcholine at both the ganglion and the target organ.
Thinking "fight or flight" means only extreme fear. The sympathetic division also activates during exercise, excitement, standing up quickly, or even eating a large meal (to a mild degree). It is a spectrum, not just a panic response.
⚠️ Be able to fill in a table comparing sympathetic and parasympathetic effects on: heart, lungs, eyes, digestive organs, and glands. This is one of the most common exam formats for this topic.
⚠️ Know the neurotransmitters: ACh at all preganglionic synapses (both divisions) and at parasympathetic target organs; noradrenaline at sympathetic target organs (with the sweat-gland exception).
⚠️ Understand dual innervation and how the balance between the two divisions maintains homeostasis. Exam questions often describe a scenario (e.g. running from danger, relaxing after dinner) and ask you to predict the organ-level effects.
⚠️ The vagus nerve (CN X) is the primary parasympathetic nerve for thoracic and abdominal organs. This connects directly to the cranial nerve material.
True or false: The sympathetic division constricts the pupils.
Fill in the blank: The neurotransmitter at parasympathetic target organs is _______.
True or false: During "fight or flight," digestive activity increases.
Fill in the blank: Most organs receive fibres from both divisions, a principle called _______.
True or false: Sympathetic preganglionic neurons release noradrenaline.
Answers: 1. False (dilates). 2. Acetylcholine (ACh). 3. False (decreases). 4. Dual innervation. 5. False (acetylcholine; noradrenaline is the postganglionic neurotransmitter).
Q: Describe the effects of sympathetic activation on the heart, lungs, and digestive system.
A: Sympathetic activation increases heart rate and force of contraction, dilates the bronchi to increase airflow, and inhibits digestive activity (reduced motility and secretion). Together these changes redirect resources toward skeletal muscles and away from non-urgent maintenance functions.
Q: A patient is given a drug that mimics acetylcholine at muscarinic receptors. Predict the effects on heart rate and pupil size.
A: Muscarinic receptors are the parasympathetic receptors at target organs. Stimulating them would slow heart rate (parasympathetic effect on the heart) and constrict the pupils (miosis, the parasympathetic effect on the eye).
Q: Explain why both divisions of the ANS use acetylcholine at the ganglionic synapse, yet produce opposite effects at the target organ.
A: At the ganglion, both divisions use ACh acting on nicotinic receptors to relay the signal to the postganglionic neuron. The difference arises at the target organ: parasympathetic postganglionic fibres release ACh onto muscarinic receptors, while sympathetic postganglionic fibres release noradrenaline onto adrenergic receptors. The different neurotransmitters and receptor types at the organ level produce the opposite effects.
Q: Why is the vagus nerve considered the most important parasympathetic nerve?
A: The vagus nerve (CN X) provides parasympathetic innervation to the heart, lungs, and most abdominal organs. It is responsible for slowing the heart rate, stimulating digestive activity, and promoting other "rest and digest" functions across a wide territory. No other parasympathetic nerve has such extensive distribution.
The ANS builds directly on the nervous system hierarchy covered in the first set of notes (the autonomic branch sits under the PNS motor division). Synaptic transmission concepts (neurotransmitter release, receptor binding, signal termination) apply at every autonomic synapse. The vagus nerve links to the cranial nerves section. Pharmacology courses expand on the adrenergic and cholinergic receptor subtypes introduced here, since so many drugs target these receptors.
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